WJEC Chemistry for A2: Study and Rev Guide

Using standard electrode potentials Some common standard electrode values, along with some of the associated colour changes are listed below: E θ / V Zn 2+ (aq) + 2e – Zn (s) −0.76 2 H + (aq) + 2e – H 2 (g) 0.00 Cu 2+ (aq) + 2e – Cu (s) Blue +0.34 I 2 (aq) + 2 e – 2 I – (aq) +0.54 Fe 3+ (aq) + e – Fe 2+ (aq) Yellow Pale green +0.77 Br 2 (aq) + 2 e – 2 Br – (aq) Orange Colourless +1.09 Cr 2 O 7 2– (aq) + 14 H + (aq) + 6e – 2 Cr 3+ (aq) + 7H 2 O(l) Orange Dark green +1.33 Cl 2 (aq) + 2 e – 2 Cl – (aq) +1.36 MnO 4 – (aq) + 8 H + (aq) + 5e – Mn 2+ (aq) + 4 H 2 O(l) Purple Palepin k/ colourless +1.51 The more positive the value of the E θ the more likely the system is to gain electrons. If you connect two half-cells together, electrons will flow from the more negative half-cell to the more positive one. The reading on the high-resistance voltmeter will be given by: Standard potential of cell = E θ (more positive) – E θ (less positive) Is a reaction feasible? We can use the E θ values to work out if a reaction is feasible. For a reaction to be possible, the EMF for the reaction must be positive. To calculate the EMF, we need to identify which half-equation is the reduction and which is the oxidation. EMF = E θ for reduction – E θ for oxidation For example, we can identify whether chloride can reduce Cu 2+ ions to copper metal using the E θ values above. The two relevant half-equations are: Cu 2+ (aq) + 2e – Cu (s) +0.34 V Cl 2 (aq) + 2 e – 2 Cl – (aq) +1.36 V This reaction involves Cu 2+ Cu, which is a reduction, and Cl – Cl 2 , which is an oxidation. The EMF for the reaction is: EMF = E θ for Cu 2+ – E θ for Cl – = 0.34 – 1.36 = –1.02 V The reaction is not feasible as the standard potential is negative. Grade boost When discussing oxidation and reduction, it is important to make sure that you clearly identify any species involved. A common error is to confuse iodide and iodine, or to mix up metal atoms. If asked to identify the strongest reducing agent, then you need to say that this is the zinc atom or zinc metal not just zinc as this could be the Zn 2+ ion. QUICKFIRE QUICKFIRE QUICKFIRE  Calculate the standard potentials that would be measured if the following pairs of half-cells were connected together: a) Zn|Zn 2+ and Cu|Cu 2+ . b) Zn|Zn 2+ and Fe 2+ , Fe 3+ |Pt. QUICKFIRE QUICKFIRE QUICKFIRE  Use the values of E θ given to show that bromide ions will react with Cl 2 but not with I 2 . 11 3.1 Redox and standard electrode potential