MCAT Physical : Biochemistry, Organic Chemistry, and Other Concepts
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Example Question #31 : Reaction Kinetics
Reaction:
Step 1: 
Step 2: 
In the reaction above, which step is the rate-determining step and what is the intermediate?
Step 2 is the rate-determining step and BC2 is the intermediate
Step 2 is the rate-determining step and B is the intermediate
Step 1 is the rate-determining step and B is the intermediate
Step 1 is the rate-determining step and BC2 is the intermediate
Step 2 is the rate-determining step and BC2 is the intermediate
In this reaction, step 2 is the rate-determining step and BC2 is the intermediate. The rate-determining step is always the slowest step in the reaction; the rate of the entire reaction depends on the speed of this step. BC2 is the intermediate because intermediates are not found in the overall reaction; they are produced and then immediately consumed.
Example Question #31 : Reaction Kinetics
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 sum of the rates of both reactions
It must be equal to the rate of the reaction in the passage
It is a new rate, unrelated to either individual reaction rate
It must be equal to the difference in the rates of both reactions
It must be equal to the rate of the second reaction
It must be equal to the rate of the reaction in the passage
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.
Example Question #32 : Reaction Kinetics
Which of the following is true about the rate-determining step of a reaction that contains two transition states?
The rate determining step can only be one step in the reaction
The rate-determining step has the lowest activation energy
The rate determining step is the fastest step in the overall reaction
The rate-determining step cannot be sped up with an enzyme
The rate-determining step has the highest activation energy
The rate-determining step has the highest activation energy
The rate-determining step is the slowest step in the reaction; however, there can be more than one rate-determining step if there are two steps that have high activation energies. Enzymes help lower the activation energy for a particular step, and thus diminish the effect of rate-determining steps to speed up the reaction. The rate-determining step generally precedes the highest energy transition state.
Example Question #33 : Reaction Kinetics
Which of the following are true concerning the rate-limiting step of a chemical reaction?
I. It is the slowest step
II. Cannot be altered by a catalyst or enzyme
III. Involves the transition state with the highest activation energy
IV. Is un-affected by a change in reactant concentration or reaction temperature
III and IV
I, II, and III
I, II, III, and IV
I only
I and III
I and III
The rate-limiting step of any reaction will always have the highest activation energy, and thus be the slowest step in a chemical reaction (hence the name). It can be altered with the help of a catalyst, by changing the concentration of reactions, and by altering the reaction temperature. These changes can affect the energy of a transition state or chemical equilibrium, altering the rate-limiting step. Only choices I and III are true.
Example Question #34 : Reaction Kinetics
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?
Carbon monoxide is a solid, so it is not included in the rate law
The overall reaction must be done twice
The slow step involves two nitrogen dioxide molecules
The fast step involves two nitrogen dioxide molecules
The slow step involves two nitrogen dioxide molecules
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.
Example Question #35 : Reaction Kinetics
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, 3, and 4
2 and 3
3 and 4
3 only
2 and 4
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.
Example Question #36 : Reaction Kinetics
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?
3
5
1
2
4
2
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.
Example Question #16 : Catalysts, Transition States, And Activation Energy
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.
After much thought, the scientist in the passage determines that the reaction depicted in the diagram must be either a radical reaction or a combustion reaction. Which of these options is more likely?
A radical reaction, as they are always exothermic
A combustion reaction, as they contain carbocation intermediates
A radical reaction, as point 3 would correspond to a radical intermediate
A combustion reaction, because they are always endothermic
A combustion reaction, as they contain radical intermediates
A radical reaction, as point 3 would correspond to a radical intermediate
A radical reaction would be a more likely candidate in this case, as step 3 on the diagram would correspond to the stable intermediate. In the radical reaction, this would correspond to a radical intermediate.
Combustion reactions typically don't have a stable intermediate, and are typically exothermic (not endothermic as the answer choice indicates).
Example Question #17 : Catalysts, Transition States, And Activation Energy
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 able to determine that the reactants in the diagram are hydrophilic compounds. Which of the following is likely to decrease the energy level of the reactants?
Resonance structures for the reactants' chemical structures
Increased occupance of d and f orbitals in the reactants' electronic structures
Increased abundance of pi bonds in the reactants' bonding patterns
Dissolution in ethane
Dissolution in carbon tetrachloride
Resonance structures for the reactants' chemical structures
Of any of these changes, only the presence of resonance structures will decrease the overall energy level of hydrophilic compounds. The two solution options both reference nonpolar solvents (though carbon tetrachloride has polar bonds, their geometry cancels out any net dipole moment). Additionally, pi bonds are high energy bonds, and d and f orbitals are high energy orbitals.
Resonace allows greater charge distribution and stability, thus reducing energy level.
Example Question #21 : Catalysts, Transition States, And Activation Energy
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
2 and 4
2 only
4 only
1 only
1 and 5
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.
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