2019 H2 Chem P1 Q14 — Kinetics Solution
Question
Which statements correctly describe the situation when [substrate] = x?
1 The initial rate of reaction is affected by increasing [substrate].
2 The order of reaction with respect to [substrate] is 1.
3 There are no more enzyme active sites available.
Video Solution
Explanation
The answer is D (3 only).
At [substrate] = x the graph has levelled off: the initial rate stays constant as [substrate] increases.
• Statement 1 is false: on the flat part of the curve, increasing [substrate] does not change the initial rate.
• Statement 2 is false: since the rate does not depend on [substrate], the order with respect to substrate is zero. First order applies only to the straight-line part near the origin, where rate is proportional to [substrate].
• Statement 3 is true: all the enzyme active sites are occupied (the enzyme is saturated), so extra substrate molecules cannot form more enzyme-substrate complex, and the rate is limited by the amount of enzyme.
• A is wrong: it includes statement 1, which is false.
• B is wrong: statement 1 is false, and it leaves out the true statement 3.
• C is wrong: statement 2 is false (the order is zero at x), and it leaves out statement 3.
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