2020 P1 Q10 - Deduce Concentration Time Graph for Zero Order Reaction

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 10.

2020 p1 q10 question

We are required to deduce which graph represents variaton of [I2] with time, given reaction is zero order with respect to iodine.

Let's go through the concepts involving rate equation, which is given by the following expression:

2020 p1 q10 rate equation

rate = k [A]m [B]n


k is the rate constant affected by temperature and activation energy. Higher temperature or lower activation energy will increase k.

[A] and [B] are the concentrations of reactants A and B respectively.

m and n are the order of reaction with respect to A and B respectively.

In A Level Chemistry syllabus we only consider order zero, 1 or 2 for kinetics.

Check out the following video lesson for a more detailed discussion of rate equation and how order of reaction affects rate.

Since the reaction is zero order with respect to iodine, let's see what the rate equation would look like.

2020 p1 q10 zero order reaction

rate = k [I2]0 = k

This means rate is a constant regardless of iodine concentration.

The rate is also the gradient of graph of iodine concentration against time.

Since rate is a constant, the gradient of this graph will be a constant too.

Let's run through the options and see which is the best answer.

2020 p1 q10 option A

Option A is where iodine concentration is constant and gradient is zero.

This is applicable when iodine is the catalyst or non-reactant.

2020 p1 q10 option B

Option B is a straight line with constant gradient.

This means rate is constant and independent of iodine concentration, hence will be the best answer for this question.

2020 p1 q10 option C

Option C is where the initial gradient/rate is fast when iodine concentration is high, and rate decreases as iodine concentration decreases.

This graph is applicable for both first and second order reactions.

To differentiate the two, we can find the half life of the reaction.

If half life is constant, the reaction is first order with respect to reactant.

If half life is not constant, the reaction is second order with respect to reactant.

2020 p1 q10 option D

Option D is where the initial gradient/rate is slow but increases as iodine concentration decreases, but eventually decreases as iodine concentration drops to very low levels.

This 'S' shaped curve is typical of auto-catalysed reactions, where one of the products formed catalyses its own reaction.

At the first part of the reaction, the rate is dominated by increasing product/catalyst concentration, hence increases when there are more products formed.

At the second part of the reaction, rate is dominated by decreasing reactant concentration, hence decreases when amount of reactants decreases.

Therefore the answer to this question will be option B.

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

Back to list of questions for 2020 A Level H2 Chemistry Paper 1.

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