2019 A Level H2 Chemistry Paper 1 Question 14 - Deduce Order for Enzyme Catalysed Reaction


Let's take a look at question 14 for 2019 A Levels H2 Chemistry Paper 1.

2019 p1 q14 question

For an enzyme catalysed reaction, we notice the following relationship between initial rate and substrate concentration.

1. Rate is directly proportionate to substrate concentration at low concentration

When concentration of substrate is low, there are enough enzymes to catalyse the reaction.

Hence an increase in [substrate] will result in increase in rate of reaction.

Order of reaction with respect to substrate is one and the rate equation is as follows:

rate = k [substrate]

2. Rate is independent of substrate concentration at high concentration

When concentration of substrate is high (greater than concentration of enzyme), saturation occurs.

All the enzyme active sites are fully utilised and there are no more enzymes left to catalyse any additional substrate.

Hence an increase in [substrate] will not affect rate of reaction anymore.

2019 p1 q14 saturation

Order of reaction with respect to substrate is zero and the rate equation is:

rate = k

2019 p1 q14 zero order wrt substrate

Hence we can now look at the statements and deduce if they are true or not when [substrate] = x:

1. The initial rate of reaction is affected by increasing [substrate] - we know rate is constant when concentration of substrate is x and beyond, hence this is false.

2. The order of reaction with respect to [substrate] is 1 - this is false as we have discussed previously order of reaction is zero at saturation instead.

3. There are no more enzyme active sites available - true as this would mean the rate of reaction remains constant as all the enzymes are fully utilised.

Only statement 3 is true so the answer to this question will be option D.

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

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