Chemistry 2005 JAMB Past Questions - Question 4: When iron is exposed to moisture and it rusts, the value of ΔG for the reaction is

Correct Answer

D

Explanation

AG is negative which makes the reaction feasible.
The formation of rust on iron involves the oxidation of iron in the presence of oxygen and water. The overall reaction can be represented as:

\[4 \text{Fe} + 3 \text{O}_2 + 6 \text{H}_2\text{O} \rightarrow 4 \text{Fe(OH)}_3\]

The change in Gibbs free energy (\(\Delta G\)) for a reaction is related to the standard Gibbs free energy change (\(\Delta G^\circ\)) by the equation:

\[\Delta G = \Delta G^\circ + RT \ln Q\]

where:
- \(\Delta G\) is the Gibbs free energy change under non-standard conditions.
- \(\Delta G^\circ\) is the standard Gibbs free energy change.
- \(R\) is the gas constant (approximately 8.314 J/(mol·K)).
- \(T\) is the temperature in Kelvin.
- \(Q\) is the reaction quotient.

For the rusting of iron, if we assume standard conditions (\(\Delta G^\circ\)), the actual \(\Delta G\) will depend on the temperature and the concentrations of reactants and products, which are reflected in the reaction quotient (\(Q\)). However, the overall process of rusting iron is a spontaneous one under standard conditions, meaning that \(\Delta G^\circ\) is negative.

The actual numerical value of \(\Delta G\) for the rusting of iron will depend on the specific conditions of the reaction. If the system is at equilibrium (\(\Delta G = 0\)), then \(Q = K_{\text{eq}}\) (the equilibrium constant) and \(\Delta G^\circ = -RT \ln K_{\text{eq}}\).

In summary, the value of \(\Delta G\) for the rusting of iron will depend on the specific conditions of the reaction, but under standard conditions (\(\Delta G^\circ\)), it is negative, indicating that the process is thermodynamically favorable.