MCAT Physical : Reaction Kinetics
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Example Questions
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.
Example Question #41 : Reaction Kinetics
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) __________.
cannot be determined
enzyme
intermediate
hormone
intermediate
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).
Example Question #1 : Energetics
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.
Upon further review, the scientist realizes that the reaction in question involved formation of a carbocation that quickly reacted again to form stable products. At which point would we most likely find this carbocation in the above diagram?
5
3
1
2
4
3
Point 3 is where you would expect to find a relatively stable intermediate. An intermediate is more stable than a transition state, but not as stable as the original reactants and final products. Stability is inversely proportional to energy, thus we are looking for the point that is between the highest and lowest energies in the reaction. By this logic, point 1 is the reactants, 2 and 4 are transition states, 3 is a stable intermediate, and 5 is the products.
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