AQA A-Level Chemistry Paper 3, 2021: Question 18
1 mark · Medium difficulty · Multiple Choice
This question explores why certain chemical reactions may not occur, even if the reactants are mixed in the correct molar ratio. Specifically, it tests your understanding of activation energy in the context of a hydrogen–oxygen mixture at room temperature. You're asked to identify the main reason why water is not formed spontaneously.
Practise this questionQuestion
Question text
18 A mixture of 2 dm3 of hydrogen and 1 dm3 of oxygen is at room temperature.
Which statement is correct?
[1 mark]
There is no reaction to form water because the molecules do not
A
collide with sufficient energy.
There is no reaction to form water because the molecules do not
B
collide with sufficient frequency.
The mean velocity of the hydrogen molecules is less than that of
C
the oxygen molecules.
D The partial pressure of each gas is the same.
Mark scheme
Show the mark scheme
There is no reaction to form water because the molecules do
18 A 1
not collide with sufficient energy.
How to answer it
Multiple Choice Explained: Why doesn’t hydrogen and oxygen react at room temperature?
Correct Answer: A – There is no reaction to form water because the molecules do not collide with sufficient energy.
Why? At room temperature, the kinetic energy of molecules is too low to overcome the high activation energy for the reaction between hydrogen and oxygen. This is why a spark or flame is usually needed to start the reaction – to provide the necessary energy.
Why B is incorrect:
“...do not collide with sufficient frequency” is not correct. Gas molecules at room temperature do collide frequently. The issue is not frequency, but energy – their collisions aren't energetic enough to initiate reaction.
Why C is incorrect:
“The mean velocity of hydrogen is less than oxygen” is the wrong way round. Hydrogen is much lighter than oxygen, so it moves faster at the same temperature according to kinetic theory.
Why D is incorrect:
“The partial pressure of each gas is the same” is incorrect because there is twice as much hydrogen (2 dm³) as oxygen (1 dm³). Therefore, hydrogen contributes a greater partial pressure in the mixture.
Topics
Physical Chemistry · 3.1.5 Kinetics
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.