Electrochemistry — Practice

Question 1

Write the cell notation and calculate the standard cell potential for the reaction Ni(s) + Cu2+(aq) gives Ni2+(aq) + Cu(s). Take E(Ni2+/Ni) = -0.25 V and E(Cu2+/Cu) = +0.34 V.

Accepted Answer

Put the oxidised metal on the left, then subtract anode from cathode. Answer: Ni(s) | Ni2+(aq) || Cu2+(aq) | Cu(s), with E(cell) = 0.59 V.

Question 2

The conductivity of a 0.05 M solution of an electrolyte is 0.0035 S/cm at 298 K. Calculate its molar conductivity.

Accepted Answer

Multiply by 1000 to convert litres to cubic centimetres before dividing by the molarity. Answer: 70 S cm2/mol.

Question 3

How many faradays are needed to deposit 1 mol of chromium from a solution containing Cr3+ ions?

Accepted Answer

Read the electron count straight off the charge on the ion. Answer: 3 F, which is 2.89 x 10^5 C.

Question 4

Calculate the emf at 298 K of the cell Zn(s) | Zn2+(0.1 M) || Cu2+(0.01 M) | Cu(s), given E(cell, standard) = 1.10 V.

Accepted Answer

Build Q from the overall reaction with n = 2, then apply the Nernst equation. Answer: 1.07 V.

Question 5

For a cell reaction transferring 2 electrons, E(cell, standard) = 0.46 V at 298 K. Find the standard Gibbs energy and the equilibrium constant.

Accepted Answer

Use DrG = -nFE first, then convert to K through the 0.0591 relation. Answer: DrG = -88.8 kJ/mol and K = 3.6 x 10^15.

Question 6

The limiting molar conductivities of NaCl, HCl and CH3COONa are 126.4, 425.9 and 91.0 S cm2/mol. Find the limiting molar conductivity of acetic acid.

Accepted Answer

Combine the three so that the sodium and chloride contributions cancel. Answer: 390.5 S cm2/mol.

Question 7

A current of 2 amperes is passed through molten AlCl3 for 30 minutes. What mass of aluminium is deposited? Take Al = 27.

Accepted Answer

Find the charge, convert to moles of electrons, then divide by three. Answer: 0.336 g.

Question 8

Write the anode and cathode reactions of the hydrogen-oxygen fuel cell in alkaline medium and state its overall reaction.

Accepted Answer

The electrolyte is concentrated NaOH, so hydroxide ion appears in both half-reactions. Answer: Anode: 2H2 + 4OH- gives 4H2O + 4e-; cathode: O2 + 2H2O + 4e- gives 4OH-; overall 2H2 + O2 gives 2H2O.

Question 9

The emf of the cell Zn(s) | Zn2+(x M) || Cu2+(1.0 M) | Cu(s) is 1.16 V at 298 K, and E(cell, standard) is 1.10 V. Calculate x.

Accepted Answer

Rearrange the Nernst equation for the logarithm and solve for the unknown concentration. Answer: x = 9.1 x 10^-3 M.

Question 10

The molar conductivity of 0.1 M acetic acid is 5.2 S cm2/mol and its limiting molar conductivity is 390.5 S cm2/mol. Find the degree of dissociation, the dissociation constant and the pH of the solution.

Accepted Answer

Get alpha first, then Ka from Ostwald's law, and read the hydrogen ion concentration as c times alpha. Answer: alpha = 0.0133, Ka = 1.79 x 10^-5, pH = 2.88.

Question 11

Using Table 2.1, decide whether an aqueous solution of Fe3+ can oxidise both I- and Br-, and explain the difference between the two cases.

Accepted Answer

Compare each halogen couple with the Fe3+/Fe2+ value of 0.77 V. Answer: It oxidises I- (E(cell) = +0.23 V) but not Br- (E(cell) = -0.32 V), because bromine is the stronger oxidising agent of the two halogens.

Practice — Electrochemistry

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