Question

HL Questions on Electrochemical cells (2) (AHL) - Electrolysis H.504 lectricity was passed through two different electrolytic cells connected in series. One of the cells contained dilute suffuric acid solution and 250 cm3 of oxygen (measured at STP) was evolved at the positive electrode (anode). The second cell contained molten lead bromide. (I mole of gas occupies 22.7 dm3 at STP) (a) Deduce the volume of hydrogen that was released (measured at STP) at the negative electrode (cathode) in the first cell (b) Deduce the mass of lead that was formed in the second cell (c) During the electrolysis the current was supposed to have been kept constant at 0.500 A but in fact it fluctuated to give an average reading of 0.450 A. Describe how this would affect the answer to part (b).

Answer 1

O_2 at anode = 250 cm^2 nodes of o_2 = 250/22.7 times 10^3 1.1 times 10^-2 For 1 mole O_2, charge required = 4 F = 1.1 times 10^-2, charge required } where F = Faraday = 96500 C = 4F times 1.1 times 10^-2 = 4.4 times 10^-2 F For 1 mole H_2 charge required = 2F moles of H_2 formed = 4.4 times 10^-2 F/2 F = 2.2 times 10^-2 vol. (H_2) at cathode = 2.2 times 10^-2 times 22.7 times 10^3 = 449.7 cm^3 Charge flowing = 4.4 times 10^-2 F For 1 mole Pb, q = 2F amount of Pb formed = 4.9 times 10^-2 R/2 R = 2.2 times 10^-2 mol = 2.2 times 10^-2 times 207.2 M_Pb = 207.2 = 4.56 g Pb \r\n q_theoretical = 4.4 times 10^-2 F = ip times t t = 4.4 times 10^-2 F/0.5 = 8.8 times 10^-2 F sec