For Supervisor s 2 9 0 3 11 Level 2 Chemistry, 2005 90311 Describe oxidation-reduction reactions Credits: Three 2.00 pm Wednesday 23 November 2005 Check that the National Student Number (NSN) on your admission slip is the same as the number at the top of this page. A Periodic Table is provided on the Resource SHEET in your Level 2 Chemistry package. You should answer ALL the questions in this booklet. If you need more space for any answer, use the page(s) provided at the back of this booklet and clearly number the question. Check that this booklet has pages 2 8 in the correct order and that none of these pages is blank. YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION. For Achievement Describe oxidation-reduction reactions. Achievement Criteria Achievement with Merit Apply oxidation-reduction principles. Overall Level of Performance Achievement with Excellence Discuss oxidation-reduction processes. New Zealand Qualifications Authority, 2005 All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
2 You are advised to spend 35 minutes answering the questions in this booklet. QUESTION ONE Chlorine is a non-metallic element and forms compounds with many other elements. Determine the oxidation number of chlorine in each of the following species. HCl Cl 2 O (c) HClO 3 (d) HClO 4
3 QUESTION TWO When magnesium metal is added to a solution of hydrochloric acid, bubbles of gas are observed and the magnesium disappears after a few minutes. Identify the reductant in this reaction and justify your answer. Reductant: Justification: Identify the species that is reduced in this reaction and justify your answer. Species reduced: Justification: (c) Write half-equations and the overall balanced equation for the reaction that occurs.
QUESTION THREE Aluminium is a metal that has many uses. It is obtained from aluminium oxide by electrolysis. A simplified diagram of the cell used to produce aluminium is shown below: + electrical energy carbon anodes carbon cathode molten aluminium dissolved aluminium oxide The cathode is the carbon lining of a steel tank. (i) What type of reaction occurs at the cathode oxidation or reduction? (ii) Write a balanced half-equation to show the reaction occurring at the cathode. Carbon anodes are placed into the dissolved aluminium oxide. Write a balanced half-equation to show the reaction occurring at the anode.
5 QUESTION FOUR Sulfur dioxide gas is bubbled through an acidified solution of potassium permanganate. Describe the expected observation(s). Sulfur dioxide, SO 2, is a reducing agent in this reaction and sulfate ions, SO 4 2, are formed. Explain why sulfur dioxide acts as a reducing agent by referring to the oxidation numbers of sulfur in both sulfur dioxide and sulfate ions. (c) Write a balanced half-equation to show the formation of sulfate ions from sulfur dioxide. (d) Permanganate ions, MnO 4, from the potassium permanganate solution are the oxidising agent in the reaction that occurs. Write a balanced half-equation for the reaction of the permanganate ions. (e) Combine the half-equations to write a balanced equation for the reaction occurring between sulfur dioxide and acidified permanganate ions.
QUESTION FIVE Nitrate ions, NO 3, in concentrated nitric acid, can act as an oxidising agent and brown nitrogen dioxide gas, NO 2, is formed. Complete and balance the half-equation for the nitrate ions acting as an oxidising agent. NO 3 (aq) NO 2 (g) Copper metal can be oxidised by nitrate ions from concentrated nitric acid and copper(ii) ions are formed. Complete and balance the half-equation for the copper atoms being oxidised to form copper(ii) ions. Cu(s) Cu 2+ (aq) (c) Combine these half-equations to write a balanced equation for the reaction between copper metal and concentrated nitric acid. QUESTION SIX One way to have a pollution-free car is to use hydrogen as the fuel, since the combustion of hydrogen forms water vapour only, along with the energy produced. Discuss how hydrogen can be made by the electrolysis of water using inert (unreactive) electrodes such as carbon or platinum. In your answer refer to any other product, the electrodes (anode and cathode), as well as relevant oxidation-reduction half-equations.
QUESTION SEVEN Group 17 elements, the halogens, act as oxidants in reactions. Aqueous chlorine, Cl 2 (aq), can react with a solution containing iodide ions, I (aq). Write balanced half-equations for the oxidation and reduction reactions that occur. Use these to write a balanced equation for the overall oxidation-reduction reaction that occurs. oxidation: reduction: overall equation: Use the balanced equation to predict expected observations for this reaction, and justify these observations by referring to the species involved.
8 Extra paper for continuation of answers if required. Clearly number the question. Question number Number