IB Chemistry SL R1.2 Energy cycles in reactions questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type
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Question 1

Multiple choiceEasy1 mark

An exothermic reaction is carried out with and without a catalyst. The catalyst provides an alternative pathway, while the reactants and products are unchanged. How does ΔH\Delta H compare?

Show answer and explanation

Answer: B. Enthalpy change depends on the enthalpies of the initial and final states, not on the pathway. Since those states are unchanged, ΔH\Delta H is unchanged.

A: confuses the catalyst’s effect on activation energy with making the reaction enthalpy less negative.
C: assumes the alternative pathway changes the energy released, although the initial and final states are unchanged.
D: treats an alternative pathway as the reverse reaction; reversing a reaction reverses the sign of ΔH\Delta H

Remember: Hess’s law makes ΔH\Delta H pathway-independent.

Question 2

Paper 2Medium6 marks

Carbon monoxide can be formed by the incomplete combustion of graphite and can be oxidized further to carbon dioxide. At 298 K, the standard enthalpy changes for the following reactions are:

C(graphite, s) + O₂(g) → CO₂(g)  ΔH° = −393.5 kJ mol⁻¹

CO(g) + ½O₂(g) → CO₂(g)  ΔH° = −283.0 kJ mol⁻¹

(a)

Write a balanced equation, including state symbols, for the formation of exactly 1 mol of CO(g) from its elements in their standard states.

[1]
(b)

Construct an energy cycle using the information above and your equation in (a), then calculate the standard enthalpy change for the formation of CO(g). Give your answer in kJ mol⁻¹.

[3]
(c)

Using the sign of your answer in (b), explain what it indicates about the direction of energy transfer and the relative enthalpy of the products and reactants.

[2]

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