Eduqas Chemistry for A Level Year 2: Student Bk

11 PI1.1 Redox and standard electrode potential Redox reactions Redox reactions are reactions where are electrons are transferred from one species to another. They are combinations of Oxidation and Reduction: Oxidation is a process where electrons are lost . OIL (Oxidation is Loss of electrons) Reduction is a process where electrons are gained . RIG (Reduction is Gain of electrons) These two processes always occur together, giving a redox reaction ( Red uction- Ox idation). They must occur at the same time, as the electrons gained during reduction must have come from a different material. We call the material that donates the electrons the reducing agent, and as it loses electrons in the process the reducing agent is oxidised. An oxidising agent is a species that oxidises another species, and is itself reduced in the process. A reducing agent is a species that reduces another species, and is itself oxidised in the process. Oxidation and reduction half-equations A full equation describes what happens to all the substances in a chemical reaction; however, it can be useful to focus on what happens to each substance separately. To do this we can split a full ionic equation into two ion-electron half-equations which separately describe what happens to each of the substances present. One half-equation represents electrons being lost (oxidation) and one half-equation represents electrons being gained (reduction). Worked example The ionic equation for the reduction of Cu 2+ by metallic zinc is: Zn (s) + Cu 2+ (aq) Cu (s) + Zn 2+ (aq) In this reaction we can split the changes into what happens to the zinc and what happens to the copper: The zinc changes from Zn (s) to Zn 2+ (aq). To do this it must lose two electrons. Zn (s) Zn 2+ (aq) + 2e – OXIDATION The copper changes from Cu 2+ (aq) to Cu (s). To do this it must gain two electrons. Cu 2+ (aq) + 2e – Cu (s) REDUCTION The half-equations must be balanced in terms of atoms and charge, so it is important to remember that each electron has a negative charge. YOU SHOULD KNOW › › › ››› oxidation is the loss of electrons ››› reduction is the gain of electrons Oxidation numbers in Topic C1.1 on page 12 of the year 1 book. Link Key Terms An oxidising agent is a substance that takes electrons from another substance and so it is reduced. A reducing agent is a substance that gives electrons to another substance and so it is oxidised. 1 Knowledge check Write ion-electron half-equations for the oxidation and reduction processes in these reactions. 1. Mg (s) + Fe 2+ (aq) Mg 2+ (aq) + Fe (s) 2. Zn (s) + 2 H + (aq) Zn 2+ (aq) + H 2 (g) The zinc placed into copper sulfate solution rapidly changes from a silvery surface to brown as copper metal covers the zinc.