MCAT Physical : Physical Chemistry
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Example Questions
Example Question #114 : Biochemistry, Organic Chemistry, And Other Concepts
Consider the energy diagram of a three-step chemical process shown below. Which step would have the fastest rate?
The reaction with the fastest rate will have the lowest activation energy; the energy difference between the starting compound and the top of the energy barrier that must be overcome to proceed to the product should be the smallest. Of the given choices, the reaction proceeding from C to D has the lowest energy barrier, and hence will have the lowest activation energy and fastest reaction rate.
In contrast, the energy required to move from point A to point B has the largest energy barrier, and can be assumed to be the rate-limiting step.
Example Question #1 : Factors Affecting Reaction Rate
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.
Reaction rates depend on reaction concentrations, as well as the reaction constant 
I. Temperature
II. Activation energy
III. Reactant concentrations
III only
II and III
I, II, and III
I and II
I only
I and II
If you thought all of these factors are used in the Arrhenius equation, consider that the rate constant is independent of the reactant concentrations. The concentration effect is considered in the rate law by multiplying the concentrations by the constant, but the concentrations themselves are independent of the actual Arrhenius calculation.
The calculation shows that 
Example Question #116 : Biochemistry, Organic Chemistry, And Other Concepts
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.
The scientist in the passage calculates that, under experimental conditions, the gievn reaction releases 
Under the exact same conditions, another unknown reaction is studied. This second reaction releases 
Which of the following must be true?
The second reaction will always progress faster
The first reaction will progress faster at the experimental conditions, but may be slower at other conditions
The first reaction will always progress faster
We cannot predict the relative rates of these two reactions with the information given
The second reaction will progress faster at the experimental conditions, but may be slower at other conditions
We cannot predict the relative rates of these two reactions with the information given
The amount of energy released or absorbed by a reaction is independent of its rate. Rate is principally determined by the activation energy, while the inherent bond energies of reactants and products dictate thermodynamic changes. One cannot be used to predict the other.
Example Question #1 : Factors Affecting Reaction Rate
In any given reaction, the rate-determining step __________.
represents the step with the lowest activation energy
may change if temperature is changed or pressure is changed
represents the endothermic component of the reaction
cannot be modified by use of a catalyst
may change if temperature is changed or pressure is changed
Reaction rates, including the rate-determining step, can be altered with a change in conditions. This includes a change in temperature, a change in pressure, or the addition of a catalyst.
Example Question #2 : Factors Affecting Reaction Rate
For a multi-step chemical reaction, which of the following will reduce the reaction rate?
A step that involves the limiting reagent
A step with a high activation energy
A step with a high energy transition state
A step that involves the excess reagent
A step with a high activation energy
For a multi-step reaction, the rate-determining step is the step with the highest activation energy. Be careful not to confuse this with the energy transition state.
Example Question #3 : Factors Affecting Reaction Rate
What is the role of a catalyst in an reaction?
To increase the rate of reaction by lowering the activation energy
To decrease the rate of reaction by lowering the activation energy
To increase the rate of reaction by increasing the activation energy
To decrease the rate of reaction by increasing the activation energy
To increase the rate of reaction by lowering the activation energy
A catalyst is used to increase the rate of a reaction. By definition, it does so without being consumed during the reaction. Thus the choices that talk about decreasing the rate of reaction are wrong. Catalysts decrease the activation energy of a reaction.
Example Question #1 : Catalysts, Transition States, And Activation Energy
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
decrease
increase
not affect
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.
Example Question #2 : Catalysts, Transition States, And Activation Energy
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 
Carbonic anhydrase catalyzes the reaction of CO2 (g) + H2O (l) 
Decrease
Increase
Not change
Increase
This question asks us how the rate of the reaction would change if temperature were increased. Increasing the temperature increases the relative velocity of each reactant, increasing the chance that two reactants collide and are able to form a product with the help of carbonic anhydrase. This can also be seen with the following equation.
Increasing the temperature decreases the denominator because eE/RT becomes e0 = 1 as the temperature increases. The overall effect is an increasing in reaction rate.
Note however, that the temperature can only increase up to a point. Once the temperature becomes too high, the enzyme would denature and no longer work.
Example Question #3 : Catalysts, Transition States, And Activation Energy
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
Decrease
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.
Example Question #4 : Catalysts, Transition States, And Activation Energy
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?
Increase
Decrease
No change
Decrease
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.
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