9.1 Qualitative Analysis

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1 Chemistry Form 4 Page 44 Ms. R. Buttigieg Test for positive Ions (Cations) 9.1 Qualitative Analysis 1) Flame Tests Nichrome wire is dipped in concentrated hydrochloric acid, then in the salt being tested. Ion Na + K + Ca 2+ Cu 2+ Flame colour Brilliant yellow Lilac Brick red Bluish green confirmatory indicative 2) Add Sodium hydroxide solution to a solution of the salt. Ion Result Equation Fe ++ A dirty green precipitate Fe NH 4 Cu ++ Mg ++ Ca ++ Pb ++ Al +++ A reddish brown precipitate Ammonia gas is evolved (given off ) on boiling. Check by smell, and by concentrated hydrochloric acid (dense white fumes) and by damp red litmus (becomes blue) Blue precipitate (turning black on boiling) White gelatinous precipitate, insoluble in excess NaOH. White precipitate, insoluble in excess NaOH. White precipitate, soluble in excess NaOH. White precipitate, soluble in excess NaOH. All the above are precipitation reactions. The positive metal ion meets the hydroxide ion (OH - ) in solution and forms an insoluble metal hydroxide which falls as a precipitate. Note that lead (Pb) and aluminium (Al) hydroxide (white precipitates) are soluble in excess NaOH. Both lead and aluminium hydroxides are amphoteric hydroxides (acidic and basic) and so react with an alkali to form a soluble salt. 3) Add Potassium Iodide solution (KI) to a solution containing lead (II) ions. A canary yellow precipitate forms. Pb ++ (aq) + 2I - (aq) PbI 2 (s)

2 Chemistry Form 4 Page 45 Ms. R. Buttigieg Test for negative Ions (Anions) 1) To 1 spatula load of the solid add about 3 cm 3 of dilute hydrochloric acid (HCl). Warm if no reaction occurs in the cold. Ion Result Equation Sulphite Ion -- SO 3 Sulphur dioxide evolved on warming. Check by smell; it is acidic; turns acidified potassium dichromate from Carbonate Ion CO 3 -- orange to green. Carbon dioxide evolved. Check using limewater which turns milky 2) Test for the Sulphate Ion Add dilute hydrochloric acid followed by barium chloride solution. White precipitate of barium sulphate forms. Ba ++ (aq) + SO 4 -- (aq) BaSO 4 (s) 3) Add dilute nitric acid followed by silver nitrate solution to a solution of the salt. Ion Result Equation Chloride Ion White precipitate Cl - Bromide Ion Cream coloured precipitate Br - Iodide Ion I - Pale yellow precipitate 4) Test for the Nitrate Ion NO 3 - Add aluminium and alkali (NaOH). If the nitrate ion is present, ammonia gas is given off. The ammonium ion (NH 4 + ) also gives off ammonia with this test. So as not to confuse the two: First NaOH is added Contents are boiled If ammonia is given off at this stage, ammonium ion (NH 4 + ) is present. o If not, aluminium is added. o If ammonia is given now, the nitrate ion is present.

3 Chemistry Form 4 Page 46 Ms. R. Buttigieg Tests for Gases Gas Oxygen O 2 Hydrogen H 2 Carbon dioxide CO 2 Test and information Relights a glowing splint If lit burns with a squeaky pop if mixed with air 2H 2 (g) + O 2 (g) 2H 2 O(g) Gives a white precipitate when bubbled through limewater; prolonged bubbling causes the precipitate to disappear: CO 2 (aq) + Ca(OH) 2 (aq) CaCO 3 (s) + H 2 O(l) Chlorine Cl 2 Ammonia NH 3 Nitrogen dioxide NO 2 Sulphur dioxide SO 2 CaCO 3 (s) + CO 2 (aq) + H 2 O(l) Ca(HCO 3 ) 2 (aq) Bleaches damp litmus paper Turns moist red litmus blue it is the only common alkaline gas. Forms a white smoke if an open bottle of concentrated HCl is held near to it. Characteristic smell nitrogen dioxide (brown fumes) o holding a copper turning in the fumes intensifies the brown colouration o turns moist starch iodide paper blue but does not affect silver nitrate solution when passed into acidified potassium dichromate solution turns it green. This question is about qualitative tests on pairs of substances. a) Solutions of calcium chloride and magnesium chloride give the same result when sodium hydroxide solution is added to them, first a few drops then in excess. (i) Describe the result of this test and give an equation for one of the reactions. (ii) Describe another test, including the observations, that would enable you to distinguish between the two compounds. (6 marks) b) Solutions of aluminium nitrate and lead nitrate also give the same result with sodium hydroxide solution. (i) Describe the result of this test. (ii) Describe another test, including the observations that would enable you to distinguish between the two solutions. Give an equation for any reaction that takes place. (6 marks) c) Two tests that would enable you to distinguish between sodium sulphite and sodium sulphate are. Test 1. adding dilute hydrochloric acid to samples of the solids. Test 2. adding acidified barium chloride solution to a solution of each compound. Give the results of each test for both substances and write balanced equations for any reactions taking place. (8 marks)

4 Chemistry Form 4 Page 47 Ms. R. Buttigieg 9.2 Action of Heat on Materials Elements Metals ~ Magnesium Burns with a brilliant white flame to form white ashes of magnesium oxide. It therefore forms a solid product which weighs more than the original substance 2Mg (s) + O 2 (g) 2MgO (s) white Copper turns black as a layer of black copper oxide forms on its surface reacts more slowly than magnesium. Non-metals ~ copper + oxygen copper(ii) oxide. 2Cu (s) + O 2(g) 2CuO (s) Carbon Carbon monoxide or carbon dioxide are produced depending on levels of oxygen present. 2C (s) + O 2 (g) 2CO (g) C (s) + O 2 (g) CO 2 (g) Sulphur This is a yellow powder that when heated gently melts into an amber coloured liquid. Then it begins to burn with a blue flame and produces sulphur dioxide. Compounds S (s) + O 2 (g) SO 2 (g) ARE THERMALLY STABLE: E.G. NaCl, Na 2 CO 3 Sodium chloride and sodium carbonate are thermally stable so nothing happens when they are heated. DECOMPOSE (into two or more simpler substances) Carbonates Of Group 1 - Sodium hydrogen Carbonate NaHCO3 Sodium hydrogen carbonate sodium carbonate + carbon dioxide + water 2 NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O (l) Sodium hydrogen carbonate is used in cakes. When it decomposes, the carbon dioxide raises the cake.

5 Chemistry Form 4 Page 48 Ms. R. Buttigieg Other Carbonates - Copper Carbonate CuCO3, Calcium carbonate CaCO 3 Copper carbonate Copper oxide + carbon dioxide Green black turns lime water milky CuCO 3 (s) CuO (s) + CO 2 (g) Calcium carbonate Calcium oxide + carbon dioxide CaCO 3 (s) CaO (s) + CO 2 (g) Nitrates Of Group 1 - Sodium nitrate NaNO3 Sodium nitrate Sodium nitrite + oxygen NaNO 3 (s) NaNO 2 (s) + O 2 (g) Other Nitrates - Lead Nitrate Pb(NO3)2 Lead (II) nitrate Lead (II) oxide + Nitrogen dioxide (g) + oxygen (g) White crystals yellow solid brown gas relights a glowing splint Pb(NO3)2 (s) 2PbO (s) + NO 2 (g) + O 2 (g) A crackling sound is also heard Sugar when heated gently it melts, then it decomposes. Some of the carbon catches fire or turns black with a strong smell of charring. C 12 H 22 O C (s) + 11H 2 O (l) UNDERGO A REVERSIBLE CHANGE Silica gel. Silica gel can adsorb about 40 percent of its weight in moisture and can take the relative humidity in a closed container down to about 40 percent. Once saturated, you can drive the moisture off and reuse silica gel by heating it above 300 degrees F (150 C). Hydrated copper sulphate is blue and changes to white upon heating and back to blue when water is added. CuSO 4.5H 2 O (s) CuSO 4 (s) + 5H 2 O (l) Blue white UNDERGO A PHYSICAL CHANGE Zinc Oxide is white when cold and yellow when hot. Revision of the combustion of a hydrocarbon e.g. butane (covered in Unit 3, Topic 3.2 ). A knowledge of the changes in mass (increase, decrease, unchanged), accompanying the action of heat on substances.

6 Chemistry Form 4 Page 49 Ms. R. Buttigieg 1. This question is about the action of heat on materials. Consider the following list of substances: MAGNESIUM, HYDRATED COPPER (II) SULPHATE, SODIUM CHLORIDE, SODIUM HYDROGENCARBONATE, SODIUM NITRATE, SUCROSE (SUGAR), ZINC OXIDE. Using only substances in this list, give the name of one substance in each case which, when heated, behaves as described below. a) liberates oxygen as the only gas. b) is thermally stable. c) forms a solid product which weighs more than the original substance. d) changes to yellow while hot but becomes white again on cooling. e) undergoes a reversible chemical change. f) gives off a gas which turns lime water milky. g) gives off clouds of water vapour and then turns black with a strong smell of charring. (7 marks) 2. Are compounds of reactive or unreactive metals more stable? 3. Copper (II) carbonate is heated in a hard glass tube. a) How would you test for the gas given off? b) Write an equation for the reaction 4. Why is sodium hydrogen carbonate used in cakes? Explain and write any equations involved. 5. Which substance can be heated to give off: Water Carbon dioxide Nitrogen dioxide Oxygen only

7 Chemistry Form 4 Page 50 Ms. R. Buttigieg 6. State what you would SEE when each of the following tests are carried out, which would enable you to distinguish between each pair of substances. (a) A flame test is carried out on salts containing Na + and K + ions. Na + gives a colour, while K + gives a colour. (b) Acidified silver nitrate solution is added to solutions containing Cl and I ions. (i) With Cl a precipitate is formed, (ii) With I a precipitate is formed. (c) Sodium hydroxide solution is added to solutions containing Cu 2+ and Fe 2+ ions. (i) With Cu 2+ a precipitate is formed (ii) With Fe 2+ a precipitate is formed. (d) Give an ionic equation to show the formation of one of the precipitates formed in (c). (8 marks) 7. Complete When sulphur is heated gently, it melts into an coloured liquid. Then it burns with a flame. The equation is. When lead (II) nitrate is heated strongly it makes a/n sound, and a/n coloured gas and a gas which (test) is given off. The white crystals turn into coloured powder. The equation is. Zinc oxide is when cold and when hot. 8. Give a test that you could carry out to test for the presence of four (4) named gases of your choice

8 Chemistry Form 4 Page 51 Ms. R. Buttigieg 9.3 Volumetric Analysis Volumetric Analysis is a means of estimating quantities of certain materials (often acids and alkalis) by an analytical process which involves measurement of volumes of solutions, using pipettes, burettes, and (for approximate measurements) measuring cylinders. Weighings may also be involved. Definition of standard solution and concentration. Concentration is the amount of a substance per unit volume of solution. A standard solution is a solution of known concentration E.g. if a solution is known to contain 12g of NaCl in 1dm 3 concentration is: 12g dm -3 of solution, we say that the A molar (M) solution of a compound is one, which contains one mole of the compound in 1 dm 3 of the solution. The practical steps in preparing a standard solution, including all precautions to ensure accuracy. Preparation of a standard solution of sodium carbonate Anhydrous sodium carbonate (Na 2 CO 3 ) has a molar mass of 106 g mol -1. A 0.1 M solution is made up, using a 250 cm 3 volumetric flask. For 250 cm 3 of 0.1 M sodium carbonate solution, the mass required is: 106 x 0.1 x 250 / 1000 = 2.65 g 1. Using a balance, measure accurately 2.65 g of pure anhydrous sodium carbonate on a clock glass. 2. Slowly transfer the sodium carbonate with stirring, to about 50 cm 3 of deionised water in a clean 250 cm 3 beaker. To ensure that all the sodium carbonate is transferred, use a wash bottle to rinse the clock glass with deionised water, and add the rinsings to the beaker. 3. Continue stirring the mixture with a stirring rod until the sodium carbonate has fully dissolved. 4. Using a wash bottle, wash off the solution on the stirring rod with deionised water into the beaker. 5. Pour the solution through a clean funnel into the 250 cm 3 volumetric flask. 6. Using a wash bottle, rinse out the beaker several times with deionised water, and add the rinsings to the solution in the flask.

Chemistry Form 4 Page 52 Ms. R. Buttigieg 7.

the water dropwise, using a dropping pipette, until the bottom of the

Stopper the flask and invert it several times to ensure a homogeneous

9 Chemistry Form 4 Page 52 Ms. R. Buttigieg 7. Rinse the funnel with deionised water, allowing the water to run into the flask. 8. Fill the flask to within about 1 cm of the calibration mark, and then add the water dropwise, using a dropping pipette, until the bottom of the meniscus just rests on the calibration mark. 9. Stopper the flask and invert it several times to ensure a homogeneous (evenly mixed) solution. Label the flask. 1. What is a standard solution? 2. Which substance is used to prepare a standard solution? Why? 3. What units of concentration do we use? 4. M is read as and means

10 Chemistry Form 4 Page 53 Ms. R. Buttigieg 5. A solution has a concentration of 1mol/dm 3. What does this mean? 6. A solution of sulphuric acid has a concentration of 0.1 mol dm -3. What is the number of moles present in 250 cm 3 of the solution? What is the mass of sulphuric acid present in 250 cm 3 of the solution? What is the number of H 2 SO 4 molecule present in 250cm 3 of the solution. Titrations Chemistry for You pg , GCSE Chemistry pg The correct technique of carrying out a titration, emphasising the solutions/liquid to be used for washing specific items of apparatus, selecting an appropriate indicator, recording burette readings and carrying out sufficient number of titres to ensure accuracy. 7. What do you understand by the word titration? 8. What is the end-point of a titration? 9. Name an indicator used in acid-alkali titrations and give its colour in alkali, neutral and acid solutions. 10. Before filling the burette with acid, what is the burette washed with? Why? 11. Before using the pipette what do you wash it with? Why? 12. Before putting the alkali in the flask, what do we wash it with? Why?

11 Chemistry Form 4 Page 54 Ms. R. Buttigieg 13. In an experiment 25cm 3 of sodium hydroxide solution was reacted with sulphuric acid of concentration 0.1 mol dm -3. The following are the results. 1 st time 2 nd time 3 rd time Final burette reading 22cm 3 21cm cm 3 Initial burette reading 0cm 3 0cm 3 0cm 3 Volume of acid used a) Fill the table and find the average volume of acid used. b) Find the concentration of NaOH (aq) Section B questions 1a) A solution of sodium hydroxide, NaOH, was prepared by dissolving 5g sodium hydroxide per 250cm 3 solution. i) What is the relative formula mass of sodium hydroxide? ii) How many moles of sodium hydroxide are dissolved in solution? iii) What is the molar concentration of the solution? (3 marks) b) An excess of iron (II) sulphate solution is added to 100cm3 of sodium hydroxide solution of molar concentration 2M (2mol dm-3). The equation for the reaction is Calculate: FeSO4(aq) + 2NaOH(aq) Fe(OH)2(s) + Na2SO4(aq) i) the number of moles of NaOH in solution, ii) the number of moles of Fe(OH)2 precipitated,

12 Chemistry Form 4 Page 55 Ms. R. Buttigieg iii) the mass of Fe(OH)2 precipitated. (5 marks) 2 (a) Describe, in detail, how you would prepare 250cm 3 of a standard solution of anhydrous sodium carbonate. Include all practical steps and precautions required to ensure accuracy. (10 marks) (b) 25cm 3 of a sodium carbonate solution of molar concentration 0.2M (0.2 mol dm -3 ) are required 20cm 3 of dilute hydrochloric acid solution to neutralise it. (i) Calculate the mass of anhydrous sodium carbonate used to prepare 250cm 3 of the standard solution. (3 marks) (ii) Give the equation for the reaction of sodium carbonate with hydrochloric acid. (2 marks) (i) Calculate the molar concentration (molarity) of the hydrochloric acid. (5 marks) 3. A solution of hydrochloric acid was titrated against a standard solution of sodium carbonate of molar concentration 0.1 mol dm-3 (0.1M) using a suitable indicator. (a) For this titration procedure, give: (i) a labelled diagram of the titration set up; (3 marks) (ii) the names of the three main items of apparatus; (3 marks) (iii) the name of a suitable indicator and its colour change at the end point; (3 marks) (iv) three precautions or practical steps which must be taken to obtain accurate titre values. (3 marks) (b) It was found that 25cm 3 of the standard sodium carbonate solution was exactly neutralized by 30 cm 3 of the hydrochloric acid solution. The equation for the reaction is: Na2CO3 + 2HCl 2NaCl + H2O + CO2 (i) Calculate the number of moles in 25cm3 of the sodium carbonate solution. (3 marks) (ii) Use the acid : carbonate mole ratio of the equation to find the number of moles of hydrochloric acid that must have reacted. (2 marks) (iii) Use your answer to part (ii) to find the Molarity of the hydrochloric acid solution. (3 marks)

Acid, Bases and Salts (IGCSE Chemistry Syllabus )

Acid, Bases and Salts (IGCSE Chemistry Syllabus 2016-2018) Acid o A compound when dissolved in water produces hydrogen ions (H + ) ; proton (H + ) donor o It turns blue damp litmus paper to red o ph 1