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MCAT Chemistry Question — Rate Law
- by
- Mar 21, 2017
- MCAT Question of the Day
- Reviewed By: Liz Flagge
The reaction A + B → C proceeds at a rate that varies with the initial concentration of the reactants, as follows:
|
Trial |
[A]t=0 |
[B]t=0 |
Rate t=0 (M/s) |
|
1 |
0.1 M |
0.5 M |
5 x 10-4 |
|
2 |
0.2 M |
1.0 M |
1 x 10-3 |
|
3 |
0.1 M |
4.0 M |
4 x 10-3 |
What is the rate law for the reaction above?
- Rate = k[A]
- Rate = k[B]
- Rate = k[A][B]
- Rate = k[A][B]2
Click for Explanation
To determine the rate law, you must compare trials to see how different initial reactant concentrations affect the initial rate of the reaction. Comparing trial 1 and trial 3, the initial rate in trial 3 increased by a factor of 8. Since [A] is constant and [B] increases by a factor of 8, the reaction rate is first order with respect to B.
Comparing trial 1 and trial 2, the initial rate of trial 2 increase by a factor 2. Since [B] also increases by a factor of 2 and the reaction rate is first order with respect to B, the increase in the reaction rate is due to [B] alone. Thus, the reaction is zero order with respect to A. Thus, the rate law can be described by answer choice B.
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