WJEC Chemistry for A2: Study and Rev Guide

Equations and half-equations In a redox reaction, one species is being reduced and another is being oxidised. We can usually divide a full chemical equation into two half- equations, one showing the oxidation and one showing the reduction. In the example below, the Cu 2+ (aq) is being reduced and the Mg (s) is being oxidised: Cu 2+ (aq) + Mg (s) Cu (s) + Mg 2+ (aq) The Cu 2+ (aq) is converted into Cu (s). To do this it must have gained two electrons so the ion-electron half-equation is: Cu 2+ (aq) + 2e – Cu (s) The Mg (s) is converted into Mg 2+ (aq). To do this it must have lost two electrons so the ion-electron half-equation is: Mg (s) Mg 2+ (aq) + 2e – Electrochemical cells Half-equations are not just a theory – it is possible to separate a redox reaction so oxidation happens in one place and reduction happens somewhere else. To do this we need to set up two half-cells where the separate processes will happen, and join them together in a complete circuit. er mix up a diagram ectrochemical cell ell diagram. A cell is a representation written in ne line of not a drawing. e boost riting cell diagrams, metals at both ith a salt bridge in dle (shown by two lines, ||). A vertical ws each change of ith a comma between in the same physical e metal with the gative E θ goes on Equations and half-equations In a redox reaction, one species is being reduced and another is being oxidised. We can usually divide a full chemical equation into two half-equations, one showing the oxidation and one showing the reduction. In the example below the Cu 2+ (aq) is being reduced and the Mg (s) is being oxidised: Cu 2+ (aq) + Mg (s) ↓ Cu (s) + Mg 2+ (aq) The Cu 2+ (aq) is converted into Cu (s). To do this it must have gained two electrons so the ion-electron half-equation is: Cu 2+ (aq) + 2e – ↓ Cu (s) The Mg (s) is converted into Mg 2+ (aq). To do this it must have lost two electrons so the ion-electron half-equation is: Mg (s) ↓ Mg 2+ (aq) + 2e – Electrochemical cells Half-equations ar not just a theory – it is possible to separate a redox reaction so oxidation happens in on place and reduction happens somewhere else. To do this we need to set up two half-cells where the separate processes will happen, and join them together in a complete circuit. Mg(s)|Mg 2+ (aq)||Cu 2+ (aq)|Cu(s) cell The high-resistance voltmeter gives a reading of the EMF produced by the cell. The salt bridge completes the circuit by allowing ions to move, without the two solutions mixing. V magnesium electrode copper electrode 1 mol dm –3 Mg 2+ (aq) 1 mol dm –3 Cu 2+ (aq) high-resistance voltmeter + salt bridge istry: Study and Revision Guide Mg(s)|Mg 2+ (aq)||Cu 2+ (aq)|Cu(s) cell The high-resistance voltmeter gives a reading of the EMF produced by the cell. The salt bridge completes the circuit by allowing ions to move, without the two solutions mixing. Pointer Do not mix up a diagram of an electrochemical cell with a cell diagram. A cell diagram is a representation of a cell written in one line of text and not a drawing. Pointer If you need to label a diagram of a cell fully, you’ll need to remember that the half-cell with the most positive E θ value will be the positive electrode, and electrons ow along the wire towards this half-cell. Grade boost When writing cell diagrams, put the metals at both ends, with a salt bridge in the middle (shown by two vertical lines, ||). A vertical line shows each change of state, with a comma between species in the same physical state. The metal with the most negative E θ goes on the left. 9 3.1 Redox and standard electrode potential