15.3.3 The Effects of Pressure and Volume on Equilibrium

Question 1

The Effects of Pressure and Volume on Equilibrium

Answer 1

• A change in total pressure that does not affect the partial pressures of the components of a system does not apply a stress, and therefore does not affect the equilibrium.
• A change in the volume of a system affects the partial pressures or concentrations of the components of the system; therefore, the system will respond to counteract the stress.
• If the sum of the stoichiometric coefficients of the products of a balanced chemical reaction equals the sum of the stoichiometric coefficients of the reactants, the system cannot respond to a change in volume.

Question 2

note

Answer 2

• A chemical system can be taken out of equilibrium by adding or removing a reagent. By Le Châtelier’s principle, the system will respond to this stress by shifting the equilibrium to counteract the stress.
• If a system is at equilibrium at a total pressure of one
  atmosphere, what will happen if the pressure is doubled?
• When argon was added, the total pressure of the system increased, but the partial pressures of NO 2 and N 2 O 4 remained the same. Partial pressures are unaffected by the number of molecules of other non-reacting gases in the container
• How will a system at equilibrium respond to a doubling of volume (thus decreasing concentration)?
• If the dimerization of cyclopentadiene (Cp) is conducted at double the volume with the moles remaining constant, the final molarity will be half the original molarity.
• Since Q > K, the reaction shifts left.
• To reestablish equilibrium, Q must decrease, [Cp] must increase and [Cp 2 ] must decrease. This is established by shifting the equilibrium to the left, as predicted.
• In the case of the hydrogen gas (H 2 ) and iodine gas (I 2 ), two moles of reactants form two moles of products.
• What will happen to the equilibrium if the volume of the container is doubled?
• The system can’t respond to this change, since reacting one mole of H 2 with one mole of I 2 still results in two total moles of gas. Le Châtelier’s principle predicts that the reaction remains at equilibrium after the volume is doubled.

Question 3

Which of the following statements about Le Châtelier’s principle is not correct?

Answer 3

Adding an inert gas (that is not part of the equilibrium system) will cause a change in the total pressure of the system and will cause a new equilibrium to be established.

Question 4

Which of the following statements is not correct?

Answer 4

The effect of a stress on the total pressure of a system is important when investigating changes in Q.

Question 5

Which of the following statements about partial pressures and total pressure in a system is not correct?

Answer 5

If you change the volume of an equilibrium system, the value for Q will change as well.

Question 6

Look at the reaction that we examined in the lesson:

N2O4(g) ↔ 2NO2(g)

Which of the following statements about this system is not true?

Answer 6

Adding an inert gas, such as argon, will cause the volume to change.

Question 7

Look at the following equilibrium reaction.

N2O4(g) ↔ 2NO2(g)

Which statement about this equilibrium system is not correct?

Answer 7

You can double the total pressure of the system by adding more reaction reagent, adding another gas, or reducing the volume by a factor of 2. All these pressure changes will cause the value of Q to change.

Question 8

Suppose that the volume for the equilibrium system, N2O4(g) ↔ 2NO2(g), was changed from 4 L to 2 L.

What happens to the value of Q as the system is shifted away from equilibrium by changing the volume? Predict which direction the reaction will proceed to reach equilibrium again.

Answer 8

The value of Q would increase by a factor of 2. In other words, the new value for Q is four times the original value for Q. Therefore, the reverse reaction would occur in order to reach a new equilibrium.

Question 9

Which statement about this reaction is not correct?

N2O4(g) ↔ 2NO2(g)

Answer 9

At equilibrium, Q is equal to Kc. A decrease in the volume of the system will not change the value of Q.

Question 10

Suppose that you have the following equilibrium reaction of four gases, A, B, C, and D.

aA(g) + bB(g) ↔ cC(g) + dD(g)

Which statement about this system is not correct?

Answer 10

Adding another gas, E, will not affect the total pressure of the system.

Question 11

If a gas is compressed from 5 L to 1 L and the temperature remains constant, what happens to the pressure?

Answer 11

The final pressure is 5 times the original pressure.

Question 12

Which of the following statements correctly defines Boyle’s law and explains how it is related to this lesson?

Answer 12

For a fixed mass of gas at constant temperature, the product of the volume and the (partial) pressure of the gas is a constant value. As the volume changes, the pressure of the gas changes by the inverse of the change in volume.