Question detail
Calculate the enthalpy change for the reaction using Hess's law if the enthalpy of formation for A is -200 kJ/mol and for B is -150 kJ/mol, and the enthalpy of combustion for C is -500 kJ/mol. What is the enthalpy change for the reaction A + B → C?
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. -450 kJ/mol
- B. -350 kJ/mol
- C. -500 kJ/mol
- D. -600 kJ/mol
Answer
-450 kJ/mol
Explanation
The correct option is -450 kJ/mol. -450 kJ/mol is the best answer because it directly supports the AQA A-Level Chemistry objective to use Hess's law to calculate reaction enthalpy changes. This reasoning is anchored to Applications of Hess's law in Energetics, and it separates Hess's law 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 Hess's Law Calculations
Students often forget to include the correct signs for enthalpy changes when applying Hess's law, leading to incorrect final values.
To fix this, remember that exothermic reactions have negative enthalpy changes and endothermic reactions have positive enthalpy changes. Use the formula ΔH = Σ(ΔH_f products) - Σ(ΔH_f reactants) and ensure you apply the correct signs during substitution. For example, if you have ΔH_f for products as -100 kJ/mol and for reactants as -50 kJ/mol, the calculation would be: ΔH = (-100) - (-50) = -100 + 50 = -50 kJ/mol. Thus, the final answer is -50 kJ/mol, indicating an exothermic reaction.
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