CAIE A-Level Chemistry 23.4.2 Gibbs Free-Energy Calculations

Practise calculating ΔG, ΔH, ΔS or a crossover temperature from numerical data and Gibbs plots.

Exam points

calculate ΔH and ΔS for the balanced reaction before substituting into the Gibbs equation
convert J to kJ or kJ to J explicitly and always use absolute temperature in kelvin
from a ΔG-versus-T graph, use intercept for ΔH, negative gradient for ΔS and crossing for ΔG=0

[Maximum number: 2]

Whether or not a chemical reaction is spontaneous (feasible) can be deduced by calculating the change in free energy, $\Delta G^{\ominus}$ , at a given temperature.

\mathrm{MgCO}_{3}(\mathrm{~s}) \rightarrow \mathrm{MgO}(\mathrm{~s})+\mathrm{CO}_{2}(\mathrm{~g})

\begin{aligned} & \Delta H^{\ominus}=+117 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \Delta S^{\ominus}=+175 \mathrm{JK}^{-1} \mathrm{~mol}^{-1} \end{aligned}

Calculate the value of $\Delta G^{\ominus}$ at 298 K for the above reaction.

$\begin{aligned} \Delta G^{\ominus}=\Delta H^{\ominus}-\mathrm{T} \Delta S^{\ominus} =117-((298 \times 175) / 1000) =(+) 64.85\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) \end{aligned}$

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23.4.2—Calculations: the equation ΔG question 1