MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
Chemical Kinetics
Answers to Concept Checks
· 5.1
1. Two molecules of A2B come together in a combination reaction to form an intermediate, A4B2, which subsequently decomposes to produce the final products, two molecules of A2 and one molecule of B2.
2. The rate-determining step is the slowest step of a reaction. It determines the overall rate of the reaction because the reaction can only proceed as fast as the rate at which this step occurs.
3. The activation energy is the minimum energy needed for a chemical reaction to occur.
4. Both theories require a certain activation energy to be overcome in order for a reaction to occur (therefore not all reactions will occur). The transition state theory focuses on forming a high-energy activated complex that can then proceed forward or backward, forming the products or reverting to the reactants, respectively. The collision theory focuses on the energy and orientation of reactants, and considers each potential reaction to be “all-or-nothing” (either there is enough energy to form the products, or there is not).
· 5.2
1.
|
Conditions |
Zero-Order |
First-Order |
Second-Order |
|
Temperature lowered |
rate decreased |
rate decreased |
rate decreased |
|
All reactants’ concentrations doubled |
rate unaffected |
rate doubled |
rate multiplied by 4 |
|
Catalyst added |
rate increased |
rate increased |
rate increased |
2. This question asks for the rate law and rate order for the following reaction:
A + B + C → D
|
Trial |
[A]initial (M) |
[B]initial (M) |
[C]initial (M) |
|
|
1 |
1.00 |
1.00 |
1.00 |
2.0 |
|
2 |
1.00 |
2.00 |
1.00 |
2.1 |
|
3 |
2.00 |
1.00 |
1.00 |
15.9 |
|
4 |
2.00 |
1.00 |
2.00 |
32.2 |
Start by writing the generic rate law for the reaction: rate = k[A]x[B]y[C]z.
In a complex rate law problem, always check for the possibility of a reagent that has no impact on the rate law. Looking at Trials 1 and 2, the concentration of B is doubled with no change in the rate. Thus, reagent B has no impact on the rate law, and its exponent is zero. The rate law can be updated to rate = k[A]x[B]0[C]z.
Next, compare Trials 1 and 3. The concentration of A doubles, the concentrations of B and C remain constant, and the rate increases by a factor of approximately 8. This results in the proportionality 
The rate law can now be updated to rate = k[A]3[B]0[C]z.
Finally, compare Trials 3 and 4. The concentration of C doubles, the concentrations of A and B remain constant, and the rate approximately doubles. This results in the proportionality 
The rate law can now be updated to rate = k[A]3[B]0[C]1.
Thus, the final rate law is: rate = k[A]3[B]0[C]1 = k[A]3[C]. The rate order is 3 + 0 + 1 = 4.