Multiple Choice
Calculate ΔG°rxn at 25°C for the reaction: 2CH4(g) → C2H6(g) + H2(g). Given the following standard Gibbs free energies of formation: ΔG°f(CH4) = -50.8 kJ/mol, ΔG°f(C2H6) = -32.8 kJ/mol, ΔG°f(H2) = 0 kJ/mol.
8. Thermochemistry
Thermochemical Equations
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Choose the thermochemical equation that illustrates ΔH°f for K2SO4(s).
A
2 K+(aq) + SO4 2-(aq) → K2SO4(aq)
B
2 K(s) + S(s, rhombic) + 2 O2(g) → K2SO4(s)
C
K2S(s) + 2 O2(g) → K2SO4(s)
D
K2SO4(s) → 2 K(s) + S(s, rhombic) + 2 O2(g)
Verified step by step guidance1
Understand the concept of standard enthalpy of formation (ΔH°f). It is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.
Identify the elements in their standard states for the formation of K2SO4(s). Potassium (K) is a solid, sulfur (S) is a solid in its rhombic form, and oxygen (O2) is a diatomic gas.
Write the balanced chemical equation for the formation of K2SO4(s) from its elements in their standard states. This involves combining 2 moles of solid potassium, 1 mole of rhombic sulfur, and 2 moles of gaseous oxygen to form 1 mole of solid K2SO4.
Recognize that the thermochemical equation illustrating ΔH°f for K2SO4(s) should show the formation of K2SO4(s) from its elements, not the decomposition or transformation of other compounds.
Select the equation that correctly represents the formation of K2SO4(s) from its elements: 2 K(s) + S(s, rhombic) + 2 O2(g) → K2SO4(s). This equation matches the definition of ΔH°f.
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