Catalysts, Transition States, and Activation Energy
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MCAT Physical › Catalysts, Transition States, and Activation Energy
Consider the following overall reaction:
The rate law for this reaction is determined to be:
What can we conclude about the reaction, based on the rate law?
The slow step involves two nitrogen dioxide molecules
The fast step involves two nitrogen dioxide molecules
The overall reaction must be done twice
Carbon monoxide is a solid, so it is not included in the rate law
Explanation
Since the rate law does not match the overall reaction, we can assume that the reaction has multiple steps.
In a multistep reaction, the slowest step will determine the rate law. As a result, we can conclude that the overall reaction has a slow step, in which two nitrogen dioxide molecules react. The coefficient becomes the exponent. Note that this is only the case because we are working with the identified slow step, which can only be determined experimentally.
Carbon monoxide only enters into the equation in the faster step, and is not included in the overall rate law.
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
In the reaction diagram, which point is most instrumental in determining the rate of the forward reaction?
2
1
3
4
5
Explanation
Point 2 is at the highest peak on the chart. The difference between the energy level at this point and the energy level of the reactants is the activation energy. This is the energy that the reactants must overcome to proceed to the lower energy products. Overcoming this point takes energy, and is the primary limiting factor in the speed of any reaction.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3 
If the reaction series presented above is occurring during alkalosis, H2CO3 may be considered a(n) __________.
intermediate
enzyme
cannot be determined
hormone
Explanation
First, we need to see what the information the question provides us, namely that the reaction is occurring during alkalosis. In alkalosis, we know that the H+ concentration is too low, thus the reaction must be favoring the products in order to reduce the effects of alkalosis (Le Chatlier’s Principle). The H+ is low on the products side, so the reaction shifts to the right. Next, we need to determine where in the reaction H2CO3 is and what is happening to it. We can see that H2CO3 is formed from CO2 and H2O, but then is used up to create HCO3- and H+. In the scenario of alkalosis, H2CO3 will be formed then used (the definition of an intermediate).
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
At which point(s) on the above graph would you expect to find bonds forming and breaking?
2 and 4
2 and 3
3 and 4
3 only
2, 3, and 4
Explanation
It may be tempting to include points 2, 3, and 4, but only points 2 and 4 actually have bonds in the process of forming and breaking. This lack of stability is what gives these points the highest relative energy levels on the diagram. Point 3 is a relatively stable intermediate, stabilized by the fact that it does not have bonds in active flux.
If the reactants and/or products in a chemical reaction are gases, the reaction rate can be determined by measuring the change of pressure as the reaction proceeds. Consider the following reaction and pressure vs. reaction rate data below.
| Trial | PXY(torr) | PZ(torr) | Rate (torr/s) |
|---|---|---|---|
| 1 | 100 | 200 | 0.16 |
| 2 | 200 | 200 | 0.32 |
| 3 | 200 | 100 | 0.04 |
| 4 | 200 | 150 | 0.14 |
If an inhibitory catalyst were added to the reaction __________.
the activation energy would increase
the activation energy would decrease
the concentration of YZ would increase at equilibrium
the concentration of YZ would decrease at equilibrium
Explanation
A catalyst affects activation energy; an inhibitory catalyst increases activation energy. Catalysts do not affect equilibrium concentrations of products or reactants.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3 
If the pH of the blood increases above 8, how would the activity of carbonic anhydrase change?
Decrease
Increase
Not change
Explanation
Extreme temperatures and pH levels decrease the activity of enzymes because they become denatured. In the body, most enzymes work optimally around a pH of 7.4. Increasing the pH too high would denature a protein because amino acids that are normally protonated at physiological pH (i.e. acidic residues) would become deprotonated. Lack of protonation would cause collapse of the tertiary and quaternary structures, leading to a decrease in enzyme function.
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
The scientist in the passage is attempting to modify the reaction as it is ongoing, and adds a catalyst to the vessel. Which points will not move with the addition of a catalyst?
1 and 5
1 only
4 only
2 and 4
2 only
Explanation
Points 1 and 5 are the energy levels of the reactants and products, respectively. These levels do not change with the action of a catalyst, and instead are fixed by the thermodynamic nature of the chemical species involved. A catalyst would lower activation energies by providing an alternate route to reach the products from the reactants, and thus would likely affect points 2, 3, and 4.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3 
If the pH of the blood decreases below 7, how would the concentration of HCO3- change?
Decrease
Increase
No change
Explanation
Extreme temperatures and pH levels decrease the activity of enzymes because they become denatured. In the body, most enzymes work optimally around a pH of 7.4. Decreasing the pH too low would denature a protein because amino acids that are normally deprotonated at physiological pH (i.e. basic residues) would become protonated. Protonation would cause changes in tertiary and quaternary structures, leading to a decrease in enzyme function, thus the concentration of the product in the catalyzed reaction would decrease as well.
A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:
In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.
Consider that the reaction in the passage is the first step of a larger process. This process then goes on to a second, faster reaction between water and a solid metal. What is true of the overall rate of this entire process?
It must be equal to the rate of the reaction in the passage
It must be equal to the rate of the second reaction
It must be equal to the sum of the rates of both reactions
It must be equal to the difference in the rates of both reactions
It is a new rate, unrelated to either individual reaction rate
Explanation
The question specifies that the reaction in the passage is the slowest step of this overall process. This means that it is the rate-limiting step, and its rate defines the overall reaction rate of the process.
Water is formed according to the rate in the passage. The second step cannot continue until this water is formed; thus, the second step cannot work at a faster rate than the first. The overall reaction only goes as fast as the rate of the limiting step.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3 
Increasing the concentration of the carbonic anhydrase would __________ the rate constant of the forward reaction.
not affect
increase
decrease
Explanation
The concentration of the enzyme is independent of the rate constant because the enzyme can only catalyze the conversion of reactants to products at a specific rate. Increasing the concentration of the enzyme, however, would increase the absolute number of reactions occurring simultaneously; thus, the rate of reaction (but not the rate constant) would increase.