Eduqas Chemistry for A Level Year 2: Student Bk

13 PI1.1 Redox and standard electrode potential 2. A gas in contact with a solution of non-metal ions, with an inert metal electrode Since non-metals are not conductors, we must use an inert platinum electrode to allow electrons to flow in or out of the half-cell. This is typically used for a hydrogen electrode (H 2 (g) ǀ H + (aq)) or oxygen (O 2 (g) ǀ OH – (aq)) half-cells. The gas is bubbled over the inert electrode which is dipping into a solution of the ions. These changes do not cause any apparent colour change. 3. A solution containing ions of a metal in two different oxidation states, again using an inert metal electrode Again there is no conductor in the system so we must use an inert platinum electrode to allow electrons to flow in or out of the half-cell. This is typically used for the transition metals, where the metal may have several oxidation states. Good examples are Fe 2+ /Fe 3+ and Mn 2+ /MnO 4 – . Both these half- cells cause colour changes when oxidation or reduction occurs. Fe 2+ is pale green and Fe 3+ is yellow/orange; Mn 2+ is colourless, MnO 4 – is purple. Representing half-cells We can show half-cells using cell diagrams. These list all the essential substances present in a half-cell, starting with the metal that is used to conduct electrons into or out of the half-cell. Each substance is then listed, with vertical lines separating substances in different physical states (solid, liquid, gas or solution) and commas separating substances in the same physical state. Mg (s) ǀ Mg 2+ (aq) This is the metal/metal ion half-cell for magnesium. Pt (s) ǀ H 2 (g) ǀ H + (aq) This is a gas/solution half-cell containing an inert platinum electrode. As this is for hydrogen, it is the standard hydrogen electrode if it is under standard conditions. Pt (s) ǀ Mn 2+ (aq), MnO 4 – (aq) This is a mixed solution half-cell containing an inert platinum electrode. In this case a comma separates the two manganese-containing ions in aqueous solution as they are in the same physical state. The cell involving copper and zinc electrodes is called the Daniell cell after its inventor John Daniell. Inert Pt electrode A mixture of: 1 mol dm –3 Fe 2+ (aq) 1 mol dm –3 Fe 3+ (aq) & S C Stretch & Challenge Platinum electrodes are sometimes described as platinum black electrodes. These have tiny grains of platinum over the surface of the metal, which makes the surface appear much darker. These small grains give a very high surface area which allows the gas to be absorbed more effectively on the surface so that it can get involved in the reaction. Examtip When drawing labelled diagrams of standard half-cells the conditions must be stated clearly in the labels, with all solutions having a concentration of 1.00 mol dm –3 and all gases being at a pressure of 1 atm. Inert Pt electrode 1 atm. H 2 in 1 mol dm –3 H + (aq)