Beatty 10. QA2 – modified 6020 June 2015 Answers

1 Beatty Secondary School Science Department (Chemistry Unit) Chemistry 6092 Name: ___________________________________ ( ) Date: ________________ Class: 4E___ Practical 10: QA2 – modified 6020 June 2015 You are provided with a mixture of two solids, J and K, which are both salts. J is water soluble and K is insoluble. Procedure Carry out the following tests on the mixture, recording all your observations in the table. Include in your observations the identities of any gas produced in the reaction. Add 10 cm3 of distilled water to the mixture. Stopper the boiling tube and shake the mixture for a minute. Filter the mixture and keep the residue for tests later. Tests on the filtrate: Divide the solution intro three equal portions in three test tubes. test no. test observations 1 To the first portion of the solution, add aqueous sodium hydroxide until no further change. No visible reaction / solution remains colourless. [1] [1] 2 To the second portion of the solution, add a few drops of dilute nitric acid and about 1 cm3 of aqueous silver nitrate solution. Yellow ppt formed. [1] [1] 3 To the third portion of the solution, add about 1 cm3 of hydrogen peroxide solution. Add a few drops of starch to the mixture. Test the gas given off. Effervescence is observed [1] Gas produced relights a glowing splint [1]. Gas is oxygen [1] Test tube feels warm Solution turns yellow / brown [1] Blue-black solution is formed [1] [5] 23

2 Tests on the residue: test no. test observations 4 Use a spatula to transfer some of the residue into a test-tube. (a) Add about 3 cm3 of dilute hydrochloric acid to the test-tube. Test the gas given off. (b) To the solution add about 1 cm3 of dilute sulfuric acid. residue is white solid. White solid dissolves to form a colourless solution [1] Effervescence is observed. Gas produced forms a white ppt in limewater [1]. Gas is carbon dioxide [1] White ppt formed [1]. [4] Questions: (a)(i) Suggest an identity of solid J. sodium iodide / potassium iodide (a)(ii) Suggest an extension to the experiment to distinguish the identity of the cation present in solid J. State the expected observation. [You need not carry out the experiment]. Conduct a flame test on the solid J [1]. If the splint burns with a lilac flame, K+ ions is present. If the splint burns with a yellow-orange flame, Na+ ions is present. (b) Identify the anion of solid K. carbonate ion / CO32− (c) Explain the role of hydrogen peroxide and the observations seen in Test 3. Hydrogen peroxide acts as an oxid ising agent which oxidises iodide ions to form aqueous iodine which gives the yellow/brown colouration. [1] Starch will turn blue-black in the presence of iodine molecules. [1] Oxygen is formed when hydrogen peroxide is reduced. Hence effervescence is seen and oxygen will relight a glowing splint. [1]

3 (d) An insoluble powder X is said to be a catalyst for the decomposition of aqueous hydrogen peroxide. 2H2O2(aq) → 2H2O(l) + O2(g) A catalyst is a substance which increases the rate of a reaction but is not used up in the reaction. Given a sample of X and aqueous hydrogen peroxide, describe how you would show that both of the following statements are true. 1 X increases the rate of decomposition of aqueous hydrogen peroxide. 2 X is not used up in the decomposition of aqueous hydrogen peroxide. You can assume all the apparatus normally found in a school laboratory is available. Suggested Answers: 1. Measure 50 cm3 of 1 mol/dm3 of aqueous hydrogen peroxide using a measuring cylinder and pour it into the conical flask. 2. Connect the gas syringe to the delivery tube as shown in the diagram. Oxygen gas given off will be collected in the gas syringe. 3. Record the time taken, t1 to first collect 20 cm3 of oxygen gas. 4. Repeat Steps 1 to 3, but measure 0.2 g of solid X using an electronic balance and place it into the conical flask containing aqueous hydrogen peroxide. Replace the stopper immediately. 5. Record the time taken, t2 to first collect 20 cm 3 of oxygen gas in the reaction containing X. 6. Compare the time taken, t 1 and t2. If t2 is lesser than t 1, X increases the rate of decomposition of aqueous hydrogen peroxide. 7. Filter the reaction mixture to obtain the residue. 8. Wash the residue with cold deionised water to remove any soluble impurities. Dry the residue with sheets of filter paper. 9. Measure the mass of the dry residue obtained at the end of the reaction. If the mass remains the same, X is not used up in the decomposition of aqueous hydrogen peroxide.