Multiple Choice
Calculate the standard free-energy change (ΔG°) at 25 °C for the reaction: Mg(s) + Fe²⁺(aq) → Mg²⁺(aq) + Fe(s). Given: E°(Mg²⁺/Mg) = -2.37 V and E°(Fe²⁺/Fe) = -0.44 V.
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19. Chemical Thermodynamics
Gibbs Free Energy Calculations
Multiple Choice
Calculate ΔG°rxn for the reaction: CaCO3(s) --> CaO(s) + CO2(g). Use the following reaction and given ΔG°rxn value: Ca(s) + CO2(g) + 1/2 O2(g) --> CaCO3(s) ΔG°rxn = -734.4 kJ. Assume ΔG°f for CaO(s) is -604.0 kJ/mol and ΔG°f for CO2(g) is -394.4 kJ/mol.
A
ΔG°rxn = -130.0 kJ
B
ΔG°rxn = 36.0 kJ
C
ΔG°rxn = -36.0 kJ
D
ΔG°rxn = 130.0 kJ
Verified step by step guidance1
Identify the given reaction: CaCO3(s) --> CaO(s) + CO2(g). We need to calculate the standard Gibbs free energy change (ΔG°rxn) for this reaction.
Use the provided reaction: Ca(s) + CO2(g) + 1/2 O2(g) --> CaCO3(s) with ΔG°rxn = -734.4 kJ. This reaction is the reverse of the decomposition reaction we are interested in.
Apply Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction. Reverse the given reaction to match the desired reaction: CaCO3(s) --> Ca(s) + CO2(g) + 1/2 O2(g). The ΔG°rxn for this reversed reaction is +734.4 kJ.
Calculate the ΔG°rxn for the target reaction by adding the ΔG°f values of the products and subtracting the ΔG°f value of the reactant: ΔG°rxn = [ΔG°f(CaO) + ΔG°f(CO2)] - ΔG°f(CaCO3).
Substitute the given ΔG°f values into the equation: ΔG°rxn = [-604.0 kJ/mol + (-394.4 kJ/mol)] - 0 kJ/mol (since ΔG°f for elements in their standard state is zero). Calculate the result to find the ΔG°rxn for the decomposition of CaCO3.
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