Electrochemical Cells — practice questions

For a magnesium–copper galvanic cell, identify the anode and cathode, state which species is oxidised and which reduced, and calculate E°cell. (E° Mg²⁺/Mg = −2.37 V; E° Cu²⁺/Cu = +0.34 V)

Explain why a salt bridge is necessary in a galvanic cell, describing what happens without one and which way the ions move.

A student measures 1.05 V from a nickel–silver cell initially, but after several hours the reading has fallen to 0.92 V, and the cell eventually stops. Justify why the voltage falls, and compare this behaviour with what would happen if the same reaction were carried out by simply adding nickel powder to silver nitrate solution.

For the electrolysis of molten magnesium chloride, write the half equation at each electrode, state the sign of each electrode, and state what would be observed.

Explain why sodium metal is produced when molten sodium chloride is electrolysed, but hydrogen is produced when aqueous sodium chloride is electrolysed. Use E° values in your answer.

In the electrolysis of concentrated brine, E° values predict that water should be oxidised at the anode in preference to chloride, yet chlorine gas is the observed product. Justify why the prediction fails, and evaluate what this shows about using E° to predict electrolysis products.

State three ways in which an electrochemical cell and an electrolytic cell differ, and two ways in which they are the same.

Explain why the anode is negative in an electrochemical cell but positive in an electrolytic cell, even though oxidation occurs at the anode in both.

Compare and contrast the oxidation–reduction processes in a lead–acid car battery when it is discharging and when it is being charged. Justify your answer with half equations, and evaluate what this example shows about the relationship between the two cell types.