WJEC Chemistry for A2: Study and Rev Guide

Standard electrode potentials, E θ Each electrochemical cell has a different tendency to gain or lose electrons. It is useful to have a way to measure and compare these and so we use the standard electrode potential (E θ ) . To measure the value of E θ for any half- cell, we need to connect it to the standard hydrogen electrode under standard conditions, and measure the potential difference using a high-resistance voltmeter. Standard hydrogen electrode The standard hydrogen electrode is a half-cell where H 2 gas at a pressure of 1 atm bubbles over an inert platinum electrode dipped in 1 mol dm –3 H + (aq) at a temperature of 298K. Key Stand poten potent any ha to the electro condit Gr Rem hav cell a hi and use Alw con • 1 co • 1 a ga • 29  Draw sho cell t to m elect follo Fe 3+ Q Standard el ctrode potentials, E θ Each el ctrochemical cell has different tendency to gain or l se el ctrons, and t is useful to have a way to measure and compare these and so we use the standard el ctrode potential (E θ ) . To measure the value of E θ for any half- cell, we need to c nnect it to the standard hydrogen lectrode under standar conditions, and measure the potential differenc using a hig resistance voltmeter. Standard hydrogen electrode The standard hydrogen electrode is a half-cell where H 2 gas at a pressure of 1 atm bubbles over an inert platinum electrode dipped in 1 mol dm –3 H + (aq) at a temperature of 298 K. This is the standard half-cell which all others are compared to. Its standard electrode potential is de ned as being exactly 0 volts. We can measure the standard electrode potentials for three types of half-cell: A metal-solution half-cell: A piece of metal acting as an electrode dipping into a solution containing ions of the same metal. Examples include Cu | Cu 2+ and Zn | Zn 2+ . A gas-solution half-cell: A gas is bubbled over an inert platinum electrode dipping in a solution containing ions of the gas. Examples include the standard hydrogen electrode and X 2 (g)|X – (aq) (X=Cl, Br or I). A mixed ion half-cell: This half-cell also uses an inert platinum electrode. We place this electrode in a solution containing different ions of the same element in different oxidation states. Examples include Fe 2+ , Fe 3+ and Mn 2+ , MnO 4 – . Note that commas are used here as both ions are in the same physical state. A cell diagram for these two half-cells joined together would be Pt | Fe 2+ , Fe 3+ || MnO 4 – , Mn 2+ | Pt. magnesium electrode copper electrode Pt electrode H 2 (g) (1 atm) 1 mol dm –3 Mg 2+ (aq) 1 mol dm –3 Cu 2+ (aq) 1 mol dm –3 H + (aq) salt bridge to other half-cell salt bridge wire to other half-cell CH5: R dox an This is the standard half-cell w ich all others are compared to. Its stan ard electrode potential is defined as being exactly 0 volts. We can measure the stan ard electrode po entials fo three types of half-cell: A metal–solution half-cell: A piece of metal acting as an electrode dippi g into a solution contai ing ions of the same metal. Examples include Cu|Cu 2+ and Zn| Zn 2+ . A gas–solution half-cell: A gas is ubbled over an inert platinum electrode dipping in a solution containing ions of the gas. Examples include the standard hydrogen electrode and X 2 (g)|X – (aq) (X=Cl, Br or I). A mixed ion half-cell: This half-cell also uses an inert platinum electrode. We place this electrode in a solution containing different ions of the same element in different oxidation states. Examples include Fe 2+ , Fe 3+ and Mn 2+ , MnO 4 – . Note that commas are used here as both ions are in the same physical state. A cell diagram for these two half-cells joined together would be Pt | Fe 2+ , Fe 3+ || MnO 4 – , Mn 2+ | Pt. Grade boost Remember to be clear if you are asked to label a diagram of a cell – the voltmeter must be a high-resistance voltmeter and always specify the metal used to make the electrode. Always include the standard conditions: 1 mol dm –3 for the concentrations of solutions 1 atm for the pressures of gas 298 K for the temperature. QUICKFIRE ICKFIRE  Draw a labelled diagram to show the electrochemical cell that should be used to measure the standard electrode potential for the following reaction: Fe 3+ (aq) + e Fe 2+ (aq). Standard electrode potential (E θ ) = the potential difference when any half-cell is connected to the standard hydrogen electrode under standard conditions. Key Term 10 A2 Chemistry: Study and Revision Guide