The ΔHof for CO2(g), CO(g) and H2O(g) are −393.5, −110.5 and −241.8 kJ mol−1 respectively. The standard enthalpy change (in kJ) for the reaction: CO2(g) + H2(g) → CO(g) + H2O(g) is :
Correct answer: B. 41.2
- A. 524.1
- B. 41.2
- C. −262.5
- D. −41.2
Explanation
The standard enthalpy change for a reaction can be calculated using the formula: ΔHreaction = ΣΔHf (products) - ΣΔHf (reactants). For the given reaction, CO2(g) + H2(g) → CO(g) + H2O(g), the calculation proceeds as follows:ΔHreaction = [ΔHf (CO) + ΔHf (H2O,g)] - [ΔHf (CO2,g)]= (−110.5 − 241.8) + 393.5 = 41.2 kJ mol−1Therefore, the correct answer is 41.2. Other options result from misapplication of the formula or incorrect arithmetic operations, leading to errors in either the sign or magnitude of the calculated enthalpy change.
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About Hess's Law
Hess's law states that the enthalpy change of a reaction is independent of the route because enthalpy is a state function. Calculations involve reversing and adding thermochemical equations, using standard enthalpies of formation or combustion, and finding an unknown reaction enthalpy without confusing enthalpy change with reaction rate.
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