MCAT Physical : Equilibrium
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Example Questions
Example Question #11 : Le Chatelier's Principle And Common Ion Effect
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 
Assuming that reacting CO2 is in the gas phase, increasing the pressure would shift the carbonic anhydrase reaction to the __________.
no change
left
right
right
This is a Le Chatlier shift problem. When the equilibrium of a chemical reaction is disturbed, the reaction shifts to the side to minimize the change. With pressure, the reaction is shifted to the side with fewer moles of gas. In the carbonic anhydrase reaction, the only gas present would be CO2, a reactant. Decreasing pressure would thus shift the reaction toward the products (right).
Example Question #22 : Equilibrium
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 
While the kidney is able to compensate for many acid/base changes in our bodies, vomiting is a temporary cause of acid/base imbalance. While vomiting may allow our bodies to get rid of toxic substances, it also causes us to lose gastric acid, which influences blood pH. How would the loss of gastric acid change the pH of our blood?
Decrease pH
Not change in pH
Increase pH
Increase pH
This is an undercover Le Chatlier shift problem. The question tells us that vomiting causes us to lose gastric acid. In the equation that we can see above, losing H+ (in HCl) would pull the reaction to the right, increasing the concentration of HCO3-. Increasing the concentration of the base HCO3- increases the pH, leading the blood to become more basic.
Example Question #11 : Le Chatelier's Principle And Common Ion Effect
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 
What happens to the pH of our blood if we hyperventilate?
Not change
Increase
Decrease
Increase
This is an undercover Le Chatlier shift problem. If we hyperventilate, we expel more CO2, thus pulling the reaction to the left and decreasing the concentration of H2CO3. As the concentration of the acid decreases, the blood becomes more basic, leading to respiratory alkalosis.
Example Question #12 : Le Chatelier's Principle And Common Ion Effect
Le Chatlier's principle states that when a stressor is introduced to a system, the system will shift its equilibrium state as a way of countering the stress. What are the factors (stressors) that apply to Le Chatlier's principle?
Changes in temperature, pressure, or concentration
Changes in temperature, concentration, or amount of catalyst
Changes in the amount of catalyst added or pressure only
Changes in temperature or concentration only
Changes in temperature, pressure, or concentration
Le Chatelier's principle emphasizes three main stressors: changes in temperature, pressure, and concentration of reactants and products. If any of these three stressors are added, the Keq will shift in a way that counters this added stress. Temperature and concentration changes will affect any reaction, while pressure changes will only have a marked effect on reactions involving at least one gaseous compound.
Example Question #25 : Equilibrium
The Haber process for creating ammonia is written below.
This reaction takes place in a glass container, and is allowed to progress to equilibrium. Which of the following manipulations to the system will NOT shift the equilibrium to the left?
Increasing the pressure in the container.
Increasing the temperature.
Removing some nitrogen gas.
Adding ammonia to the container.
Increasing the pressure in the container.
Le Chatlier's principle states that when a system at equilibrium is stressed, the equilibrium will shift accordingly in order to reduce the stress. There are a variety of ways to stress the system and elicit a shift.
1) Addition or removal of a reactant or product.
2) Changing the pressure of a system containing gases.
3) Changing the temperature of the system.
By increasing the pressure of the system, the reaction will shift in the direction which results in fewer gas molecules in the container. Since there are only two gas molecules on the product side and four gas molecules on the reactant side, we would predict a pressure increase to shift the equilibrium to the right.
Increasing temperature and adding ammonia both equate to adding product, which will shift the reaction away from the products and toward the reactants. Similarly, removing a reactant, such as nitrogen, will cause more of that reactant to be produced, also shifting the reaction to the left.
Example Question #11 : Le Chatelier's Principle And Common Ion Effect
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 attempts to modify the conditions for the reactions by placing the reactants in a sealed, variable-volume vessel at 1atm, and allowing it to reach equilibrium. She then decreases the volume of the vessel, increasing the pressure to 5atm. Which of the following is most likely to occur?
The reactant concentrations will increase relative to their original equilibrium concentrations
The reactant concentrations will decrease relative to their original equilibrium concentrations
The reactant concentrations will remain the same as their original equilibrium concentrations
The reactant concentrations will first increase, then decrease relative to their equilibrium concentrations
The reactant concentrations will first decrease, then increase relative to their equilibrium concentrations
The reactant concentrations will increase relative to their original equilibrium concentrations
By changing the volume of the container, the scientist has changed the partial pressure of nitrogen gas in the vessel. As a result, the equilibrium shifts to the left, increasing the concentration of the reactants.
According to Le Chatelier's principle, decreasing pressure will shift equilibrium away from any gaseous compounds. The only gas in the reaction is the nitrogen product. Decreasing volume will, thus, shift equilibrium toward the reactants.
Example Question #21 : Equilibrium
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 attempts to modify the conditions for the reaction by placing the reactants in a sealed vessel at 1atm, and allowing it to reach equilibrium. She then adds helium gas to the vessel, increasing the pressure to 5atm. Which of the following is most likely to occur?
The reactant concentrations will remain the same as their original equilibrium concentrations
The reactant concentrations will increase relative to their original equilibrium concentrations
The reactant concentrations will decrease relative to their original equilibrium concentrations
The reactant concentrations will first increase, then decrease relative to their original equilibrium concentrations
The reactant concentrations will first decrease, then increase relative their original equilibrium concentrations
The reactant concentrations will remain the same as their original equilibrium concentrations
The addition of an inert gas like helium does not change the partial pressure of the products or reactants. As a result, the equilibrium concentrations do not change.
Example Question #28 : Equilibrium
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
In studying the acid 
The concentration of 
The concentration of 
The concentration of 
The concentration of 
The concentration of 
The concentration of
In exothermic process heat is released during the reaction.
Heat can be considered a product in this situation, and thus increasing the amount of heat in the vessel after the system has reached equilibrium will drive the reaction to the left. This property is a derivative of Le Chatelier's principle.
Example Question #791 : Mcat Physical Sciences
In the given reaction, which of the following changes takes place if the temperature of the system is increased?
More than one of these changes occurs
None of these changes occurs
This reaction is exothermic, since heat is released in the reaction. In exothermic reactions, decreasing the temperature favors the forward (exothermic) reaction, while increasing the temperature favors the reverse (endothermic) reaction. Similarly, for an endothermic reaction decreasing the temperature favors the reverse (exothermic) reaction, while increasing the temperature favors the forwards (endothermic) reaction.
In this question, increasing the temperature will favor the reverse reaction, increasing the reactant concentration and decreasing the product concentration.
Example Question #11 : Le Chatelier's Principle And Common Ion Effect
This reaction is allowed to proceed to equilibrium in a 
The reaction will shift to the left
The reaction will shift to the side containing a greater number of gas particles
More 
No change will occur
More 
More
This reaction is at equilibrium until it is disturbed by a sudden volume change. Since the volume is decreased, the pressure on the container's contents will increase. According to Le Chatelier's principle, an increase in pressure will cause the reaction to shift toward the side that contains fewer gas particles.
In this reaction, there are three moles of gas in the reactants and two moles of gas in the products. A decrease in volume, or increase in pressure, will shift the reaction to the right, toward the product 
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