Topic 4 Acids and Bases Part A Unit-based exercise Unit 14 Acids and alkalis Fill in the blanks 1 hydrochloric 2 Sulphuric 3 Ethanoic 4 sour 5 red; yellow 6 colourless; red 7 bases 8 dissociate; dissociation 9 hydrogen 10 mobile ions 11 basicity 12 monobasic 13 dibasic 14 a) salt; hydrogen b) salt; water; carbon dioxide c) salt; water; carbon dioxide d) salt; water e) salt; water 15 salt; water 16 alkali 17 bitter 18 ammonia 19 precipitates 20 complex 1
21 hygroscopic 22 deliquescent True or false 23 F Concentrated sulphuric acid should be diluted by adding the acid to water slowly while stirring. 24 T 25 F Solid citric acid has NO effect on zinc. The aqueous solution of citric acid shows typical properties of an acid while solid citric acid does not. 26 F An acid is a hydrogen-containing substance that gives hydrogen ions as the only type of positive ions when dissolved in water. 27 T The aqueous solution of citric acid can conduct electricity. 28 F The basicity of an acid is the maximum number of hydrogen ions produced by one acid molecule. For example, ethanoic acid (CH 3 COOH) is monobasic because only the hydrogen atom in the COOH group can undergo dissociation. 29 T 30 F Phosphoric acid (H 3 PO 4 (aq)) is a tribasic acid. 31 T Concentrated sulphuric acid is hygroscopic. It can absorb water vapour from the air. 32 T When solid citric acid dissolves in water, only a few molecules dissociate to give ions. citric acid(s) + water citric acid(aq) citric acid(aq) H + (aq) + citrate ion(aq) Hence an aqueous solution of citric acid contains mobile citric acid molecules. 33 T 34 F Phenolphthalein is colourless in acidic solutions. 35 F Many metal hydroxides are INSOLUBLE in water. 36 T 2
37 F The discolorations and tight scale buildup that occur in toilet bowls are mostly calcium carbonate deposits from hard water. Calcium carbonate reacts wtih some acids to form water soluble substances. The solid type toilet bowl cleansers are mostly sodium hydrogensulphate while the liquid type contain hydrochloric acid. 38 T Sodium hydroxide absorbs carbon dioxide when exposed to air. Sodium hydrogencarbonate is formed and it undergoes dehydration to give sodium carbonate. 39 F The properties of solutions of alkalis depend on the presence of mobile hydroxide ions. 40 T Ca 2+ (aq) + 2OH (aq) Ca(OH) 2 (s) white precipitate 41 F Iron(II) hydroxide does NOT dissolve in excess dilute aqueous ammonia. 42 F Copper(II) hydroxide does NOT dissolve in excess dilute sodium hydroxide solution. It dissolves in excess dilute aqueous ammonia to give a deep blue solution. 43 T Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) reddish brown precipitate 44 F Concentrated nitric acid tends to decompose to nitrogen dioxide gas and oxygen gas. 4HNO 3 (aq) 4NO 2 (g) + O 2 (g) + 2H 2 O(l) 45 F Concentrated sulphuric acid is NOT used to dry ammonia gas because the acid would react with ammonia gas. H 2 SO 4 (l) + 2NH 3 (g) (NH 4 ) 2 SO 4 (s) Multiple choice questions 46 A 47 C Option A Caustic soda is sodium hydroxide. Opiton B Drain cleansers usually contain sodium hydroxide. Option D Slaked lime is calcium hydroxide. 48 C Option A Dilute acids react with carbonates to give carbon dioxide gas. Option B Dilute acids have a sour taste. Option C Dilute acids conduct electricity due to the presence of mobile ions. Option D Dilute acids react with reactive metals only. 49 C Option A Acidic solutions turn litmus solution red. Option B Dilute solutions of alkalis have a slippery feel. Option D Typical acids show NO reaction with copper. 3
50 B Option C Hydrochloric acid can turn methyl orange red. Option D Concentrated hydrochloric acid is NOT stored in brown bottles. 51 B Option A Sulphuric acid is used to make fertilizers, but it is NOT used as fertilizers. Option B Concentrated sulphuric acid is hygroscopic. Option C Concentrated sulphuric acid should be diluted by adding the acid to water slowly while stirring. Option D Dilute sulphuric acid shows NO oxidizing property. 52 D Option A Nitric acid is NOT a drying agent. Option B Sulphuric acid, NOT nitric acid, is used in car batteries. Option C Dilute nitric acid is an oxidizing agent. It reacts with copper to give nitrogen monoxide, NOT hydrogen. Option D Concentrated nitric acid is corrosive. 53 C Aqueous solution of ethanoic acid can conduct electricity. Ethanoic acid is an electrolyte. 54 C When dilute sulphuric acid reacts with calcium carbonate, insoluble calcium sulphate forms. The calcium sulphate covers the surface of calcium carbonate and prevents further reaction. 55 A Option A Vinegar contains ethanoic acid. It shows NO reaction with copper. 56 A 57 D Option B Dilute nitric acid is an oxidizing agent. It reacts with copper to give nitrogen monoxide gas. Option C Copper(II) oxide and dilute nitric acid undergo neutralization when mixed. Option D Copper(II) hydroxide and vinegar undergo neutralization when mixed. 58 C Solid citric acid does NOT react with magnesium because it does not contain hydrogen ions. The aqueous solution of citric acid shows typical properties of an acid while solid citric acid does not. 59 C Phosphoric acid (H 3 PO 4 ) is a tribasic acid. 60 A Carbon is a reducing agent. It can reduce FeO(s) to Fe(s). 61 D Option A Glass cleansers usually contain ammonia. 62 A Glass cleansers usually contain ammonia. 63 A Option A Sodium hydroxide is manufactured by the electrolysis of concentrated sodium chloride solution. 4 Option C Sodium hydroxide absorbs carbon dioxide when exposed to air. Sodium hydrogencarbonate is formed and it undergoes dehydration to give sodium carbonate.
64 A Option A When ammonia gas dissolves in water, it reacts with water to give ammonium ions and hydroxide ions. However, ammonia does not react with water completely. Only very few hydroxide ions are formed. NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH (aq) Hence aqueous ammonia contains both ammonia molecules and hydroxide ions. Option B Metals tarnish because they react with the air to form a layer of oxide or sulphide. Acids can be used to remove this layer. One of the oxide layers most difficultly removed is rust. Rust removers usually contain an acid such as hydrochloric acid or phosphoric acid. Option C Aqueous ammonia reacts with lead(ii) nitrate solution to give a white precipitate, lead(ii) hydroxide. Option D Aqueous ammonia gives a yellow solution with methyl orange. 65 C Dilute sodium hydroxide solution does NOT give a white precipitate with potassium chloride solution. The mixture is a colourless solution. 66 B Aluminium hydroxide dissolves in excess dilute sodium hydroxide solution due to the formation of a soluble complex salt. Al(OH) 3 (s) + OH (aq) [Al(OH) 4 ] (aq) aluminate ion colourless solution 67 B 68 D Option Solution Observation A ammonium chloride no precipitate B iron(ii) sulphate green precipitate Fe(OH) 2 C magnesium chloride white precipitate Mg(OH) 2 D nickel(ii) sulphate green precipitate Ni(OH) 2 69 D The white precipitate is magnesium hydroxide. 70 D Option Solution Observation A calcium nitrate no precipitate B copper(ii) sulphate pale blue precipitate Cu(OH) 2 C lead(ii) nitrate white precipitate Pb(OH) 2 D zinc sulphate white precipitate Zn(OH) 2 5
71 C Option Hydroxide Solubility in excess NaOH(aq) / NH 3 (aq) A Al(OH) 3 soluble in excess NaOH(aq) B Cu(OH) 2 soluble in excess NH 3 (aq) C Fe(OH) 2 insoluble in both excess NaOH(aq) and NH 3 (aq) D Zn(OH) 2 soluble in both excess NaOH(aq) and NH 3 (aq) 72 C Option Compound X Addition of NH 3 (aq) to dilute solution of X A Fe 2 (SO 4 ) 3 reddish brown precipitate Fe(OH) 3 B MgCl 2 white precipitate Mg(OH) 2 C NiSO 4 green precipitate Ni(OH) 2 D NH 4 Cl no precipitate 73 A Concentrated sodium hydroxide solution is corrosive. 74 B Concentrated nitric acid is an oxidizing agent. 75 C 76 A Option A Concentrated hydrochloric acid and concentrated nitric acid are volatile while concentrated sulphuric acid is NOT. Option B Concentrated sulphuric acid is NOT stored in brown bottles. Option D Concentrated sulphuric acid is NOT used to dry ammonia gas because the acid would react with ammonia gas. H 2 SO 4 (l) + 2NH 3 (g) (NH 4 ) 2 SO 4 (s) 77 A The aqueous solution of compound X gives a gas with dilute hydrochloric acid, hence X is probably a carbonate, rather than a sulphate. Solution of X does NOT give a precipitate with dilute sodium hydroxide solution. Hence X probably contains ammonium ions, rather than iron(ii) ions. X is ammonium carbonate. 78 A From test 1, it can be deduced that X is a chloride. Ag + (aq) + Cl (aq) AgCl(s) From test 2, it can be deduced that X contains iron(ii) ions, rather than iron(iii) ions. Fe 2+ (aq) + 2OH (aq) Fe 3+ (aq) + 3OH (aq) Fe(OH) 2 (s) green precipitate Fe(OH) 3 (s) reddish brown precipitate 6
79 B Option Substances Upon mixing the substances A magnesium and ethanoic acid hydrogen gas is produced B magnesium hydroxide and dilute hydrochloric acid only a salt and water are produced C magnesium carbonate and dilute sulphuric acid carbon dioxide gas is produced D magnesium and steam hydrogen gas is produced magnesium hydroxide and dilute hydrochloric acid would NOT produce a gas when mixed. 80 C Silver nitrate solution gives a white precipitate with dilute hydrochloric acid. Ag + (aq) + Cl (aq) AgCl(s) 81 D NaOH(aq) gives a precipitate Fe 2+ (aq) ions, but NOT K + (aq) ions. 82 C Zinc can be obtained by heating zinc oxide with carbon. Carbon dioxide gas is formed in the process. zinc oxide + carbon zinc + carbon dioxide 83 D The aqueous solution of citric acid shows typical properties of an acid while the citric acid crystals do not. When citric acid crystals (jar 3) are mixed with water (jar 5), the aqueous solution would give carbon dioxide gas with solid hydrogencarbonate (jar 1). HCO 3 (aq) + H + (aq) H 2 O(l) + CO 2 (g) 84 A (2) Carbonic acid is formed when carbon dioxide gas dissolves in water. Hence rainwater is slightly acidic. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) carbonic acid 85 D 86 A (1) Acids are covalent compounds when pure. For example, pure sulphuric acid and nitric acid are colourless liquids consisting of molecules. (2) An acid may NOT contain both hydrogen and oxygen, e.g. HCl(aq). (3) Typical acids would NOT react with copper. 87 B (2) Sulphuric acid is used to make fertilizers, but it is NOT used as fertilizers. 88 B (2) Sulphuric acid, NOT hydrochloric acid, is used in car batteries. 89 D (3) When solid citric acid dissolves in water, only a few molecules dissociate to give ions. citric acid(s) + water citric acid(aq) citric acid(aq) H + (aq) + citrate ion(aq) 7
Hence an aqueous solution of citric acid contains mobile citric acid molecules. 90 B (2) Vinegar contains ethanoic acid. It shows NO reaction with copper. 91 D (2) Carbonic acid is formed when carbon dioxide dissolves in water. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) carbonic acid (3) Sulphurous acid is formed when sulphur dioxide dissolves in water. SO 2 (g) + H 2 O(l) H 2 SO 3 (aq) sulphurous acid 92 D (1) Magnesium reacts with dilute sulphuric acid to give hydrogen gas. (2) and (3) Sodium carbonate and potassium hydrogencarbonate react with dilute sulphuric acid to give carbon dioxide gas. 93 B (2) Very dilute nitric acid acts like a typical acid. It shows NO reaction with copper. 94 D All the three metals (calcium, iron and magnesium) react with acids to give hydrogen. 95 D (1) Heating ammonium sulphate solution with dilute sodium hydroxide solution gives ammonia gas. NH 4 + (aq) + OH (aq) NH 3 (g) + H 2 O(l) (2) Iron(II) sulphate solution gives a green precipitate with dilute sodium hydroxide solution. Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) (3) Citric acid would undergo neutralization reaction with dilute sodium hydroxide solution. 96 D (1) When ammonia gas dissolves in water, it reacts with water to give ammonium ions and hydroxide ions. However, ammonia does not react with water completely. Only very few hydroxide ions are formed. NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH (aq) Hence aqueous ammonia contains both ammonia molecules and ammonium ions. 8
97 B (1) Cu(OH) 2 dissolves in excess dilute aqueous ammonia to give a deep blue solution. Cu(OH) 2 (s) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) (2) Pb(OH) 2 is insoluble in excess dilute aqueous ammonia. (3) Zn(OH) 2 dissolves in excess dilute aqueous ammonia to give a colourless solution. Zn(OH) 2 (s) + 4NH 3 (aq) [Zn(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) 98 A (1) Heating ammonium chloride with calcium hydroxide solution liberates ammonia gas. 2NH 4 Cl(s) + Ca(OH) 2 (aq) 2NH 3 (g) + CaCl 2 (aq) + 2H 2 O(l) 99 A (3) Addition of dilute sodium hydroxide solution to zinc sulphate solution gives a white precipitate, Zn(OH) 2. Zn(OH) 2 dissolves in excess dilute sodium hydroxide solution to give a colourless solution. Zn(OH) 2 (s) + 2OH (aq) [Zn(OH) 4 ] 2 (aq) 100 A (2) Heating solutions of ammonium chloride and sodium hydroxide liberates ammonia. NH 4 Cl(aq) + NaOH(aq) NH 3 (g) + NaCl(aq) + H 2 O(l) 101 A (1) Magnesium chloride solution gives a white precipitate with silver nitrate solution. Ag + (aq) + Cl (aq) AgCl(s) (2) Magnesium chloride solution gives a white precipitate with dilute aqueous ammonia. Mg 2+ (aq) + 2OH (aq) Mg(OH) 2 (s) (3) Copper is less reactive than magnesium. It CANNOT displace magnesium from magnesium chloride solution. 102 B (1) Zinc is more reactive than silver, but less reactive than sodium. Zinc granules can displace silver from silver nitrate solution. There is NO reaction between zinc granules and sodium nitrate solution. (2) Both sodium nitrate solution and silver nitrate solution are colourless. (3) Only silver nitrate solution gives a white precipitate with dilute hydrochloric acid. Ag + (aq) + Cl (aq) AgCl(s) 103 A (1) NH 3 (aq) and NiSO 4 (aq) would form a green precipitate when mixed. Ni 2+ (aq) + 2OH (aq) Ni(OH) 2 (s) (2) NaOH(aq) and FeSO 4 (aq) would form a green precipitate when mixed. Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) (3) There is no observable change when NH 3 (aq) and NaCl(aq) are mixed. 9
104 D (1) Ammonia gas would react with dilute sulphuric acid. 105 B 2NH 3 (g) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) (2) Carbon dioxide gas would give a white precipitate with calcium hydroxide solution. CO 2 (g) + Ca(OH) 2 (aq) CaCO 3 (s) + H 2 O(l) (3) Sulphur dioxide gas would react with sodium hydroxide solution. SO 2 (g) + NaOH(aq) NaHSO 3 (aq) 106 C Aqueous solution of ethanoic acid can conduct electricity. Hence ethanoic acid is an electrolyte. Ethanoic acid is a covalent compound when pure. Pure ethanoic acid consists of molecules. 107 D When dilute sulphuric acid reacts with calcium carbonate, insoluble calcium sulphate forms. The calcium sulphate covers the surface of calcium carbonate and prevents further reaction. 108 B Citric acid shows its acidic properties in the presence of water. When solid citric acid dissolves in water, only a few molecules dissociate to give ions. citric acid(s) + water citric acid(aq) citric acid(aq) H + (aq) + citrate ion(aq) Hence an aqueous solution of citric acid contains mobile citric acid molecules. 109 D 110 D 111 C Dilute nitric acid is an oxidizing agent. It reacts with copper to give nitrogen monoxide, NOT hydrogen. 112 A Concentrated nitric acid decomposes according to the following chemical equation: 4HNO 3 (aq) 4NO 2 (g) + O 2 (g) + 2H 2 O(l) 113 D Many metal hydroxides are insoluble in water. 114 B When ammonia gas dissolves in water, it reacts with water to give ammonium ions and hydroxide ions. NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH (aq) Dilute aqueous ammonia conducts electricity due to the presence of mobile ions. 115 C Anhydrous calcium chloride is NOT used to dry ammonia gas because calcium chloride would react with ammonia gas. 10 CaCl 2 (s) + 4NH 3 (g) CaCl 2 4NH 3 (s)
Unit 15 Molarity, ph scale and strengths of acids and alkalis Fill in the blanks 1 molarity 2 hydrogen ions 3 neutral 4 less 5 increases 6 hydroxide ions 7 hydrogen ions; hydrogensulphate ions; sulphate ions 8 hydrogen ions; ethanoate ions; ethanoic acid molecules 9 ammonium ions; hydroxide ions; ammonia molecules 10 concentrated; strong True or false 11 F The ph of a solution decreases as the concentration of hydrogen ions in the solution increases. 12 T 13 F The ph of dilute aqueous ammonia is greater than 7, but less than 14. 14 T 15 F Aqueous solution of ethanoic acid can conduct electricity. Hence ethanoic acid is an electrolyte. 16 T When citric acid crystals dissolve in water, only a few molecules dissociate to give ions. Hence citric acid is a weak acid. 17 F When we describe acids as strong and weak, we are talking about the extent of their dissocation in water. When we talk about concentration, we are referring to the amount of an acid in a unit volume of solution. 18 T For example, 5 mol dm 3 nitric acid is a concentrated solution of a strong acid while 0.1 mol dm 3 nitric acid is a dilute solution of a strong acid. 19 F 1 mol dm 3 HCl(aq) has a higher concentration of hydrogen ions than 1 mol dm 3 CH 3 COOH(aq) does. Hence the ph of HCl(aq) is lower than that of CH 3 COOH(aq). 11
20 F NaOH(aq) + HCl(aq) NaCl(aq) + H 2 O(l) NH 3 (aq) + HCl(aq) NH 4 Cl(aq) According to the equations, 1 mole of NaOH / NH 3 requires 1 mole of HCl for complete neutralization. Hence 10 cm 3 of 1 mol dm 3 NaOH(aq) and 10 cm 3 of 1 mol dm 3 NH 3 (aq) require the same number of moles of HCl for complete neutralization. Multiple choice questions 21 B Number of moles of KNO 3 molarity of KNO 3 solution x volume of solution 40 0.25 mol dm 3 x 1.0 x 10 2 mol 22 D Molar mass of Na 2 CO 3 (2 x 23.0 + 12.0 + 3 x 16.0) g mol 1 106.0 g mol 1 mass Number of moles of Na 2 CO 3 molar mass 13.8 g 106.0 g mol 1 0.130 mol Volume of solution 500 Molarity of Na 2 CO 3 solution number of moles of Na 2CO 3 volume of solution 0.130 mol ( 500 1 000 ) dm3 0.260 mol dm 3 the molarity of the sodium carbonate solution is 0.260 mol dm 3. 23 D Molar mass of (COOH) 2 2H 2 O [2 x (12.0 + 2 x 16.0 + 1.0) + 2 x (2 x 1.0 + 16.0)] g mol 1 126.0 g mol 1 mass Number of moles of (COOH) 2 2H 2 O present molar mass 3.15 g 126.0 g mol 1 0.0250 mol 12 Molarity of ethanedioic acid solution number of moles of (COOH) 2 2H 2 O volume of solution 0.0250 mol 250.0 ( 1 000 ) dm3 0.100 mol dm 3 the molarity of the ethanedioic acid solution is 0.100 mol dm 3.
24 B Molar mass of Ca(OH) 2 [40.1 + 2 x (16.0 + 1.0)] g mol 1 74.1 g mol 1 Molarity of calcium hydroxide solution number of moles of Ca(OH) 2 volume of solution 0.150 mol dm 3 number of moles of Ca(OH) 2 250.0 ( 1 000 ) dm3 Number of moles of Ca(OH) 2 0.150 mol dm 3 x 250.0 0.0375 mol Mass of Ca(OH) 2 number of moles of Ca(OH) 2 x molar mass of Ca(OH) 2 0.0375 mol x 74.1 g mol 1 2.78 g 2.78 g of calcium hydroxide are present. 25 D Molar mass of (NH 4 ) 2 SO 4 [2 x (14.0 + 4 x 1.0) + 32.1 + 4 x 16.0)] g mol 1 132.1 g mol 1 Molarity of ammonium sulphate solution number of moles of (NH 4) 2 SO 4 volume of solution 1.02 mol dm 3 number of moles of (NH 4) 2 SO 4 1.50 dm 3 Number of moles of (NH 4 ) 2 SO 4 1.02 mol dm 3 x 1.50 dm 3 1.53 mol Mass of (NH 4 ) 2 SO 4 number of moles of (NH 4 ) 2 SO 4 x molar mass of (NH 4 ) 2 SO 4 1.53 mol x 132.1 g mol 1 202 g 202 g of ammonium sulphate are present. 26 C Molar mass of KCl (39.0 + 35.5) g mol 1 74.5 g mol 1 mass Number of moles of KCl present molar mass 44.7 g 74.5 g mol 1 0.600 mol Molarity of potassium chloride solution 2.40 mol dm 3 0.600 mol Volume of solution 2.40 mol dm 3 0.250 dm 3 250 cm 3 the volume of the solution is 250 cm 3. number of moles of KCl volume of solution 0.600 mol volume of solution 13
27 B Option Number of moles of NaOH molarity of solution x volume of solution A B C D 3.0 mol dm 3 x 2.5 mol dm 3 x 1.5 mol dm 3 x 1.0 mol dm 3 x 100 0.30 mol 200 0.50 mol 300 0.45 mol 400 0.40 mol 200 cm 3 of 2.5 mol dm 3 NaOH(aq) contain the greatest number of moles of solute. 28 D Option A B C D Number of moles of MgSO 4 molarity of solution x volume of solution 0.40 mol dm 3 x 1.0 dm 3 0.40 mol 0.30 mol dm 3 x 1.5 dm 3 0.45 mol 0.20 mol dm 3 x 2.0 dm 3 0.40 mol 0.10 mol dm 3 x 2.5 dm 3 0.25 mol 2.5 dm 3 of 0.10 mol dm 3 MgSO 4 (aq) contain the smallest number of moles of solute. 29 D Number of moles of CaCl 2 in 250.0 cm 3 of 0.300 mol dm 3 solution molarity of solution x volume of solution 0.300 mol dm 3 x 250.0 0.0750 mol Number of moles of CaCl 2 in 150.0 cm 3 of 0.180 mol dm 3 solution molarity of solution x volume of solution 0.180 mol dm 3 x 150.0 0.0270 mol Total number of moles of CaCl 2 in the resulting solution (0.0750 + 0.0270) mol 0.102 mol Total volume of the resulting solution (250.0 + 150.0) cm 3 400.0 cm 3 Concentration of the resulting solution ( 0.102 mol 400.0 1 000 ) dm3 0.255 mol dm 3 the concentration of the resulting solution is 2.55 x 10 1 mol dm 3. 14
30 B Number of moles of NaOH in 100.0 cm 3 of 3.00 mol dm 3 solution molarity of solution x volume of solution 3.00 mol dm 3 x 100.0 0.300 mol Number of moles of NaOH in 50.0 cm 3 of 1.20 mol dm 3 solution molarity of solution x volume of solution 1.20 mol dm 3 x 50.0 0.0600 mol Total number of moles of NaOH in the resulting solution (0.300 + 0.0600) mol 0.360 mol Total volume of the resulting solution (100.0 + 50.0) cm 3 150.0 cm 3 Concentration of the resulting solution ( 0.360 mol 150.0 1 000 ) dm3 2.40 mol dm 3 the concentration of the resulting solution is 2.40 mol dm 3. 31 A ph of citric acid solution log 10 [H + ] 1.85 i.e. log 10 [H + ] 1.85 [H + ] 10 1.85 0.0141 mol dm 3 concentration of hydrogen ions in the citric acid solution is 1.41 x 10 2 mol dm 3. 32 B ph of milk log 10 [H + ] 6.70 i.e. log 10 [H + ] 6.70 [H + ] 10 6.70 2.00 x 10 7 mol dm 3 concentration of hydrogen ions in the sample of milk is 2.00 x 10 7 mol dm 3. 15
33 B Sulphuric acid dissociates completely according to the following equation: H 2 SO 4 (aq) 2H + (aq) + SO 2 4 (aq) 0.0200 mol dm 3? mol dm 3 According to the equation, 1 mole of H 2 SO 4 dissociates to give 2 moles of hydrogen ions. i.e. concentration of hydrogen ions 2 x 0.0200 mol dm 3 0.0400 mol dm 3 ph of acid log 10 (0.0400) ( 1.40) 1.40 ph of the sulphuric acid sample is 1.40. 34 A ph of sample of urine log 10 (7.94 x 10 7 ) ( 6.10) 6.10 ph of the sample of urine is 6.10. 35 C ph of sewage before treatment 6.75 ph of sewage after treatment 7.00 Concentration of hydrogen ions in sewage before treatment 10 6.75 1.78 x 10 7 mol dm 3 Concentration of hydrogen ions in sewage after treatment 10 7.00 mol dm 3 Change in concentration of hydrogen ions (1.78 x 10 7 10 7 ) mol dm 3 0.78 x 10 7 mol dm 3 the change in the concentration of hydrogen ions in the sewage is 7.80 x 10 8 mol dm 3. 36 B Limewater is saturated calcium hydroxide solution. 37 D Option A Baking soda solution is hydrogencarbonate solution. Its ph is about 9. Option D Soft drinks contain carbonic acid. 38 C Option B Glass cleanser is alkaline. Its ph is greater than 7. Option D The ph of sea water is about 8. 39 D Oven cleanser contains sodium hydroxide. 40 D When ammonia gas dissolves in water, it reacts with water to give ammonium ions (NH 4 + (aq)) and hydroxide ions (OH (aq)). NH 3 (g) + water NH 3 (aq) + H 2 O(l) NH 3 (aq) NH 4 + (aq) + OH (aq) Thus aqueous ammonia contains ammonia molecules, water molecules and a few ammonium ions and hydroxide ions. 16
41 C In general, non-metals react with oxygen to form acidic oxides and metals react with oxygen to form basic oxides. Option A Calcium oxide is slightly basic. It reacts slightly with cold water to form calcium hydroxide, which is very slightly soluble. So, only a slightly alkaline solution is formed. CaO(s) + H 2 O(l) Ca(OH) 2 (s) Option C Potassium oxide is strongly basic. It dissolves in water to form potassium hydroxide solution. K 2 O(s) + H 2 O(l) 2KOH(aq) Hence the ph of the solution of potassium oxide is higher than that of calcium oxide. Options B and D Oxides of carbon and sulphur are acidic. The ph values of the solutions of both oxides are less than 7. 42 D Concentration of hydrogen ions in solution X 10 1 mol dm 3 0.1 mol dm 3 Concentration of hydrogen ions in solution Y 10 2 mol dm 3 0.01 mol dm 3 the concentration of hydrogen ions in solution X is ten times that in solution Y. 43 D (1) Baking soda solution is hydrogencarbonate solution. Its ph is about 9. (2) The ph of lemon juice is about 2. (3) The ph of milk is about 6. 44 D (1) and (2) The ph of 0.010 mol dm 3 HCl(aq) is higher than that of 0.100 mol dm 3 HCl(aq). (3) and (4) The ph of 0.100 mol dm 3 NaOH(aq) is higher than that of 0.010 mol dm 3 NaOH(aq). 45 A Options B, C and D Concentrated nitric acid, lemon juice and vinegar are acidic. There was no colour change when they were added to the red solution. 46 D Option A The concentration of hydrogen ions in lemon juice is higher than that in wine. Option B The concentration of hydrogen ions in lemon juice is higher than that in baking soda solution. Option C Option D concentration of hydrogen ions in wine concentration of hydrogen ions in drain cleanser 10 4 mol dm 3 10 12 mol dm 3 the concentration of hydrogen ions is 10 8 times greater in wine than in drain cleanser. concentration of hydrogen ions in distilled water concentration of hydrogen ions in baking soda solution 10 7 mol dm 3 10 9 mol dm 3 the concentration of hydrogen ions is 100 times greater in distilled water than in baking soda solution. 47 B When the right amounts of an acid (ph < 7) and an alkali (ph > 7) are mixed, they react completely to produce a salt and water only (ph 7). 17
48 A 49 A The electrical conductivity of a solution, and hence the brightness of the bulb, is proportional to the concentration of mobile ions. Option Acid Remark Concentration of ions A 25 cm 3 of 0.5 mol dm 3 H 2 SO 4 (aq) H 2 SO 4 (aq) is a strong acid H 2 SO 4 (aq) 2H + (aq) + SO 2 4 (aq) 1.5 mol dm 3 B 25 cm 3 of 0.5 mol dm 3 HCl(aq) HCl(aq) is a strong acid HCl(aq) H + (aq) + Cl (aq) 1.0 mol dm 3 C 25 cm 3 of 0.5 mol dm 3 CH 3 COOH(aq) CH 3 COOH(aq) is a weak acid CH 3 COOH(aq) CH 3 COO (aq) + H + (aq) < 1.0 mol dm 3 D 25 cm 3 of 0.5 mol dm 3 NH 3 (aq) NH 3 (aq) is a weak alkali NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) < 1.0 mol dm 3 25 cm 3 of 0.5 mol dm 3 H 2 SO 4 (aq) have the highest concentration of mobile ions and thus will make the bulb the brightest. 50 C Options A, B and D During the reaction of hydrochloric acid and ethanoic acid with marble chips, marble chips would react with hydrogen ions in the acids. Option C CaCO 3 (s) + 2HCl(aq) Hydrochloric acid is a strong acid that completely dissociates in water. Ethanoic acid is weak acid that only partially dissociates in water. Therefore 1 mol dm 3 hydrochloric acid has a higher concentration of hydrogen ions than 1 mol dm 3 ethanoic acid does. The reaction rate between marble chips and ethanoic acid is thus lower and the reaction takes a longer time to complete. CaCO 3 (s) + 2CH 3 COOH(aq) CaCl 2 (aq) + H 2 O(l) + CO 2 (g) (CH 3 COO) 2 Ca(aq) + H 2 O(l) + CO 2 (g) According to the equations, 2 moles of HCl / CH 3 COOH require 1 mole of CaCO 3 for complete reaction. 100 cm 3 of 1 mol dm 3 hydrochloric acid and 100 cm 3 of 1 mol dm 3 ethanoic acid require the same number of moles of CaCO 3 for complete reaction. Hence the mass of marble chips left over would be the same for both acids. 51 A Carbonic acid is a weak acid that only partially dissociates in water. H 2 CO 3 (aq) 2H + (aq) + CO 3 2 (aq) The concentration of H 2 CO 3 (aq) is the highest in 0.10 mol dm 3 carbonic acid. 52 C Both nitric acid and hydrochloric acid are monobasic strong acids. HNO 3 (aq) HCl(aq) H + (aq) + NO 3 (aq) H + (aq) + Cl (aq) Therefore the concentration of hydrogen ions in 1.0 mol dm 3 nitric acid and 1.0 mol dm 3 hydrochloric acid is the same. 18
Mixing 100 cm 3 of the nitric acid with 100 cm 3 of the hydrochloric acid would NOT result in a change in the concentration of hydrogen ions, i.e. would NOT result in a change in ph. 53 A When we describe acids as strong and weak, we are talking about the extent of their dissociation in water. When we talk about concentration, we are referring to the amount of an acid in a unit volume of solution. Sulphuric acid is a strong acid. Hence 0.1 mol dm 3 sulphuric acid is a dilute solution of a strong acid. 54 B The ph of NaCl(aq) is 7. Its ph remains constant upon dilution. 55 A During the reaction between magnesium and an acid, magnesium reacts with hydrogen ions in the acid. From the diagrams, it can be deduced that magnesium reacts more slowly with 0.1 mol dm 3 HA(aq). The concentration of hydrogen ions in 0.1 mol dm 3 HA(aq) is lower than that in 0.1 mol dm 3 HB(aq). Hence acid HA is probably weaker than acid HB. 56 A (3) Washing soda is hydrated sodium carbonate (Na 2 CO 3 10H 2 O). Its solution is alkaline, with a ph > 7. 57 A (2) and (3) When we describe acids as strong and weak, we are talking about the extent of their dissociation in water. When we talk about concentration, we are referring to the amount of an acid in a unit volume of solution. For example, 5 mol dm 3 nitric acid is a concentrated solution of a strong acid while 0.1 mol dm 3 nitric acid is a dilute solution of a strong acid. 58 A In general, non-metals react with oxygen to form acidic oxides and metals react with oxygen to form basic oxides. (1) and (2) Calcium oxide and magnesium oxide are slightly basic. They react with cold water to form hydroxides which are slightly soluble. Solutions with a ph > 7 are formed. CaO(s) + H 2 O(l) MgO(s) + H 2 O(l) Ca(OH) 2 (s) Mg(OH) 2 (s) 59 B (1) Hydrochloric acid is a strong acid while ethanoic acid is a weak acid. (2) The ph is a measure of the concentration of hydrogen ions in a solution. Therefore acid solutions of the same ph should have the same concentration of hydrogen ions. (3) Hydrochloric acid almost completely dissociates in water to give hydrogen ions and chloride ions. Ethanoic acid only partially dissociates in water, forming very few hydrogen ions. Hence hydrochloric acid and ethanoic acid having the same ph should have different concentrations. 60 C (1) Adding 1 mol dm 3 hydrochloric acid to the ethanoic acid makes the ethanoic acid more acidic, thus decreasing the ph of the ethanoic acid. (2) and (3) Solid sodium carbonate and magnesium react with the ethanoic acid, thus both will increase the ph of the ethanoic acid. 19
61 A (1) SO 2 (g) will neutralize the dilute sodium hydroxide solution, making the solution less alkaline. Thus the ph of the solution will decrease. (2) NaCl(s) is neutral. It will NOT lower the ph of the dilute sodium hydroxide solution. (3) When NH 3 (g) dissolves in water, it reacts with water to give ammonium ions and hydroxide ions. It will NOT lower the ph of the dilute sodium hydroxide solution. 62 B (1) The electrical conductivity of a solution is proportional to the concentration of mobile ions. Hydrochloric acid is a strong acid while ethanoic acid is a weak acid. Therefore 1 mol dm 3 hydrochloric acid has a higher concentration of mobile ions than 1 mol dm 3 ethanoic acid does. Hence the electrical conductivity of 1 mol dm 3 hydrochloric acid is higher than that of 1 mol dm 3 ethanoic acid. (2) CH 3 COOH(aq) + NaOH(aq) CH 3 COONa(aq) + H 2 O(l) HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) According to the equations, 1 mole of CH 3 COOH / HCl requires 1 mole of NaOH for complete neutralization. 1 mol dm 3 ethanoic acid and 1 mol dm 3 hydrochloric acid of the same volume require the same number of moles of NaOH for complete neutralization. (3) For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. For neutralization in which either the acid or alkali or both are weak, the heat released is less than 57 kj for 1 mole of water produced. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. Ethanoic acid is a weak acid while hydrochloric acid is a strong acid. Hence the temperature rise for the neutralization between 1 mol dm 3 ethanoic acid and NaOH(aq) is lower than that between 1 mol dm 3 hydrochloric acid and NaOH(aq). 63 C (1) NaOH(aq) is a strong alkali while NH 3 (aq) is a weak alkali. Hence 1 mol dm 3 NaOH(aq) is more alkaline than 1 mol dm 3 NH 3 (aq). The ph of 1 mol dm 3 NaOH(aq) is higher than that of 1 mol dm 3 NH 3 (aq). (2) The electrical conductivity of a solution is proportional to the concentration of mobile ions. NaOH(aq) is a strong alkali while NH 3 (aq) is a weak alkali. Therefore 1 mol dm 3 NaOH(aq) has a higher concentration of mobile ions than 1 mol dm 3 NH 3 (aq) does. Hence the electrical conductivity of 1 mol dm 3 NaOH(aq) is higher than that of 1 mol dm 3 NH 3 (aq). 20
(3) For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. For neutralization in which either the acid or alkali or both are weak, the heat released is less than 57 kj for 1 mole of water produced. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. NaOH(aq) is a strong alkali while NH 3 (aq) is a weak alkali. Hence the temperature rise for the neutralization between 1 mol dm 3 larger than that between 1 mol dm 3 NH 3 (aq) and HCl(aq). NaOH(aq) and HCl(aq) is 64 D Carbon dioxide reacts with water to form carbonic acid. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) Carbonic acid is a weak acid. It only partially dissociates in water. H 2 CO 3 (aq) 2H + (aq) + CO 3 2 (aq) (1) The resulting solution contains mobile ions and thus conducts electricity better than water. (2) Carbonic acid undergoes neutralization with dilute sodium hydroxide solution. 65 D (2) Aqueous solution of citric acid can conduct electricity. Hence citric acid is an electrolyte. 66 A 67 D Very dilute nitric acid acts as a typical acid. It reacts with magnesium. When we describe acids as strong and weak, we are talking about the extent of their dissocation in water. When we talk about concentration, we are referring to the amount of an acid in a unit volume of solution. For example, 5 mol dm 3 nitric acid is a concentrated solution of a strong acid while 0.1 mol dm 3 nitric acid is a dilute solution of a strong acid. 68 C Adding 1 mol dm 3 NaCl(aq) to 50 cm 3 of 1 mol dm 3 HCl(aq) increases the total volume of the acid. Hence the concentration of hydrogen ions in the acid decreases. Thus the ph of the acid is affected. 69 D Ethanoic acid is a weak acid. The basicity of ethanoic acid is 1. 21
70 B Sulphuric acid is a strong acid because it almost completely dissociates in water. Ethanoic acid is a weak acid because it only partly dissociates in water. 71 B Solid citric acid does NOT react with magnesium because it does not contain hydrogen ions. The aqueous solution of citric acid shows typical properties of an acid while solid citric acid does not. 72 B A solution can conduct electricity due to the presence of mobile ions. When solid citric acid dissolves in water, some of the molecules dissociate to give ions. citric acid(aq) H + (aq) + citrate ion(aq) Hence an aqueous solution of citric acid can conduct electricity. 73 C The electrical conductivity of a solution is proportional to the concentration of mobile ions. NaOH(aq) is a strong alkali while NH 3 (aq) is a weak alkali. Therefore 1 mol dm 3 NaOH(aq) has a higher concentration of mobile ions than 1 mol dm 3 NH 3 (aq) does. Hence the electrical conductivity of 1 mol dm 3 NaOH(aq) is higher than that of 1 mol dm 3 NH 3 (aq). When ammonia gas dissolves in water, it reacts with water to give ammonium ions and hydroxide ions. However, ammonia does not react with water completely. Only very few hydroxide ions are formed. NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH (aq) Hence there are many mobile ammonia molecules in aqueous ammonia. 74 C Both 1 mol dm 3 NaOH(aq) and 1 mol dm 3 NH 3 (aq) form a reddish brown precipitate with Fe 2 (SO 4 ) 3 (aq). 75 C CH 3 COOH(aq) + NaOH(aq) CH 3 COONa(aq) + H 2 O(l) HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) According to the equations, 1 mole of CH 3 COOH / HCl requires 1 mole of NaOH for complete neutralization. Hence 10 cm 3 of 1 mol dm 3 ethanoic acid and 10 cm 3 of 1 mol dm 3 hydrochloric acid require the same number of moles of sodium hydroxide for complete neutralization. Unit 16 Salts and neutralization Fill in the blanks 1 neutralization 2 exothermic 3 sodium hydrogensulphate; sodium sulphate 22
4 calcium; lead(ii); barium 5 lead(ii); silver True or false 6 T 7 F In neutralization, salt and water are the only products. However, potassium carbonate and dilute hydrochloric acid react to give a salt, carbon dioxide gas and water. Therefore the reaction is NOT a neutralization reaction. 8 T 9 F Heat is absorbed during the sublimation of iodine. Hence this is NOT an exothermic process. 10 F The reaction between dilute hydrochloric acid and dilute sodium hydroxide solution produces only one salt, sodium chloride. Sodium chloride is a normal salt. 11 T Sulphur dioxide would react with sodium hydroxide solution. SO 2 (g) + NaOH(aq) NaHSO 3 (aq) 12 T A molecule of sulphuric acid can produce two hydrogen ions. If dilute sulphuric acid is allowed to react with dilute sodium hydroxide solution, two kinds of salt can form. When one of the hydrogen ions is replaced, the salt formed is sodium hydrogensulphate (NaHSO 4 ). It is an acid salt. When both hydrogen ions are replaced, the salt formed is sodium sulphate (Na 2 SO 4 ). It is a normal salt. acid (sulphuric acid) acid salt (sodium hydrogensulphate) normal salt (sodium sulphate) H + H + SO 4 2 H + Na + SO 4 2 Na + Na + SO 4 2 replacing H + replacing H + Na + Na + (from sodium hydroxide solution) (from sodium hydroxide solution) 23
13 T A molecule of carbonic acid can produce two hydrogen ions. During the reaction between carbonic acid and dilute potassium hydroxide solution, the acid salt potassium hydrogen carbonate (KHCO 3 ) forms when one of the hydrogen ions is replaced. acid (carbonic acid) acid salt (potassium hydrogencarbonate) H + H + CO 3 2 H + K + CO 3 2 replacing H + K + (from potassium hydroxide solution) 14 F The ph of a salt solution may NOT be 7. Most acid salts give an acidic solution when dissolved in water. For example, sodium hydrogensulphate (NaHSO 4 ) solution is acidic. This is because the hydrogensulphate ion (HSO 4 ) can dissociate to give hydrogen ion. 15 F Some salts are formed from the neutralization between an acid and an insoluble metal hydroxide / an insoluble metal oxide. For example, copper(ii) oxide is an insoluble metal oxide. It reacts with dilute sulphuric acid to give copper(ii) sulphate and water. H 2 SO 4 (aq) + CuO(s) CuSO 4 (aq) + H 2 O(l) 16 F A few acid salts give alkaline solutions when dissolved in water. 17 T Ammonium compounds are soluble in water. 18 F Silver iodide is yellow in colour. 19 T Lead(II) nitrate solution gives a white precipitate with dilute hydrochloric acid. Pb 2+ (aq) + 2Cl (aq) PbCl 2 (s) 20 F Zinc chloride solution gives a white precipitate with sodium carbonate solution. NO gas is evolved in the reaction. ZnCl 2 (aq) + Na 2 CO 3 (aq) ZnCO 3 (s) + 2NaCl(aq) 21 F When dilute sulphuric acid reacts with lead, insoluble lead(ii) sulphate forms. The lead(ii) sulphate covers the surface of lead and prevents further reaction. 24
22 F Farmers neutralize acidic soil by adding quicklime. 23 T Ammonium sulphate gives an acidic solution when dissolved in water. 24 F Sodium hydroxide is highly corrosive. It is NEVER used as an active ingredient in antacids. 25 F A lot of heat is released when concentrated hydrochloric acid reacts with concentrated aqueous ammonia. This will cause skin burn. Furthermore, concentrated aqueous ammonia is corrosive. The hand should be washed immediately with plenty of water. Multiple choice questions 26 A Iron(III) hydroxide reacts with dilute hydrochloric acid to give iron(iii) chloride and water. The resulting solution is yellow-brown in colour. Fe(OH) 3 (s) + 3HCl(aq) FeCl 3 (aq) + 3H 2 O(l) 27 D Only Zn(OH) 2 dissolves in excess dilute sodium hydroxide solution due to the formation of a soluble complex salt. 28 B Insoluble CaCO 3 is formed in the reaction between solutions of Na 2 CO 3 and Ca(NO 3 ) 2. 29 D For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. For neutralization in which either the acid or alkali or both are weak, the heat released is less than 57 kj for 1 mole of water produced. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. Option Solutions mixed Strength of acid and alkali Temperature rise 25 cm 3 of 1 mol dm 3 HCl(aq) and 25 cm 3 of 1 mol dm 3 KOH(aq) neutralization between a strong acid and a strong alkali T A 25 cm 3 of 1 mol dm 3 CH 3 COOH(aq) and 25 cm 3 of 1 mol dm 3 NH 3 (aq) neutralization between a weak acid and a weak alkali < T B 25 cm 3 of 1 mol dm 3 CH 3 COOH(aq) and 25 cm 3 of 1 mol dm 3 NaOH(aq) neutralization between a weak acid and a strong alkali < T C 25 cm 3 of 1 mol dm 3 HNO 3 (aq) and 25 cm 3 of 1 mol dm 3 NH 3 (aq) neutralization between a strong acid and a weak alkali < T D 25 cm 3 of 1 mol dm 3 HNO 3 (aq) and 25 cm 3 of 1 mol dm 3 NaOH(aq) neutralization between a strong acid and a strong alkali T the temperature rise for mixing 25 cm 3 of 1 mol dm 3 HNO 3 (aq) and 25 cm 3 of 1 mol dm 3 NaOH(aq) is the same as that for mixing 25 cm 3 of 1 mol dm 3 HCl(aq) and 25 cm 3 of 1 mol dm 3 KOH(aq). 25
30 C For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. Expt. Acid and alkali mixed Number of moles of HCl / NaOH mixed molarity of solution x volume of solution Number of moles of water formed Heat released 1 100 cm 3 of 1 mol dm 3 HCl(aq) and 100 cm 3 of 1 mol dm 3 NaOH(aq) 1 mol dm 3 x 0.1 mol 100 HCl(aq) + NaOH(aq) 0.1 mol 0.1 mol NaCl(aq) + H 2 O(l) 0.1 mol 5.7 kj 2 100 cm 3 of 2 mol dm 3 HCl(aq) and 100 cm 3 of 2 mol dm 3 NaOH(aq) 2 mol dm 3 x 0.2 mol 100 HCl(aq) + NaOH(aq) 0.2 mol 0.2 mol NaCl(aq) + H 2 O(l) 0.2 mol 11.4 kj The total volumes of the two mixtures are the same. Hence the temperature rise of the first mixture is half that of the second mixture, i.e. T 1 1 2 T 2. 31 B For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. Expt. Acid and alkali mixed Number of moles of HCl / NaOH mixed molarity of solution x volume of solution Number of moles of water formed Heat released 1 100 cm 3 of 1 mol dm 3 HCl(aq) and 100 cm 3 of 1 mol dm 3 KOH(aq) 1 mol dm 3 x 0.1 mol 100 HCl(aq) + KOH(aq) 0.1 mol 0.1 mol KCl(aq) + H 2 O(l) 0.1 mol 5.7 kj 2 50 cm 3 of 1 mol dm 3 HCl(aq) and 50 cm 3 of 1 mol dm 3 KOH(aq) 1 mol dm 3 x 0.05 mol 50 HCl(aq) + KOH(aq) 0.05 mol 0.05 mol KCl(aq) + H 2 O(l) 0.05 mol 2.85 kj The first mixture (total volume 200 cm 3 ) is heated up by 5.7 kj while the second mixture (total volume 100 cm 3 ) is heated up by 2.85 kj. Hence the two mixtures show the same temperature rise, i.e. T 1 T 2. 32 D For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. For neutralization in which either the acid or alkali or both are weak, the heat released is less than 57 kj for 1 mole of water produced. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. Expt. Solutions mixed Strength of acid and alkali Temperature rise 1 100 cm 3 of 1 mol dm 3 HNO 3 (aq) and 100 cm 3 of 1 mol dm 3 NaOH(aq) neutralization between a strong acid and a strong alkali T 1 2 100 cm 3 of 1 mol dm 3 HNO 3 (aq) and 100 cm 3 of 1 mol dm 3 NH 3 (aq) neutralization between a strong acid and a weak alkali T 2 26 T 1 > T 2
33 D For the neutralization between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water produced. For neutralization in which either the acid or alkali or both are weak, the heat released is less than 57 kj for 1 mole of water produced. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. Mixture Strength of acid and alkali Temperature rise 100 cm 3 of 2 mol dm 3 HNO 3 (aq) + 100 cm 3 of 2 mol dm 3 NaOH(aq) 100 cm 3 of 2 mol dm 3 HCl(aq) + 100 cm 3 of 2 mol dm 3 NH 3 (aq) 100 cm 3 of 2 mol dm 3 HCl(aq) + 100 cm 3 of 2 mol dm 3 KOH(aq) x z > y neutralization between a strong acid and a strong alkali neutralization between a strong acid and a weak alkali neutralization between a strong acid and a strong alkali x y < x z x 34 C Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) reddish brown precipitate 35 A Sodium carbonate solution reacts with copper(ii) sulphate solution to give a blue precipitate, copper(ii) carbonate. Na 2 CO 3 (aq) + CuSO 4 (aq) CuCO 3 (s) + Na 2 SO 4 (aq) 36 D Option Solutions mixed Precipitate formed? Ionic equation for reaction involved A barium chloride solution and silver nitrate solution Ag + (aq) + Cl (aq) AgCl(s) B barium chloride solution and dilute sulphuric acid Ba 2+ (aq) + SO 4 2 (aq) BaSO 4 (s) C barium chloride solution and sodium sulphate solution Ba 2+ (aq) + SO 4 2 (aq) BaSO 4 (s) D barium chloride solution and sodium nitrate solution sodium nitrate solution would NOT give a precipitate with barium chloride solution. 37 A Option C Ammonium compounds are soluble in water. 38 A Option Ions reacted Precipitate formed? Ionic equation for reaction involved A Ba 2+ (aq) and SO 2 4 (aq) a white precipitate (BaSO 4 ) Ba 2+ (aq) + SO 2 4 (aq) BaSO 4 (s) B Cu 2+ (aq) and NO 3 (aq) no precipitate C Ni 2+ (aq) and CO 2 3 (aq) a green precipitate (NiCO 3 ) Ni 2+ (aq) + CO 2 3 (aq) NiCO 3 (s) D Na + (aq) and OH (aq) no precipitate 27
39 A K 2 CO 3 (aq) + Ba(NO 3 ) 2 (aq) BaCO 3 (s) + 2KNO 3 (aq) reactant A white precipitate X BaCO 3 (s) + 2HCl(aq) white reactant B precipitate X CO 2 (g) + BaCl 2 (aq) + H 2 O(l) colourless solution Y 40 B Copper(II) oxide reacts with dilute sulphuric acid to give copper(ii) sulphate and water. CuO(s) + H 2 SO 4 (aq) CuSO 4 (aq) + H 2 O(l) Copper(II) sulphate is soluble in water while copper is insoluble. Therefore copper can be separated from the mixture by filtration. 41 C Option A AgNO 3 (aq) forms a white precipitate (Ag 2 CO 3 ) with CO 3 2 (aq) ions. 2Ag + (aq) + CO 3 2 (aq) Ag 2 CO 3 (s) AgNO 3 (aq) also forms a white precipitate (Ag 2 SO 4 ) in concentrated solution of SO 4 2 (aq) ions. Option B Ba(NO 3 ) 2 (aq) forms a white precipitate with both SO 4 2 (aq) ions and CO 3 2 (aq) ions. Ba 2+ (aq) + SO 4 2 (aq) Ba 2+ (aq) + CO 3 2 (aq) BaSO 4 (s) BaCO 3 (s) Option C Fe(NO 3 ) 3 (aq) gives a precipitate with CO 3 2 (aq) ions, but NOT with SO 4 2 (aq) ions. Hence Fe(NO 3 ) 3 (aq) can be used to separate the anions SO 4 2 (aq) and CO 3 2 (aq). 42 D Option A Calcium carbonate gives gas bubbles with dilute sulphuric acid. However, it is insoluble in water. Option B Lead(II) sulphate does not give gas bubbles with dilute sulphuric acid. It is also insoluble in water. Option C Magnesium chloride does not give gas bubbles with dilute sulphuric acid. Option D Potassium carbonate is soluble in water and gives gas bubbles (carbon dioxide) with dilute sulphuric acid. 43 D Option A Calcium sulphate is insoluble in water. Option B Copper(II) oxide is not white in colour. It is also insoluble in water. Option C Iron(II) chloride is not white in colour. Option D Neutralization occurs when potassium hydroxide is mixed with dilute sulphuric acid. Heat is released. 44 B Copper(II) oxide is black in colour. When copper(ii) oxide is mixed with dilute sulphuric acid, a blue solution results. CuO(s) + H 2 SO 4 (aq) CuSO 4 (aq) + H 2 O(l) 28
45 C Lead(II) carbonate gives a colourless gas (carbon dioxide) with dilute nitric acid. PbCO 3 (s) + 2HNO 3 (aq) Pb(NO 3 ) 2 (aq) + H 2 O(l) + CO 2 (g) The resulting solution contains lead(ii) ions. The solution gives a white precipitate (lead(ii) chloride) with dilute hydrochloric acid. Pb 2+ (aq) + 2Cl (aq) PbCl 2 (s) 46 C Zinc reacts with dilute H 2 SO 4 (aq) (reactant X) to give a solution containing Zn 2+ (aq) ions. Zn(s) + H 2 SO 4 (aq) ZnSO 4 (aq) + H 2 (g) The solution gives a precipitate (ZnCO 3 (s)) with K 2 CO 3 (aq) (reactant Y). Zn 2+ (aq) + CO 3 2 (aq) ZnCO 3 (s) 47 A Calcium reacts with water to give calcium hydroxide and hydrogen. Ca(s) + 2H 2 O(l) Ca(OH) 2 (s) + H 2 (g) Calcium hydroxide is slightly soluble in water. Hence the clear solution contains calcium ions. It gives a white precipitate with sodium carbonate solution. Ca 2+ (aq) + CO 3 2 (aq) CaCO 3 (s) 48 B From Test 1, it can be deduced that solid X is a sulphate. A sulphate gives a white precipitate (barium sulphate) with barium nitrate solution. Ba 2+ (aq) + SO 4 2 BaSO 4 (s) From Test 2, it can be deduced that solid X contains aluminium ions rather than calcium ions. A solution containing aluminium ions gives a white precipitate (aluminium hydroxide) with dilute sodium hydroxide solution. The precipitate dissolves in excess alkali due to the formation of a complex salt. Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) + OH (aq) Al(OH) 3 (s) [Al(OH) 4 ] (aq) 49 C Lead(II) sulphate is an insoluble salt. It can be prepared by adding lead(ii) nitrate to dilute sulphuric acid. Pb 2+ (aq) + SO 4 2 (aq) PbSO 4 (s) 50 C Options A, B and D Oxides or hydroxides of copper, iron and zinc are insoluble bases. Sulphates of copper, iron and zinc are NOT prepared by an acid-alkali titration method. Option C Sodium sulphate can be prepared by reacting dilute sulphuric acid with dilute sodium hydroxide solution via an acid-alkali titration method. 51 D 29
As sulphuric acid was added, it removed both barium ions (by precipitation) and hydroxide ions (by neutralization). Ba(OH) 2 (aq) + H 2 SO 4 (aq) BaSO 4 (s) + 2H 2 O(l) At the equivalence point, all the barium ions and hydroxide ions had been removed. Hence the number of ions in the mixture falls until the equivalence point. After the equivalence point, the number of ions in the mixture rose as excess sulphuric acid was added. 52 C Option A Ammonium sulphate gives an acidic solution when dissolved in water. Farmers may add it to soil which has become too alkaline. 53 A 54 A Option C Slaked lime (calcium hydroxide) can neutralize the acidic waste. 55 B (2) This is NOT a neutralizaton reaction. In neutralization, salt and water are the only products. 56 A (1) Heat is released in neutralization reactions. (2) The product is sodium nitrate, a normal salt. (3) Sodium nitrate is an ionic compound. 57 C (1) When hydrogen chloride gas dissolves in water, almost all the hydrogen chloride molecules dissociate to give hydrogen ions and chloride ions. The solution formed contains mainly hydrogen ions (H + (aq)), chloride ions (Cl (aq)) and water molecules. HCl(aq) H + (aq) + Cl (aq) (2) A solution of hydrogen chloride in water can conduct electricity due to the presence of mobile ions. (3) A solution of hydrogen chloride and sodium hydroxide solution undergo a neutralization reaction when mixed. The reaction is exothermic. 58 D Iron reacts with dilute sulphuric acid to give iron(ii) sulphate solution and hydrogen gas. Fe(s) + H 2 SO 4 (aq) FeSO 4 (aq) + H 2 (g) 59 A (1) Heat would be released when concentrated sulphuric acid was mixed with water. (2) Dilute sulphuric acid and dilute aqueous ammonia underwent a neutralization reaction when mixed. Heat would be released. (3) A white precipitate (barium sulphate) appeared upon mixing dilute sulphuric acid and barium chloride solution. Ba 2+ (aq) + SO 4 2 (aq) BaSO 4 (s) 30