Eduqas Chemistry for A Level Year 2: Student Bk

12 Eduqas A Level Chemistry Year 2: Component 1 Cells and half-cells Writing redox reactions as two half-equations is not a theoretical process – it is possible to physically separate the two processes so that oxidation happens in one place and reduction happens somewhere else. To do this we need to set up two half-cells, one for the oxidation and one for the reduction. We join them together to complete the circuit: V SALT BRIDGE High resistance Voltmeter Wire Copper electrode Zinc electrode 1 mol dm –3 Zn 2+ (aq) 1 mol dm –3 Cu 2+ (aq) The wire allows electrons to flow from the half-cell where oxidation occurs to the half- cell where reduction occurs. We often include a high resistance voltmeter if we are measuring the potential difference in the cell. The salt bridge completes the circuit and allows ions to flow without the solutions mixing. A typical salt bridge is made of a gel soaked in a solution of potassium nitrate. A simpler salt bridge that you may use in your experimental work is made of filter paper soaked in the same solution. The entire apparatus is called a cell, with the two parts called half-cells. Half-cells Each cell is made up of two half-cells, each of which must contain both the reactants and products of the half-equation. This means the half-cell for the reduction of Cu 2+ ions to copper atoms must contain both of these. It must also contain a metal to allow electrons to flow into or out of the half-cell. There are various types of half-cell depending on the physical states of each substance in the reaction. There are three key types of electrochemical half-cell that you need to be aware of: 1. Metal / metal ions The half-cells described so far have consisted of metals in contact with metal ions, with two key examples being Zn (s) with Zn 2+ (aq) and the Cu (s) with Cu 2+ (aq) half-cells. In both cases we have a piece of metal as the electrode, with a solution containing a 1 mol dm –3 solution of the metal ions. In the case of zinc, there is no apparent colour change, but for copper the blue solution may lose colour as the copper ions are reduced. & S C Stretch & Challenge The electrochemical cells always use high-resistance voltmeters. The resistance is important as it prevents the flow of electrons. If the electrons were flowing this would cause the redox reactions to occur in each half-cell, which would change the concentrations of the solutions. This would affect the potential difference reading on the voltmeter and it would not be possible to obtain a steady value. PRACTICAL CHECK The construction of electrochemical half-cells and full cells, and the measurement of cell EMF values is a specified practical task . These can include a range of different types of half-cell. Although you do not need to perform practical tasks involving the standard hydrogen electrode you will need to be able to recall how this is constructed and used. Key Term A salt bridge is a piece of apparatus that connects the solutions in two half-cells so that the circuit can be complete and the current can flow without the solutions mixing. YOU SHOULD KNOW › › › ››› the type of half-cell needed for any half-equation ››› platinum is used as an electrode in many cases as it is unreactive and doesn’t affect the chemical reaction occurring