Question detail
Which of the following statements is true regarding the sign of ΔH°c for a combustion reaction?
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. ΔH°c is always negative because energy is released.
- B. ΔH°c is always positive because energy is absorbed.
- C. ΔH°c can be either positive or negative depending on the reactants.
- D. ΔH°c is zero because the reaction is at equilibrium.
Answer
ΔH°c is always negative because energy is released.
Explanation
The correct option is ΔH°c is always negative because energy is released.. ΔH°c is always negative because energy is released. is the best answer because it directly supports the AQA A-Level Chemistry objective to define standard enthalpy changes for formation, combustion and neutralisation where appropriate. This reasoning is anchored to Enthalpy change in Energetics, and it separates enthalpy 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
Confusing Enthalpy Changes
Students often confuse standard enthalpy changes for formation with those for combustion, leading to incorrect calculations.
To fix this, remember that the standard enthalpy change of formation refers to the enthalpy change when one mole of a compound is formed from its elements in their standard states, while the standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen. Use the correct formula for each type of enthalpy change and ensure you know the definitions clearly.
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