# 2021 P1 Q13 - Deduce Rate Equation for Hydrogen and Halogen Reaction

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 13. We are required to deduce which of the statements are correct for these reactions.

1. For the hydrogen and bromine reaction, formation of HBr slows down the rate of reaction From the rate equation for reaction 1, we can deduce that when concentration of HBr increase, rate will decrease.

Hence statement 1 is correct.

2. Only the hydrogen and iodine reaction could be a single step reaction.

For single step reaction, all reactants will be involved in that rate determining step.

Therefore the order of reaction with respect to each reactant will be the number of that reactant in the slow step, which is also the coefficient of that reactant in the balanced equation. For reaction 2 (hydrogen and iodine), the proposed rate equation for single step mechanism will be:

rate = k [H2][I2]

where order with respect to H2 is 1 and order with respect to I2 is 1.

The given rate equation in the question is exactly the same.

This means it is possible for reaction 2 to be single step. For reaction 1 (hydrogen and bromine), the proposed rate equation for single step mechanism will be:

rate = k [H2][Br2]

where order with respect to H2 is 1 and order with respect to Br2 is 1.

The given rate equation in the question is significantly different.

There are 2 rate constants k1 and k2 which suggest that there is a minimum of 2 steps in the mechanism.

Hence reaction 1 cannot be single step and statement 2 is correct.

3. Doubling concentration of halogen in each reaction doubles each rate of reaction. From the rate equations we can deduce that:

- doubling concentration of Br2 in reaction 1 will not double its rate
- doubling concentration of I2 in reaction 2 will double its rate

Hence statement 3 is incorrect.

We can now put everything together to answer the question. Only statements 1 and 2 are correct and option B is the answer to this question.

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

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

Found this A Level Chemistry video useful?