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Example Questions
Example Question #1 : Le Chatelier's Principle And Common Ion Effect
Calcium carbonate dissolves in water based on the following reaction:
Which of the following will decrease the solubility of the salt?
Add calcium chloride to the solution
Remove calcium ions from the solution
Add more calcium carbonate
Add more water to the solution
Add calcium chloride to the solution
When thinking about the solubility of a salt, it helps to use Le Chatelier's principle. The solubility of a salt is dependent on the amount of ions that are created by the precipitate in solution. As a result, we decrease the solubility by increasing the amount of ions from the salt in solution.
In this case, increasing the amount of calcium or carbonate ions will shift the reaction to the left, decreasing solubility.
The common-ion effect tells us that when an ion made by the salt is increased by another substance, the solubility of the salt will decrease. Calcium chloride will dissolve completely in solution, and will increase the amount of calcium ions. This will shift the reaction to the left, thus reducing the solubility of the precipitate.
Example Question #22 : Reaction Chemistry
Consider the reaction reaction below.
A student allows the system to reach equilibrium and then removes two moles of hydrogen gas. Which of the following will be a result?
The reaction will first shift toward the products, then toward the reactants
No change will occur
The reaction will shift to the side with fewer total moles of gas
More NH3 will be produced
The amount of N2 in the reaction vessel will increase
The amount of N2 in the reaction vessel will increase
According to Le Chatelier's principle, when a system at equilibrium is disturbed, the system will react to restore equilibrium. In other words, it will seek to undo the stress. Here, if hydrogen gas is removed, the reaction will shift toward the reactants to re-form it. In the process, more nitrogen will be produced.
Example Question #1 : 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 
The addition of H2CO3 would __________ the concentration of HCO3-.
not affect
decrease
increase
increase
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. Here, increasing H2CO3 would shift the reaction to the right to minimize the addition of the acid. Shifting the reaction to the right would thus increase the concentration of HCO3-.
Example Question #781 : Mcat Physical Sciences
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 
How would the addition of pure H2O shift the carbonic anhydrase reaction?
Rightward shift
No change
Leftward shift
No change
Remember that Le Chatlier’s principle says that pure liquids and solids do not change the equilibrium of the reaction. Regardless of where in the reaction the pure liquid or solid is present, no equilibrium shift would occur.
Example Question #21 : 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 
Assuming that reacting CO2 is in the gas phase, increasing the pressure would shift the carbonic anhydrase reaction to the __________.
right
left
no change
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 #12 : 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 
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 the amount of catalyst added or pressure only
Changes in temperature, pressure, or concentration
Changes in temperature, concentration, or amount of catalyst
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 #15 : Le Chatelier's Principle And Common Ion Effect
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.
Adding ammonia to the container.
Increasing the temperature.
Removing some nitrogen gas.
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 #13 : 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 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
The reactant concentrations will remain the same as their original equilibrium concentrations
The reactant concentrations will decrease relative to their original 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.
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