2019 A Level H2 Chemistry Paper 1 Question 13 - Explain H2O2 Decomposition via Thermodynamics

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Here's 2019 A Levels H2 Chemistry Paper 1 Question 13.

2019 p1 q13 question

We need to explain why hydrogen peroxide can be stored for several weeks without much decomposition.

Let's take a look at each statement and see if it can explain this observation.

3. The entropy change for the reaction at 20oC is negative.

We will consider this first since it is easier to deduce change in entropy.

By looking at the overall equation we see that there is an increase in the moles of gases formed.

This means there is an increase in the degree of disorder, hence entropy change is positive.

2019 p1 q13 deduce sign of entropy change

Therefore statement 3 is false.

1. The Gibbs free energy change for the reaction at 20oC is positive.

With the sign of entropy change deduced and sign of enthalpy change given, we can now deduce the sign of Gibbs free energy.

Since entropy change is positive, -TΔS is always negative.

Since ΔH is also negative, ΔG is always negative at all temperatures.

2019 p1 q13 deduce sign of gibbs free energy

Hence statement 1 is false.

2. The reaction has a very high activation energy.

Activation energy is related to kinetics and rate of reaction.

There is no kinetics information in this question hence we cannot determine if statement 2 is true or not.

However if it were true, then a very high activation energy means the rate of H2O2 decomposition is very slow.

This would explain the observation that H2O2 can be stored for several weeks.

Since statements 1 and 3 are false, then in order for us to answer this question, statement 2 has to be true.

2019 p1 q13 deduce answer

Therefore the answer to this question will be option D.

Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore

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