Question detail
Calculate the enthalpy change for the formation of 1 mole of water from its elements, given the enthalpies of formation: H2(g) = 0 kJ/mol, O2(g) = 0 kJ/mol, H2O(l) = -286 kJ/mol.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Energetics
Question
- A. -286 kJ/mol
- B. -572 kJ/mol
- C. 286 kJ/mol
- D. 572 kJ/mol
Answer
-286 kJ/mol
Explanation
The correct option is -286 kJ/mol. -286 kJ/mol is the best answer because it directly supports the AQA A-Level Chemistry objective to calculate enthalpy changes from enthalpies of formation. This reasoning is anchored to Applications of Hess's law in Energetics, and it separates calculate from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Common Mistake in Enthalpy Change Calculation
Students often forget to use the correct molar enthalpy values when calculating enthalpy changes from formation data, leading to incorrect results.
To fix this, remember to use the standard enthalpy of formation values for each reactant and product in the reaction. Apply the formula: ΔH = Σ(ΔHf products) - Σ(ΔHf reactants). Substitute the correct values, perform the calculation, and ensure you express the final answer in kJ/mol.
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