Suggested Teaching Scheme

Transcription

1 Suggested Teaching Scheme Suggested Teaching Scheme The following suggested teaching schemes are for teachers reference only. Teachers may revise them based on the time-tabling arrangement of their own schools. Scheme 1: Chemistry to be studied in Secondary 3, 4, 5 and 6 In many schools, the Chemistry curriculum is studied in Secondary 3, 4, 5 and 6. Although the distribution of periods varies from school to school, the total number of periods for the curriculum is generally around 416. A possible distribution of periods is as follows: A possible distribution of periods for S3, S4, S5 and S6 S3 S4 S5 S6 Number of teaching weeks per year Number of periods per week Total number of periods per year Total number of periods for the curriculum 416 Suggested teaching scheme for the curriculum Level S3 (56 periods) S4 (140 periods) S5 (140 periods) S6 (80 periods) Only 2 out of 3 Content Suggested number of period(s) Topic 1 Planet Earth 12 Topic 2 Microscopic World I 44 Revision on laboratory safety 1 Topic 3 Metals 39 Topic 4 Acids and Bases 45 Topic 5 Redox Reactions, Chemical Cells and Electrolysis 41 Topic 6 Microscopic World II 14 Revision on laboratory safety 1 Topic 7 Fossil Fuels and Carbon Compounds 32 Topic 8 Chemistry of Carbon Compounds 45 Topic 9 Chemical Reactions and Energy 13 Topic 10 Rate of Reaction 16 Topic 11 Chemical Equilibrium 18 Topic 12 Patterns in the Chemical World 15 Revision on laboratory safety 1 Topic 13 Industrial Chemistry 39 Topic 14 Materials Chemistry 39 Topic 15 Analytical Chemistry 40 Schools taking investigative study need to allocate an extra of 30 periods for the curriculum.

2 Topic 4 Acids and Bases Scheme 2: Chemistry to be studied in Secondary 4, 5 and 6 In some schools, the Chemistry curriculum is studied in Secondary 4, 5 and 6. The total number of periods for the curriculum is generally around 360. A possible distribution of periods is as follows: A possible distribution of periods for S4, S5 and S6 S4 S5 S6 Number of teaching weeks per year Number of periods per week Total number of periods per year Total number of periods for the curriculum 360 Suggested teaching scheme for the curriculum Level S4 (140 periods) S5 (140 periods) S6 (80 periods) Only 2 out of 3 Content Suggested number of period(s) Topic 1 Planet Earth 8 Topic 2 Microscopic World I 31 Topic 3 Metals 32 Topic 4 Acids and Bases 36 Topic 5 Redox Reactions, Chemical Cells and Electrolysis 33 Revision on laboratory safety 1 Topic 6 Microscopic World II 13 Topic 7 Fossil Fuels and Carbon Compounds 29 Topic 8 Chemistry of Carbon Compounds 41 Topic 9 Chemical Reactions and Energy 12 Topic 10 Rate of Reaction 15 Topic 11 Chemical Equilibrium 16 Topic 12 Patterns in the Chemical World 13 Revision on laboratory safety 1 Topic 13 Industrial Chemistry 39 Topic 14 Materials Chemistry 39 Topic 15 Analytical Chemistry 40 Schools taking investigative study need to allocate an extra of 30 periods for the curriculum. 2

3 Suggested Teaching Scheme Suggested number of periods for Topic 4 Chemistry for Total number of periods Suggested number of periods for each unit S3 S6 (Scheme 1) 45 Unit 14 Acids and alkalis Unit 15 Molarity, ph scale and strengths of acids and alkalis Unit 16 Salts and neutralization Unit 17 Concentration of solutions and volumetric analysis S4 S6 (Scheme 2) 36 Unit 14 Acids and alkalis Unit 15 Molarity, ph scale and strengths of acids and alkalis Unit 16 Salts and neutralization Unit 17 Concentration of solutions and volumetric analysis

4 Topic 4 Acids and Bases Teaching Plan Acids and bases are involved in numerous chemical processes, from industrial processes to biological ones. Students have already studied acids and bases in junior science courses. Here they will further study the properties of acids and bases, and the molarity concept. They should also develop an awareness of the potential hazards associated with the handling of acids and bases. Students will learn to use an instrumental method of ph measurement and acquire knowledge about strong / weak acids and alkalis. They will also learn to prepare salts by different methods and to perform volumetric analysis involving acids and alkalis. Through these experimental practices students should be able to demonstrate essential experimental techniques, to analyze data and to interpret experimental results. As an introduction to analytical chemistry, students will need to use the chemical reactions learnt to identify the species in a sample. All these serve as a basis for their further study in Topic 5 Analytical Chemistry (optional). Organization of the topic Acids and Bases Unit 14 Acids and alkalis Unit 15 Molarity, ph scale and strengths of acids and alkalis Unit 16 Salts and neutralization Unit 17 Concentration of solutions and volumetric analysis 4

5 Teaching Plan Unit 14 Acids and alkalis Section Key point(s) Suggested task(s) for students Remark Total number of period = Acids in our daily lives 14.2 Acids in the laboratory Total number of periods = Characteristics of dilute acids Acids commonly found in our daily lives Acids commonly used in the laboratory Proper procedure in diluting concentrated sulphuric acid Taste Effect on indicators Reaction with metals Reaction with carbonates Reaction with hydrogencarbonates Reaction with hydroxides and oxides of metals Electrical conductivity Activity 14.1 Investigating the properties of dilute hydrochloric acid Practice 14.1 Students have studied common acids used at home and everyday uses of acids in Unit 10 of Science (secondary 1 3). Students have studied proper procedures in diluting concentrated acids and alkalis in Unit 10 of Science (secondary 1 3). Do you know What is the colour of hydrangea flowers in acidic and alkaline soils? Reaction of magnesium with dilute hydrochloric acid Action of dilute hydrochloric acid on sodium carbonate Students have studied the use of indicators to classify solutions into acidic and alkaline in Unit 10 of Science (secondary 1 3). Continued on next page 5

6 Topic 4 Acids and Bases Section Key point(s) Suggested task(s) for students Total number of periods = 2 (Scheme 1), total number of period = 1 (Scheme 2) 14.4 The role of water for acids Water must be present for an acid to show its acidic properties Definition of an acid Hydrogen ion in aqueous solution 14.5 Basicity of an acid What basicity of an acid means Basicity of some common acids Total number of period = Bases and alkalis What bases and alkalis are 14.7 Bases and alkalis in the home 14.8 Alkalis in the laboratory Bases and alkalis commonly used in our daily lives Alkalis commonly used in the laboratory Activity 14.2 Comparing the properties of solid citric acid and its aqueous solution Practice 14.2 Total number of periods = 3 (Scheme 1), total number of periods = 2 (Scheme 2) 14.9 Characteristics of solutions of alkalis The role of water for alkalis Taste Feel Effect on indicators Reaction with metal ions Reaction with ammonium compounds Reaction with acid Electrical conductivity Properties of solutions of alkalis depend on the presence of hydroxide ions Activity 14.3 Investigating the properties of dilute solutions of alkalis Remark Studying the role of water in exhibiting the acidic properties of citric acid Do you know Baking powder Dissolving hydrogen chloride in water Students have studied common alkalis used at home and everyday uses of alkalis in Unit 10 of Science (secondary 1 3). Refer to the video clip Acids and Alkalis : net/resource_detail. php?rid= (accessed July 2014) Action of solid calcium hydroxide on ammonium chloride Dissolving ammonia in water 6 Continued on next page

7 Teaching Plan Section Key point(s) Suggested task(s) for students Total number of periods = 2 (Scheme 1), total number of period = 1 (Scheme 2) An introduction to analytical chemistry Total number of periods = 2 Using the chemical reactions learnt to identify species in a sample distinguish between chemicals Concentrated acids Introduction to concentrated hydrochloric acid, nitric acid and sulphuric acid Corrosive nature of concentrated acids and alkalis Hygroscopic and deliquescent substances How corrosive concentrated acids and alkalis are Examples of hygroscopic and deliquescent substances Drying agents Practice 14.3 Problem Solving Find & Share Corrosive nature of concentrated acids and alkalis Remark Refer to the following website for the use, manufacturing, storage and safety information of sulphuric acid: (accessed July 2014) Action of concentrated sodium hydroxide solution on meat Students have studied corrosive nature of acids, potential dangers in handling strong acids and alkalis and emergency treatment involving acids and alkalis in Unit 10 of Science (secondary 1 3). 7

8 Topic 4 Acids and Bases Unit 15 Molarity, ph scale and strengths of acids and alkalis Section Key point(s) Suggested task(s) for students Remark Total number of period = Concentration of a solution Total number of period = 1 Calculations involving molarity 15.2 The ph scale Definition of ph Calculate concentration of hydrogen ions from ph value and vice versa ph values of some common substances 15.3 Determining ph values of solutions Methods to determine ph values of solutions universal indicator solution ph meter data-logger Practice 15.1 Practice 15.2 Do you know The origin of the term ph Do you know ph of normal rainwater Students have studied ph scale as an indication of relative acidity in Unit 10 of Science (secondary 1 3). Activity 15.1 Classifying substances as acidic, alkaline or neutral using indicators Do you know Monitoring water quality Refer to the following website for simulations of measuring the ph of solutions of acids, bases and salts: iastate.edu/greenbowe/ sections/projectfolder/ flashfiles/acidbaseph/ ph_meter.html (accessed July 2014) Students have studied measurement of ph values in Unit 10 of Science (secondary 1 3). Continued on next page 8

9 Teaching Plan Section Key point(s) Suggested task(s) for students Total number of periods = 2 (Scheme 1), total number of period = 1 (Scheme 2) 15.4 Strong and weak acids 15.5 Comparing the strengths of acids Total number of period = Strong and weak alkalis 15.7 Comparing the strengths of alkalis 15.8 Strength versus concentration Definitions Comparing the dissociation of strong and weak acids in water Comparing the strength of acids by ph electrical conductivity reaction with magnesium Definitions Comparing the dissociation of strong and weak alkalis in water Comparing the strength of alkalis by ph electrical conductivity Distinguishing between the strength and concentration of acids Activity 15.2 Distinguishing between a strong acid and a weak acid Discussion Practice 15.3 Remark Strong and weak acids Comparing the ph, electrical conductivity and reaction with magnesium of acids with different strength Strong and weak alkalis 9

10 Topic 4 Acids and Bases Unit 16 Salts and neutralization Section Key point(s) Suggested task(s) for students Remark Total number of period = Acid-base reactions Neutralization of an acid and an alkali Neutralization of an acid and an insoluble metal hydroxide Neutralization of an acid and an insoluble metal oxide Practice 16.1 Refer to the video clip Neutralization : net/resource_detail. php?rid= (accessed July 2014) 16.2 Heat change during neutralization Heat released during the neutralization between a strong acid and a strong alkali Measuring the temperature change of the neutralization reaction between dilute hydrochloric acid and dilute sodium hydroxide solution Total number of periods = 2 (Scheme 1), total number of period = 1 (Scheme 2) 16.3 Formation of salts Salt formation when hydrogen ions in acids are replaced by metal ions or ammonium ions Normal salts and acid salts 16.4 Naming of salts Rules for naming salts Practice 16.2 Total number of periods = 5 (Scheme 1), total number of periods = 4 (Scheme 2) 16.5 Soluble and insoluble salts 16.6 Preparing soluble salts (except sodium, potassium and ammonium salts) Solubility of some common salts in water Preparing copper(ii) sulphate crystals Action of acids on metal / insoluble bases / insoluble carbonates using the preparation of zinc sulphate as an example Activity 16.1 Preparing magnesium sulphate crystals from the reaction between an acid and an insoluble carbonate Preparing copper(ii) sulphate crystals from the neutralization reaction between dilute sulphuric acid and copper(ii) oxide Continued on next page 10

11 Teaching Plan Section Key point(s) Suggested task(s) for students Remark 16.7 Preparing sodium, potassium and ammonium salts Titration Action of acids on alkalis / soluble carbonates using the preparation of sodium sulphate crystals as an example Activity 16.2 Preparing sodium chloride from an acidalkali titration Practice 16.3 Preparing sodium sulphate crystals from the neutralization reaction between dilute sulphuric acid and dilute sodium hydroxide solution Students have studied neutralization of acids and alkalis to get salts in Unit 10 of Science (secondary 1 3). Total number of period = Preparing insoluble salts Preparing insoluble salts by precipitation reaction using the preparation of lead(ii) chloride as an example Activity 16.3 Preparing silver chloride by precipitation Practice 16.4 Total number of periods = Uses of neutralization Soil treatment Treatment of industrial waste Production of fertilizers Treatment of an ache caused by excess acid in the stomach Find & Share Uses of neutralization Practice 16.5 Students have studied everyday uses of neutralization in Unit 10 of Science (secondary 1 3). 11

12 Topic 4 Acids and Bases Unit 17 Concentration of solutions and volumetric analysis Section Key point(s) Suggested task(s) for students Total number of periods = 2 (Scheme 1), total number of period = 1 (Scheme 2) Remark 17.1 Concentration of a solution Calculations involving concentration (in g dm 3 ) 17.2 Dilution Calculations involving dilution Total number of periods = Volumetric analysis What volumetric analysis is Apparatus used in volumetric analysis 17.4 Preparing a standard solution of an acid / alkali Total number of periods = 2 Dissolving a solid acid / alkali in water Diluting a concentrated acid / alkali of known concentration 17.5 Acid-alkali titration Steps in titration Using titration data to calculate the concentration of an unknown solution Washing apparatus in volumetric analysis Practice 17.1 Practice 17.2 Activity 17.1 Preparing solutions of known concentrations Discussion Preparing cm 3 of about 0.1 mol dm 3 ethanedioic acid solution by dissolving the solid acid Preparing cm 3 of 0.10 mol dm 3 (M) sulphuric acid by diluting 1.0 mol dm 3 (M) sulphuric acid Titration of dilute hydrochloric acid against dilute sodium hydroxide solution Refer to the following website for simulations of acid-alkali titrations: simulations/simusoft_ titration.html (accessed July 2014) Continued on next page 12

13 Teaching Plan Section Key point(s) Suggested task(s) for students Remark 17.6 ph change during a titration What equivalence point is Monitoring the ph change during an acidalkali titration using a ph meter Titration curve Activity 17.2 Following the ph change during an acidalkali titration Following the ph change during an acidalkali titration A strong acid-strong alkali titration curve Total number of period = Using an indicator in an acid-alkali titration 17.8 Equivalence point detection by temperature change Choosing a suitable indicator for an acidalkali titration Detecting the equivalence point of a titration by temperature change of the solution mixture Practice 17.3 Titration curves Total number of periods = 6 (Scheme 1), total number of periods = 5 (Scheme 2) 17.9 Steps to solve problems of acidalkali titrations Steps to solve problems involving acid-alkali titrations Calculations involving acid-alkali titrations Activity 17.3 Determining the concentration of citric acid in lemon squash Activity 17.4 Determining the concentration of ammonia in a household glass cleanser Practice 17.4 Practice 17.5 Continued on next page 13

14 Topic 4 Acids and Bases Section Key point(s) Suggested task(s) for students Remark Total number of periods = 3 (Scheme 1), total number of periods = 2 (Scheme 2) Back titration Determining the percentage by mass of aluminium hydroxide in one drug tablet by back titration Activity 17.5 Determining the mass of the active ingredient in an antacid tablet Activity 17.6 Determining the percentage by mass of calcium carbonate in eggshells by back titration Practice 17.6 Chemistry Magazine Sulphur dioxide content in wine 14

15 Teaching Notes Teaching Notes Unit 14 Acids and alkalis N1 page 2 Fizzy drinks Fizzy drinks contain carbon dioxide dissolved under pressure. Carbon dioxide exists mainly as hydrated molecules, CO 2 (aq). Only a small amount of the gas reacts with water to form carbonic acid. CO 2 (aq) + H 2 O(l) H 2 CO 3 (aq) Most fizzy drinks are made by dissolving carbon dioxide gas in the still drink under pressure. This method is used for soft drinks such as lemonade and Coca-Cola, for most bottled beers, and for some cheap sparking wines. When the cap of a bottle of fizzy drink is taken off, gaseous carbon dioxide escapes and forms bubbles which rise to the surface. Suppose a test tube with a magnesium ribbon is immersed in a beaker of freshly opened carbonated water. When dilute hydrochloric acid is added to the magnesium ribbon, an exothermic reaction occurs. The temperature is increased. Solubility of carbon dioxide in the carbonated water decreases. Hence more gas bubbles are seen in the carbonated water outside the test tube. N2 page 2 Toilet bowl cleansers The discolourations and tight scale buildup that occur in toilet bowls are mostly calcium carbonate deposits from hard water. Calcium carbonate reacts with some acids to form water soluble substances. The solid type toilet bowl cleansers are mostly sodium hydrogensulphate while the liquid type contain hydrochloric acid. In addition to the acidic substances, these cleansers may contain a carbonate or hydrogencarbonate to cause bubbling and therefore agitation to help loosen the scale and give the illusion of being a powerful cleanser. Some cleansers also contain a blue colouring substance as blue is supposed to look clean and pure. This is purely the result of psychological conditioning by the advertizing industry. Precaution: never use toilet bowl cleansers with chlorine bleaches as they react to give toxic chlorine gas. 15

16 Topic 4 Acids and Bases N8 page 9 Acid-base nature of oxides In general, non-metals react with oxygen to form acidic oxides and metals react with oxygen to form basic oxides. Carbon dioxide (CO 2 ) and phosphorus pentoxide (P 4 O 10 ) are common acidic oxides. Carbon dioxide reacts with water to form an acid and reacts with bases to form salts. CO 2 (g) + H 2 O(l) CO 2 (g) + 2NaOH(aq) H 2 CO 3 (aq) carbonic acid H 2 O(l) + Na 2 CO 3 (aq) sodium carbonate Phosphorus pentoxide reacts with water to form phosphoric acid. P 4 O 10 (s) + 6H 2 O(l) 4H 3 PO 4 (aq) phosphoric acid Sodium oxide and copper(ii) oxide are common basic oxides. Sodium oxide dissolves in water to form sodium hydroxide. Na 2 O(s) + H 2 O(l) 2NaOH(aq) sodium hydroxide Copper(II) oxide reacts with dilute sulphuric acid to form copper(ii) sulphate. CuO(s) + H 2 SO 4 (aq) CuSO 4 (aq) + H 2 O(l) copper(ii) sulphate There are some oxides, such as ZnO, PbO and Al 2 O 3, that react with acids to form salts, but also react with alkalis. These are amphoteric oxides. Aluminium oxide is insoluble in water. However, it reacts with acids to give salts. Al 2 O 3 (s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 O(l) Aluminium oxide also dissolves in sodium hydroxide solution to give a complex salt. Al 2 O 3 (s) + 2OH (aq) + 3H 2 O(l) 2[Al(OH) 4 ] (aq) tetrahydroxoaluminate ion There is another group of oxides which do not react with either acids or bases. These are neutral oxides. Common neutral oxides are carbon monoxide (CO), dinitrogen oxide (N 2 O) and nitrogen monoxide (NO). 16

17 Teaching Notes Common acidic, basic, amphoteric and neutral oxides Acidic oxides Basic oxides Amphoteric oxides Neutral oxides CO 2 Na 2 O ZnO CO NO 2 K 2 O Al 2 O 3 NO P 4 O 6 MgO PbO N 2 O P 4 O 10 CaO SnO SO 2 CuO SO 3 Fe 2 O 3 Cl 2 O Ag 2 O Look at the location of different types of oxides in the periodic table shown below: Oxides of non-metals (on the right of the periodic table) are acidic oxides. They are covalent compounds. Oxides of metals (on the left of the periodic table) are basic oxides. They are ionic compounds. The acid-base nature of oxides will be discussed in Topic 12 Patterns in the Chemical World. N10 page 11 Citric acid Examination questions often ask about citric acid. It is a weak acid. It is an electrolyte. It exists as a solid at room conditions. It contains ionizable hydrogen atoms. When citric acid is dissolved in water, citric acid molecules become mobile. When water is added to a solid mixture of citric acid and sodium hydrogencarbonate, a colourless gas (carbon dioxide) evolves. 17

18 Topic 4 Acids and Bases N11 page 15 Kitchen cleansers Many cleaning problems involve grease-bound dirt. Fats, grease and oil bind dirt to a surface, making the dirt difficult to be removed with water alone, because water and grease do not mix. The usual solution is to use a detergent, which helps grease and water mix. For very greasy surfaces, such as oven and kitchen surfaces, detergent alone is not very effective. In such cases, an alkali cleanser can be used to react with the grease. The grease, fats and oil encountered in the kitchen are all of natural origin. They contain the triesters of long-chain carboxylic acids. They are hydrolyzed by alkalis. Such reactions are called saponification. The general equation is like this: Alkalis not only break down the fats and oils but turn them into soap. Therefore they are very effective in removing fatty, greasy deposits. Oven cleansers usually contain sodium hydroxide. Drain cleansers Some drain cleansers contain solid sodium hydroxide together with a little aluminium powder. When this mixture is poured down the drain, it comes into contact with water and the following reaction occurs: 2Al(s) + 2OH (aq) + 6H 2 O(l) 2[Al(OH) 4 ] (aq) + 3H 2 (g) This reaction is very exothermic and at the high temperature reached, the grease blocking the drain melts. The cleansing action of the sodium hydroxide is much more effective. The process is aided by the hydrogen bubbles produced. 18

19 Teaching Notes N12 page 17 Nickel(II) compounds Examination questions often ask about nickel(ii) compounds. Ni 2+ (aq) is green in colour. Nickel(II) hydroxide can be precipitated by adding dilute sodium hydroxide solution to a solution of nickel(ii) salt. Ni 2+ (aq) + 2OH (aq) Ni(OH) 2 (s) green precipitate Nickel(II) carbonate can be precipitated by mixing Ni 2+ (aq) and CO 3 2 (aq). Ni 2+ (aq) + CO 3 2 (aq) NiCO 3 (s) Ni is below Fe but above Pb in the electrochemical series. N17 page 21 Reagents or methods for distinguishing between different species Examination questions often ask about the reagents (or methods) that can be used to distinguish between two species. Examples: aluminium sulphate solution and lead(ii) ethanoate solution solid sodium carbonate and solid calcium carbonate iron(ii) sulphate solution and iron(iii) sulphate solution magnesium nitrate solution and silver nitrate solution solid ammonium chloride and solid potassium chloride dilute sulphuric acid and dilute nitric acid dilute sulphuric acid and dilute nitric acid distinguished by hydrochloric acid (only lead(ii) ethanoate solution produces a white precipitate, PbCl 2 ) distinguished by testing water solubility distinguished by colour / dilute aqueous ammonia / dilute sodium hydroxide solution (they give precipitates of different colours with dilute aqueous ammonia / dilute sodium hydroxide solution) distinguished by potassium chloride solution (only silver nitrate solution produces a white precipitate, AgCl) distinguished by warming with sodium hydroxide solution / calcium hydroxide solution (only ammonium chloride gives an alkaline gas, NH 3 ) distinguished by heating with copper (only dilute nitric acid gives gas bubbles) distinguished by barium chloride solution (only dilute sulphuric acid gives a white precipitate, BaSO 4 ) 19

20 Topic 4 Acids and Bases dilute hydrochloric acid and dilute nitric acid dilute hydrochloric acid and dilute nitric acid dilute sodium hydroxide solution and dilute aqueous ammonia solution of cane sugar and solution of sodium chloride distinguished by heating with copper (only dilute nitric acid gives gas bubbles) distinguished by silver nitrate solution (only dilute hydrochloric acid gives a white precipitate, AgCl) distinguished by a solution containing aluminium ions / lead(ii) ions (only dilute sodium hydroxide solution gives a white precipitate which dissolves in excess alkali) distinguished by electrical conductivity (only solution of sodium chloride can conduct electricity) N18 page 27 Choosing a suitable drying agent Drying agent Applicable for Not applicable for Anhydrous calcium chloride Saturated hydrocarbons, haloalkanes, ethers, most esters Ammonia, amines, alcohols, aldehydes, some esters and ketones Calcium oxide Ammonia, amines, alcohols Acids, aldehydes, ketones Anhydrous calcium sulphate Universal applications Anhydrous magnesium sulphate Almost all compounds, including acids, acid derivatives, aldehydes, ketones Acid sensitive compounds Concentrated sulphuric acid For neutral and acidic gases All organic compounds, hydrogen sulphide, hydrogen iodide Silica gel Anhydrous sodium sulphate Gas drying, wide use for organic liquids Haloalkanes, fatty acids, esters, aldehydes, ketones Hydrogen fluoride References: QgAAAE_KPZ3.Lxi,nav both accessed July

21 Teaching Notes Unit 15 Molarity, ph scale and strengths of acids and alkalis N1 page 45 ph values of some common substances The following table lists the approximate ph values of some common substances. Approximate ph values of some common solutions Solution ph Solution ph Hydrochloric acid (4%) 0 Saliva Gastric juice Pure water 7.0 Lemon juice 2.1 Blood 7.4 Vinegar (4%) 2.5 Fresh egg white Rainwater (thunderstorm) Bile Milk Milk of magnesia 10.5 Urine Washing soda 12.0 Normal rainwater 5.6 Sodium hydroxide (4%) 13.0 N3 page 48 Weak acids arranged in decreasing order of acid strength Name Formula Equation Ethanedioic acid H 2 C 2 O 4 H 2 C 2 O 4 (aq) H + (aq) + HC 2 O 4 (aq) Hydrogensulphate ion HSO 4 HSO 4 (aq) H + (aq) + SO 4 2 (aq) Phosphoric acid H 3 PO 4 H 3 PO 4 (aq) H + (aq) + H 2 PO 4 (aq) Hydrofluoric acid HF HF(aq) H + (aq) + F (aq) acid strength decreasing Ethanoic acid CH 3 COOH CH 3 COOH(aq) H + (aq) + CH 3 COO (aq) Carbonic acid H 2 CO 3 H 2 CO 3 (aq) H + (aq) + HCO 3 (aq) Hydrogen sulphide H 2 S H 2 S(aq) H + (aq) + HS (aq) Dihydrogenphosphate ion H 2 PO 4 H 2 PO 4 (aq) H + (aq) + HPO 4 2 (aq) Hypochlorous acid HOCl HOCl(aq) H + (aq) + OCl (aq) Hydrocyanic acid HCN HCN(aq) H + (aq) + CN (aq) 21

22 Topic 4 Acids and Bases N4 page 50 Comparing strong acids with weak acids Examination questions often ask students to compare strong acids with weak acids. Examples: Comparing 20 cm 3 of 1 mol dm 3 CH 3 COOH(aq) with 20 cm 3 of 1 mol dm 3 HCl(aq) they contain different number of hydrogen ions; they have different ph values; they have different electrical conductivity; they react with magnesium at different rates; they give the same amount of hydrogen gas with equal mass of magnesium; both react with NH 3 (aq), each giving a salt; they require the same number of moles of NaOH for complete neutralization; they give different colour change with the same quantity of universal indicator solution; when completely neutralized by 1 mol dm 3 NaOH(aq), HCl(aq) gives a larger temperature rise than CH 3 COOH(aq). We will further discuss the heat released during a neutralization reaction in Topic 9 Chemical Reactions and Energy. Comparing 20 cm 3 of 1 mol dm 3 CH 3 COOH(aq) with 10 cm 3 of 1 mol dm 3 H 2 SO 4 (aq) they have different ph values; they have different electrical conductivity; they react with magnesium at different rates; they give the same amount of hydrogen gas with equal mass of magnesium; they require the same number of moles of NaOH for complete neutralization. Comparing 45 cm 3 of 1.2 mol dm 3 HCl(aq) with 60 cm 3 of 0.9 mol dm 3 CH 3 COOH(aq) they require the same volume of 1 mol dm 3 NaOH(aq) for complete neutralization. Comparing 100 cm 3 of 1 mol dm 3 HCl(aq) with 100 cm 3 of 1 mol dm 3 H 2 SO 4 (aq) they contain different concentration of H + (aq) ions; they react with magnesium at different rates. 22

23 Teaching Notes Unit 16 Salts and neutralization N2 page 65 Temperature rise upon mixing an acid and an alkali Examination questions often ask students to compare the temperature rises upon mixing acids and alkalis. Example Temperature rise Number of moles of water formed Heat released Explanation Mixture 1: 20 cm 3 of 1 mol dm 3 HCl(aq) mixed with 20 cm 3 of 1 mol dm 3 NaOH(aq) and Mixture 2: 40 cm 3 of 1 mol dm 3 HCl(aq) mixed with 40 cm 3 of 1 mol dm 3 NaOH(aq) T 1 = T mole 0.04 mole 1.14 kj 2.28 kj Mixture 1 (total volume 40 cm 3 ) releases 1.14 kj of heat while mixture 2 (total volume 80 cm 3 ) releases 2.28 kj of heat. Hence the two mixtures give the same temperature rise. Mixture 1: 20 cm 3 of 1 mol dm 3 HCl(aq) mixed with 20 cm 3 of 1 mol dm 3 NaOH(aq) and Mixture 2: 20 cm 3 of 2 mol dm 3 HCl(aq) mixed with 20 cm 3 of 2 mol dm 3 NaOH(aq) Mixture 1: 20 cm 3 of 1 mol dm 3 CH 3 COOH(aq) mixed with 20 cm 3 of 1 mol dm 3 NaOH(aq) and Mixture 2: 20 cm 3 of 1 mol dm 3 HCl(aq) mixed with 20 cm 3 of 1 mol dm 3 NaOH(aq) 2 T 1 = T mole 0.04 mole 1.14 kj 2.28 kj The total volumes of the two mixtures are the same. Hence the temperature rise of mixture 2 is twice that of mixture 1. T 1 < T mole 0.02 mole < 1.14 kj 1.14 kj For neutralization in which either the acid or alkali or both are weak, the heat released will be less than 57 kj for 1 mole of water formed. This is because some energy is consumed when the weak acid and weak alkali dissociate to give hydrogen ions and hydroxide ions before neutralization. CH 3 COOH(aq) is a weak acid. Hence the temperature rise of the first mixture is less than that of the second mixture. For the reaction between a strong acid and a strong alkali, the heat released is 57 kj for 1 mole of water formed. The heat change in neutralization will be further discussed in Topic 9 Chemical Reactions and Energy. 23

24 Topic 4 Acids and Bases N3 page 70 Hydrolysis of salts Besides dissolving, some salts may at the same time react with water to form new products. Such reactions are called hydrolysis. Solutions of salts may be acidic, neutral or alkaline, depending on the strengths of acids and bases from which the salts are formed. Salt of a strong acid and a strong base Salts such as sodium chloride (NaCl) and potassium nitrate (KNO 3 ) belong to this class. When sodium chloride is dissolved in water, it dissociates into sodium ions and chloride ions. The ions have no reaction with water and no hydrolysis takes place. Salt of a strong acid and a weak base Salts such as ammonium chloride (NH 4 Cl) and copper(ii) sulphate (CuSO 4 ) belong to this class. When ammonium chloride is dissolved in water, it dissociates into ammonium ions and chloride ions. Chloride ions have no reaction with water. The ammonium ions undergo hydrolysis to form ammonia molecules and hydroxonium ions. NH 4 + (aq) + H 2 O(l) NH 3 (aq) + H 3 O + (aq) The solution will be acidic due to the formation of hydroxonium ions (H 3 O + ). Salt of a weak acid and a strong base Salts such as sodium carbonate (Na 2 CO 3 ) and sodium ethanoate (CH 3 COONa) belong to this class. When sodium ethanoate is dissolved in water, it dissociates into sodium ions and ethanoate ions. The ethanoate ions undergo hydrolysis to form ethanoic acid molecules and hydroxide ions. CH 3 COO (aq) + H 2 O(l) CH 3 COOH(aq) + OH (aq) The solution will be alkaline due to the formation of hydroxide ions. The following table shows a summary of the hydrolysis of different types of salts. Strength of parent acid Strength of parent base Summary of hydrolysis of different types of salts Examples Undergo hydrolysis? ph of salt solution Strong Strong NaCl, KNO 3 No 7 (neutral) Strong Weak NH 4 Cl, CuSO 4 Yes <7 (acidic) Weak Strong Na 2 CO 3, CH 3 COONa Yes >7 (alkaline) 24

25 Teaching Notes N5 page 76 Pairs of solutions (or ions) producing a precipitate when mixed Examination questions often give pairs of solutions (or ions) and ask students to identify pairs which would produce a precipitate when mixed. Examples: NH 3 (aq) and Pb(NO 3 ) 2 (aq) Pb(NO 3 ) 2 (aq) and K 2 CO 3 (aq) Pb(NO 3 ) 2 (aq) and HCl(aq) Pb(NO 3 ) 2 (aq) and Na 2 SO 4 (aq) produce a white precipitate when mixed Examination questions often ask about lead compounds. K 2 CO 3 (aq) and CaCl 2 (aq) (NH 4 ) 2 CO 3 (aq) and CaCl 2 (aq) Ca 2+ (aq) and SO 2 4 (aq) Ni 2+ (aq) and CO 2 3 (aq) produce a green precipitate (nickel(ii) carbonate) when mixed Na 2 S 2 O 3 (aq) and H 2 SO 4 (aq) produce a yellow precipitate (sulphur) when mixed Ba(NO 3 ) 2 (aq) and ZnCl 2 (aq) NH 4 Cl(aq) and K 2 SO 4 (aq) CuSO 4 (aq) and MgCl 2 (aq) KI(aq) and NH 3 (aq) do NOT produce a precipitate when mixed Cu 2+ (aq) and NO 3 (aq) NH 4 + (aq) and OH (aq) N8 page 78 Acids in the soil The ph of soil can vary from about 4 to 8. However, most soils have a ph between 6.5 and 7.5. In chalk or limestone areas, the soil is usually alkaline. On the other hand, it is generally acidic in moorland, sandstone and forest areas. Peatbogs and clay soils are normally acidic also. For general gardening and farming purposes, the best results are obtained from a neutral or slightly acidic soil of ph 6.5 to 7.0. Only a few plants can grow well in soils which are acidic. Causes of soil acidity Soil acidity may be caused by a combination of the following factors: 1 Acid rain This factor is more important in urban areas and industrial countries. 25

26 Topic 4 Acids and Bases 2 Use of acidic fertilizers Fertilizers such as ammonium sulphate and ammonium nitrate hydrolyze to produce H 3 O + (aq), causing acidity in soil: NH 4 + (aq) + H 2 O(l) NH 3 (aq) + H 3 O + (aq) 3 Selective leaching Leaching is the washing away of minerals from soil by rain or underground water. Salts of alkali metals and alkaline earth metals are usually alkaline and responsible for soil alkalinity. They are more soluble and thus more readily leached away than salts of other metals. The soil may become acidic due to hydrolysis of metal ions such as Al 3+ and Fe 3+ remaining in the soil: Al 3+ (aq) + H 2 O(l) Fe 3+ (aq) + H 2 O(l) [Al(OH)] 2+ (aq) + H + (aq) [Fe(OH)] 2+ (aq) + H + (aq) 4 Biological activities in soil Aerobic oxidation of organic matter in soil uses up oxygen and produces carbon dioxide. Normal air contains about 21% oxygen and 0.03% carbon dioxide. In soil the percentage of oxygen may drop to as low as 15% and that of carbon dioxide may rise above 5%. Carbon dioxide increases the acidity of soil by reacting with water: CO 2 (g) + H 2 O(l) H + (aq) + HCO 3 (aq) The bacterial decomposition of organic matter also produces acidic substances other than carbon dioxide. Unit 17 Concentration of solutions and volumetric analysis N5 page 100 Experimental aspects of titration Examination questions often ask about experimental aspects of titration, such as: liquid that should be used to rinse each piece of equipment before titration; procedure for preparing a standard solution from a solid acid / alkali; procedure for diluting a concentrated acid / alkali of known concentration; procedure for preparing a burette for titration; the filter funnel should NOT remain on top of the burette after using it to fill the burette with a solution. It is because the solution clinging onto the stem of the funnel may fall into the burette and affect the titre readings. 26

27 Teaching Notes N10 page 111 End point detection by electrical conductivity measurements Reactions which show a significant change in electrical conductivity as they proceed can be studied by monitoring these changes, using a conductivity cell with dipping electrodes. Conductivity measurement serves as another method for detecting the end point in acid-alkali titrations. Consider the addition of 1.0 mol dm 3 NaOH(aq) into a beaker containing 50.0 cm 3 of 0.10 mol dm 3 HCl(aq). 1.0 mol dm 3 NaOH(aq) is added, 1 cm 3 at a time. The resistance of the solution mixture is measured after each addition. The reason for having the acid much more dilute than the alkali is to minimize dilution effects, which would tend to mask any conductivity changes. The reciprocal of the measured resistance is proportional to the conductivity. Thus, it is not essential to calculate the conductivity values at each stage. The changes in conductivity for the titration are shown in the following graph. As alkali is added, the highly mobile hydrogen ions are replaced by the slower sodium ions, since virtually unionized water is produced. The conductivity falls in a linear manner along the line XY. 27

28 Topic 4 Acids and Bases After the end point, the conductivity rises sharply along the line YZ as additional sodium ions and hydroxide ions are added. The sharp rise in conductivity in this region is primarily due to the presence of the highly mobile hydroxide ions. The slope of the line YZ is not as great as that of the line XY as hydroxide ions are not as mobile as hydrogen ions. N12 page 112 Which reactant is in excess? Examination questions may not tell students which reactant is in excess in a chemical reaction. Students need to identify that. Consider two reactions involving magnesium ribbons: Reaction Reaction mixture g of Mg cm 3 of 1 mol dm 3 HCl(aq) g of Mg cm 3 of 1 mol dm 3 H 2 SO 4 (aq) For Reaction 1: 1.5 g Number of moles of Mg present = 24.3 g mol 1 = mol Number of moles of HCl present = 1 mol dm 3 x = 0.1 mol dm3 1 mole of Mg requires 2 moles of HCl for complete reaction. Hence 0.1 mole of HCl would react with 0.05 mole of Mg. HCl is the limiting reactant. Mg does not react completely in Reaction 1. Mg is in excess. For Reaction 2: Number of moles of H 2 SO 4 present = 1 mol dm 3 x = 0.1 mol dm3 1 mole of Mg requires 1 mole of H 2 SO 4 for complete reaction. Hence mole of Mg would react with mole of H 2 SO 4. Mg is the limiting reactant. H 2 SO 4 does not react completely in Reaction 2. H 2 SO 4 is in excess. The initial rate of Reaction 2 is higher than that of Reaction 1. This is because H 2 SO 4 (aq) is a dibasic acid while HCl(aq) is a monobasic acid. 1 mol dm 3 H 2 SO 4 (aq) contains a higher concentration of hydrogen ions than 1 mol dm 3 HCl(aq). Besides calculations, we can deduce that the acid is in excess if magnesium ribbons disappear after reaction. 28

29 Teaching Notes N13 page 118 Determining the percentage by mass of calcium carbonate in a sample Examination questions often ask students to determine the percentage by mass of calcium carbonate in a sample. Alternative method 1: Let the sample react with excess dilute hydrochloric acid. Measure the volume of carbon dioxide given off with a syringe. Calculate the number of moles of CO 2 evolved. (The relationship between the volume of a gas and the number of moles will be discussed in Topic 10 Rate of Reaction.) Alternative method 2: Convert insoluble CaCO 3 to insoluble CaSO 4 via soluble Ca(NO 3 ) 2. excess HNO 3 (aq) excess Na 2 SO 4 (aq) CaCO 3 (s) Ca(NO 3 ) 2 (aq) CaSO 4 (s) Calculate the percentage by mass of CaCO 3 in the sample from the mass of CaSO 4 obtained. 29

30 Topic 4 Acids and Bases Suggested Answers page 1 1 Making use of ph paper (or meter) / litmus paper (or solution) / universal indicator solution 2 acid + alkali salt + water 3 One mole Unit 14 Acids and alkalis Practice P14.1 page 10 1 a) The oxide dissolves in the acid. b) ZnO(s) + 2HNO 3 (aq) Zn(NO 3 ) 2 (aq) + H 2 O(l) c) ZnO(s) + 2H + (aq) Zn 2+ (aq) + H 2 O(l) 2 a) Effervescence occurs. b) (NH 4 ) 2 CO 3 (aq) + 2HCl(aq) 2NH 4 Cl(aq) + H 2 O(l) + CO 2 (g) c) CO 2 3 (aq) + 2H + (aq) H 2 O(l) + CO 2 (g) P14.2 page 13 When the tablet is added to water, the citric acid in the tablet dissociates in water to give hydrogen ions. The sodium hydrogencarbonate in the tablet dissolves in water, releasing hydrogencarbonate ions. The hydrogen ions react with the hydrogencarbonate ions, forming carbon dioxide gas. H + (aq) + HCO 3 (aq) H 2 O(l) + CO 2 (g) P14.3 page 23 a) Add dilute hydrochloric acid to each substance separately. Only calcium carbonate gives a gas that turns limewater milky (the gas is carbon dioxide). b) Add dilute sodium hydroxide solution to each substance separately and warm. Only ammonium chloride gives a gas that turns moist red litmus paper blue (the gas is ammonia). 30

31 Suggested Answers Problem Solving page 23 Carry out a flame test on all the substances. Only sodium chloride gives a golden yellow flame. Dissolve the remaining three substances separately in water. Divide each solution into two portions. Add silver nitrate solution to one portion of each solution. Only the zinc chloride solution gives a white precipitate. Add dilute sodium hydroxide solution to the remaining two solutions. The lead(ii) nitrate solution gives a white precipitate which is soluble in excess alkali. The magnesium nitrate solution gives a white precipitate which does not dissolve in excess alkali. (Other possible tests are also acceptable.) Find & Share page 25 Corrosive nature of concentrated acids and alkalis Hazard warning symbol Concentrated hydrochloric acid Risk descriptions Causes burns Irritating to respiratory system Safety precautions In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident and if you feel unwell, seek medical advice immediately. Hazard warning symbols Concentrated nitric acid Risk descriptions Causes severe burns Contact with combustible material may cause fire Safety precautions Do not inhale fumes / vapour / spray. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident and if you feel unwell, seek medical advice immediately. Continued on next page 31

32 Topic 4 Acids and Bases Hazard warning symbol Concentrated sulphuric acid Risk descriptions Reacts violently with water Causes severe burns Irritating to respiratory system Safety precautions Never add water to concentrated sulphuric acid. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident and if you feel unwell, seek medical advice immediately. Hazard warning symbol Concentrated sodium hydroxide solution Risk description Causes severe burns Safety precautions Wear protective gloves and eye / face protection. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident and if you feel unwell, seek medical advice immediately. Hazard warning symbol Concentrated potassium hydroxide solution Risk descriptions Causes severe burns Safety precautions Wear protective gloves and eye / face protection. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident and if you feel unwell, seek medical advice immediately. 32

33 Suggested Answers Unit Exercise pages a) sulphuric acid b) nitric acid c) dibasic acid d) tribasic acid e) sour f) red g) hydrogen h) carbon dioxide i) salt j) water 2 a) sodium hydroxide solution b) calcium hydroxide solution c) aqueous ammonia d) slippery e) blue f) ammonia g) salt h) water i) electricity 3 Indicator dilute hydrochloric acid Colour in dilute sodium hydroxide solution Litmus solution red blue Methyl orange red yellow Phenolphthalein colourless red 33

34 Topic 4 Acids and Bases 4 Adding NaOH(aq) to solution containing Colour of precipitate formed Precipitate dissolves in excess NaOH(aq)? (Yes or No) Ionic equation(s) Ca 2+ (aq) white No Ca 2+ (aq) + 2OH (aq) Ca(OH) 2 (s) Mg 2+ (aq) white No Mg 2+ (aq) + 2OH (aq) Mg(OH) 2 (s) Al 3+ (aq) white Yes Pb 2+ (aq) white Yes Zn 2+ (aq) white Yes Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) + OH (aq) Pb 2+ (aq) + 2OH (aq) Pb(OH) 2 (s) + 2OH (aq) Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) + 2OH (aq) Al(OH) 3 (s) [Al(OH) 4 ] (aq) Pb(OH) 2 (s) [Pb(OH) 4 ] 2 (aq) Zn(OH) 2 (s) [Zn(OH) 4 ] 2 (aq) Fe 2+ (aq) green No Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) Fe 3+ (aq) reddish brown No Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) Cu 2+ (aq) pale blue No Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) 5 Adding NH 3 (aq) to solution containing Colour of precipitate formed Precipitate dissolves in excess NH 3 (aq)? (Yes or No) Ionic equation(s) Mg 2+ (aq) white No Mg 2+ (aq) + 2OH (aq) Mg(OH) 2 (s) Al 3+ (aq) white No Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) Pb 2+ (aq) white No Pb 2+ (aq) + 2OH (aq) Pb(OH) 2 (s) Zn 2+ (aq) white Yes Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) + 4NH 3 (aq) Zn(OH) 2 (s) [Zn(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) Fe 2+ (aq) green No Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) Fe 3+ (aq) reddish brown No Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) Cu 2+ (aq) pale blue Yes Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) + 4NH 3 (aq) Cu(OH) 2 (s) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) 34

35 Suggested Answers 6 CaCO 3 heat CaO CO + 2 solid A gas B H 2 O(l) Ca(OH) 2 solution C HCl(aq) CaCl 2 solution D + H 2 O liquid E excess CO 2 (g) Ca(HCO 3 ) 2 solution F 7 D 8 C CuO(s) is insoluble in water and there is no separate Cu 2+ ion or O 2 ion in water. 9 A 10 C H 3 PO 4 is a tribasic acid. 11 A 12 B FeSO 4 is soluble in water. Addition of dilute aqueous ammonia to FeSO 4 (aq) gives a precipitate, iron(ii) hydroxide. Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) The precipitate does not dissolve in excess alkali. 13 D 14 B Iron(III) hydroxide is insoluble in aqueous ammonia. Copper(II) hydroxide reacts with excess aqueous ammonia to form a soluble complex salt. Cu(OH) 2 (s) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) 15 D 16 D 17 A 35

36 Topic 4 Acids and Bases 18 (1) a) Copper does not react with dilute hydrochloric acid. b) Add magnesium to dilute hydrochloric acid. (2) a) Insoluble calcium sulphate forms when calcium carbonate reacts with dilute sulphuric acid. The calcium sulphate covers the surface of calcium carbonate and prevents further reaction. b) Add calcium carbonate to dilute hydrochloric acid / dilute nitric acid. (3) a) Sodium hydroxide is very corrosive. b) Use magnesium hydroxide / aluminium hydroxide as the active ingredient. 19 a) Gas bubbles are given off. / The zinc granules dissolve in the acid. Zn(s) + H 2 SO 4 (aq) or Zn(s) + 2H + (aq) ZnSO 4 (aq) + H 2 (g) Zn 2+ (aq) + H 2 (g) b) Effervescence occurs. / Solid copper(ii) carbonate dissolves in the acid. / A blue solution forms. CuCO 3 (s) + 2HNO 3 (aq) or CuCO 3 (s) + 2H + (aq) Cu(NO 3 ) 2 (aq) + H 2 O(l) + CO 2 (g) Cu 2+ (aq) + H 2 O(l) + CO 2 (g) c) Effervescence occurs. / Solid sodium hydrogencarbonate dissolves in the acid. NaHCO 3 (s) + HCl(aq) or NaHCO 3 (s) + H + (aq) NaCl(aq) + H 2 O(l) + CO 2 (g) Na + (aq) + H 2 O(l) + CO 2 (g) 20 Answers for the HKASLE question are not provided. 21 a) An acid is a hydrogen-containing substance that gives hydrogen ions (H + (aq)) as the only type of positive ions when dissolved in water. b) i) copper(ii) oxide and hydrochloric acid CuO(s) + 2HCl(aq) or CuO(s) + 2H + (aq) CuCl 2 (aq) + H 2 O(l) Cu 2+ (aq) + H 2 O(l) sodium carbonate and hydrochloric acid Na 2 CO 3 (s) + 2HCl(aq) or Na 2 CO 3 (s) + 2H + (aq) 2NaCl(aq) + H 2 O(l) + CO 2 (g) 2Na + (aq) + H 2 O(l) + CO 2 (g) ii) Effervescence The solid disappears / dissolves. A blue / green solution is formed. 36

37 Suggested Answers 22 a) A blue precipitate forms; the precipitate dissolves in excess dilute aqueous ammonia to give a deep blue solution. Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) + 4NH 3 (aq) Cu(OH) 2 (s) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) b) A white precipitate forms. Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) c) A white precipitate forms; the precipitate dissolves in excess dilute sodium hydroxide solution to give a colourless solution. Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) + 2OH (aq) Zn(OH) 2 (s) [Zn(OH 4 )] 2 (aq) d) Ammonia gas is given off. 2NH 4 Cl(aq) + Ca(OH) 2 (aq) 2NH 3 (g) + CaCl 2 (aq) + 2H 2 O(l) 23 a) dilute sodium hydroxide solution / calcium hydroxide b) ammonia c) ammonium d) silver chloride e) chloride 24 a) A brick-red flame is observed. b) Effervesence c) carbon dioxide d) carbonate e) A white precipitate is formed. f) calcium hydroxide 25 a) Warm with dilute sodium hydroxide solution. A gas that turns moist red litmus paper blue (ammonia) is given off. b) Add dilute hydrochloric acid. A gas that turns limewater milky (carbon dioxide) is given off. 37

38 Topic 4 Acids and Bases 26 a) Any one of the following: Add dilute aqueous ammonia to each solution. Only the aluminium nitrate solution gives a white precipitate. Add dilute sodium hydroxide solution to each solution. The aluminium nitrate solution gives a white precipitate which dissolves in excess alkali. The calcium nitrate solution gives a white precipitate which does not dissolve in excess alkali. b) Add dilute sodium hydroxide solution / dilute aqueous ammonia to each solution. The iron(ii) sulphate solution gives a green precipitate. The iron(iii) sulphate solution gives a reddish-brown precipitate. 27 a) Ag + (aq) + Cl (aq) AgCl(s) b) Filtration c) i) Copper(II) hydroxide ii) Pale blue d) Warm E. A gas that turns moist red litmus paper blue is evolved. The gas is ammonia. This shows the presence of ammonium ions in E. e) Blue; the solution contains blue Cu 2+ (aq) ions and the other ions are colourless. 28 a) Add dilute hydrochloric acid to the salt. A gas that can turn limewater milky is evolved. b) A white precipitate c) The presence of potassium cannot be shown. As in a flame test, the lilac flame of potassium will be masked by the brillant golden yellow flame of sodium. 38