Equilibrium
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MCAT Physical › Equilibrium
A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove.
The temperature outside is –10 degrees Celsius. The scientist asks the students to consider the following when answering his questions:
Gibbs Free Energy Formula:
ΔG = ΔH – TΔS
Liquid-Solid Water Phase Change Reaction:
H2O(l) ⇌ H2O(s) + X
The scientist prepares two scenarios.
Scenario 1:
The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup.
Scenario 2:
The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly.
After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment.
Scenario 3:
Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes.
Scenario 4:
The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen.
The same scientist in the passage measures the variables of another reaction in the lab. He knows that this reaction is spontaneous under standard conditions, with a standard free energy change of –43 kJ/mol. Using laboratory-calculated variables, he determines that the Gibbs Free Energy has a value of 0 kJ/mol. He then calculated the reaction quotient of this reaction, while knowing the equilibrium constant was 3 x 103. What is true of the reaction quotient?
It must be equal to 3 x 103
It must be less than 3 x 103
It must be more than 3 x 103
It must be zero
It must be one
Explanation
At equilibrium, reaction quotient and equilibrium constant are equal.
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 amount of N2 in the reaction vessel will increase
More NH3 will be produced
The reaction will first shift toward the products, then toward the reactants
The reaction will shift to the side with fewer total moles of gas
No change will occur
Explanation
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.
A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove.
The temperature outside is –10 degrees Celsius. The scientist asks the students to consider the following when answering his questions:
Gibbs Free Energy Formula:
ΔG = ΔH – TΔS
Liquid-Solid Water Phase Change Reaction:
H2O(l) ⇌ H2O(s) + X
The scientist prepares two scenarios.
Scenario 1:
The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup.
Scenario 2:
The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly.
After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment.
Scenario 3:
Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes.
Scenario 4:
The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen.
The same scientist in the passage measures the variables of another reaction in the lab. He knows that this reaction is spontaneous under standard conditions, with a standard free energy change of –43 kJ/mol. Using laboratory-calculated variables, he determines that the Gibbs Free Energy has a value of 0 kJ/mol. He then calculated the reaction quotient of this reaction, while knowing the equilibrium constant was 3 x 103. What is true of the reaction quotient?
It must be equal to 3 x 103
It must be less than 3 x 103
It must be more than 3 x 103
It must be zero
It must be one
Explanation
At equilibrium, reaction quotient and equilibrium constant are equal.
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 amount of N2 in the reaction vessel will increase
More NH3 will be produced
The reaction will first shift toward the products, then toward the reactants
The reaction will shift to the side with fewer total moles of gas
No change will occur
Explanation
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.
In the given reaction, which of the following changes takes place if the temperature of the system is increased?
None of these changes occurs
More than one of these changes occurs
Explanation
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.
Which of the following statements is false about the Keq of a reversible chemical reaction?
Pure liquid and solid concentrations are found in the Keq equation
Keq is tempurature dependent
As Keq increases, the equilibrium concentration of products in the reaction increases
Keq is not affected by catalysts
Explanation
Keq only includes the concentrations of gases and aqueous solutions. Pure solid and liquid concentrations are left out of the equation.
Keq is given by the equation below, where the concentrations expressed are the equilibrium concentrations.
Keq is a property of a given reaction at a given temperature. It is unaffected by catalysts, which only affect rate and activation energy. As the value of Keq increases, the equilibrium concentration of products must also increase, based on the equation.
Consider the following saturated solution. Assume it is at equilibrium.
Adding sodium chromate to the above solution would ___________ the solubility of lead chromate due to ___________.
decrease . . . an increase of chromate ion concentration
increase . . . an increase of chromate ion concentration
increase . . . an increase of sodium ion concentration
decrease . . . an increase of sodium ion concentration
Explanation
Adding sodium chromate increases the concentration of chromate ion in the solution, which shifts the reaction to the left due to the common ion effect. Thus, the solubility of lead chromate would decrease, as there would be an increased amount of solid lead chromate.
Which of the following adjustments would shift the equilibrium towards the left?
I. Placing the reaction vessel into an ice bath
II. Increasing the pressure in the reaction vessel
III. Increasing the amount of C and D
IV. Decreasing the amount of A and B
II, III, and IV
I, and III
I, II, and IV
III, and IV
Explanation
The following will shift the equilibrium to the left:
- decreased volume (increased pressure)
- increased temperature
- decreased amounts of A and B
- increased amounts of D and C
Of the choices, all are correct except for Roman numeral I. Placing the vessel into an ice bath decreases the temperature of the equilibrium, and thus the system would want to shift towards the side that produces heat to replace the heat lost.
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
Explanation
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.
Which of the following statements is false about the Keq of a reversible chemical reaction?
Pure liquid and solid concentrations are found in the Keq equation
Keq is tempurature dependent
As Keq increases, the equilibrium concentration of products in the reaction increases
Keq is not affected by catalysts
Explanation
Keq only includes the concentrations of gases and aqueous solutions. Pure solid and liquid concentrations are left out of the equation.
Keq is given by the equation below, where the concentrations expressed are the equilibrium concentrations.
Keq is a property of a given reaction at a given temperature. It is unaffected by catalysts, which only affect rate and activation energy. As the value of Keq increases, the equilibrium concentration of products must also increase, based on the equation.