IB Chemistry HL R1.1 Measuring enthalpy changes questions

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Question 1

Multiple choiceEasy1 mark

The figure shows the alcohol burned in a calorimetry experiment. A 0.230 g sample is combusted at constant pressure, warming 100.0 g of water from 20.0 °C to 26.0 °C. For the estimate, use the heat gained by the water as the heat released by combustion. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. What molar enthalpy change of combustion is obtained, in kJ mol⁻¹?

CH₃CH₂OH
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Answer: A. The structure gives the formula C₂H₆O, so its molar mass is 46.07 g mol⁻¹. The water gains Q = mcΔT = 100.0 × 4.18 × 6.0 = 2508 J = 2.508 kJ. The amount of ethanol is 0.230/46.07 = 0.00499 mol, giving ΔH = −2.508/0.00499 = −502 kJ mol⁻¹.

B: uses the molar mass of CO₂, 44.0 g mol⁻¹, instead of ethanol’s. C: doubles the ethanol amount by applying the coefficient 2 for CO₂ to the fuel amount. D: reverses the sign, treating heat gained by the water as a positive enthalpy change for combustion.

Remember: heat released by an exothermic reaction gives a negative enthalpy change.

Question 2

Multiple choiceEasy1 mark

A reaction takes place in a vessel immersed in a water bath. The bath cools as the reaction proceeds. Which description of the reaction is correct?

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Answer: C. As the bath cools, it loses energy to the reaction. Energy transferred from the surroundings to the reaction makes the reaction endothermic.

A: identifies the reaction as endothermic but reverses the direction of energy transfer.
B: gives the correct direction of transfer but labels energy absorbed by the reaction as exothermic.
D: treats the bath’s cooling as evidence that the reaction releases energy, reversing the transfer direction and classification.
Remember: an endothermic reaction takes in energy from its surroundings.

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