Chapter 19

Question 1

What is a homogeneous equilibrium?

Answer 1

• An equilibrium that contains species that are all of the same state

Question 2

What is a heterogeneous equilibrium?

Answer 2

• An equilibrium that contains species that are of different states or phases

Question 3

How is Kc calculated for heterogeneous equilibria? Why?

Answer 3

• Any reactants/ products in the solid or liquid states are ignored (only gases and aqueous solutions are used in the calculation)
• The concentrations of solids and liquids practically stay constant

Question 4

How can Kc be calculated when given the moles of the reactants and the volume of the container?

Answer 4

• You draw an ICE table- initial moles, change in moles and equilibrium moles (‘final’ moles)- using stoichiometry
• You divide the equilibrium moles by the volume of the container (since gases expand to fill their container)

Question 5

How are equilibria involving gases usually expressed, and why?

Answer 5

• In terms of Kp
• It is easier to find the pressure of a gas than its concentration

Question 6

How is Kp calculated for homogeneous equilibria?

Answer 6

• For the equation aA + bB <-> cC+dD, Kp= p(D)^d × p(C)^c/p(A)^a × p(B)^b

Question 7

What is partial pressure, and how is it denoted?

Answer 7

• The partial pressure of a gas is the contribution that the gas makes towards the total pressure, P
• p(A) is the partial pressure of A

Question 8

How is the partial pressure of a gas calculated?

Answer 8

• You multiply the mole fraction of the gas by the total pressure

Question 9

What is the mole fraction of a gas, and how is it denoted?

Answer 9

• The proportion of a gas to the total amount of the gas mixture
• x(A) is the mole fraction of A

Question 10

What are the 2 ways of finding the mole fraction of a gas? Explain.

Answer 10

• The number of moles of the gas can be divided by the total number of moles in the gas mixture
• Similarly, since under the same conditions the same number of moles of gases take up the same volume, the mole fraction can also be calculated by dividing the volume of the gas by the total volume of the gas mixture

Question 11

How can you check if your calculations of mole fractions and partial pressures are correct?

Answer 11

• Mole fractions should add up to 1
• Partial pressures should add up to the total pressure

Question 12

How is Kp calculated in heterogeneous equilibria?

Answer 12

• Species in states other than the gas state are ignored

Question 13

What can affect the value of an equilibrium constant (K)?

Answer 13

• Temperature

Question 14

Which factors cannot affect the equilibrium constant (K)?

Answer 14

• Pressure, concentration and the presence of a catalyst

Question 15

How does temperature affect the value of K?

Answer 15

• If the temperature increases, K will:
• increase if the forward reaction is endothermic
• decrease if the forward reaction is exothermic
• The reverse is also true

Question 16

Increasing the concentration of reactants in reactions causes Kc to remain the same. Why?

Answer 16

• This would increase the denominator
• Since that would decrease Kc, the numerator increases; the concentration of the products has to increase (the concentration of the reactants also decreases)
• This happens until Kc becomes a constant again
• (The position of equilibrium changes to maintain a constant equilibrium constant, so Le Chatelier’s principle works because of equilibrium constants, not the other way around.)

Question 17

If the pressure is increased in a system where there is 1 more mole of the reactants, Kp stays the same. Why?

Answer 17

• Replacing the partial pressures with mole fractions and the pressure would cause most of the pressures to cancel, with the exception of one in the denominator
• To prevent Kp from decreasing, the numerator increases; the mole fractions of the products increases (the mole fractions of the reactants also decrease)