Chapter 19 - Equilibrium

Question 1

How do you work out the units for Kc?

Answer 1

Substitute units into the expression for Kc

Cancel common units and show the final units on a single line

Question 2

What is homogeneous equilibria?

Answer 2

Where equilibrium species all have the same state or phase

Question 3

What is heterogeneous equilibria?

Answer 3

When equilibrium species have different states or phases

Question 4

What changes must you make to the Kc expression when it is a heterogeneous equilibrium?

Answer 4

Any species that are solids and liquids and omitted from the Kc expression (as their concentration is essentially constant)
Kc only includes species that are (g) or (aq)

Question 5

What does ICE stand for in an ICE table for experimental results?

Answer 5

Initial moles
Change moles
Equilibrium moles

Question 6

What is the mole fraction of a gas?

Answer 6

The proportion of a gas to the total volume of gases in a gas mixture

Question 7

How do you calculate mole fraction?

Answer 7

For Gas A:

mole fraction x(A) = number of moles of A/total number of moles in gas mixture

Question 8

What is partial pressure?

Answer 8

The contribution that a gas makes in a gas mixture towards the total pressure

Question 9

How do you calculate partial pressure?

Answer 9

For Gas A:

partial pressure p(A) = mole fraction of A x total pressure

Question 10

What is the expression for the equilibrium constant Kp?

Answer 10

Kp = p(C)^c p(D)^d/p(A)^a p(B)^b

Where p = equilibrium partial pressure
small c/d/a/b = balancing number in chemical equation

Question 11

What does the value of K give?

Answer 11

The exact position of the equilibrium

Question 12

What happens to K at a set temperature when concentration or pressure changes, or a catalyst is present?

Answer 12

K value remains constant

Question 13

How does K change with increasing temperature if the forward reaction is exothermic? (and why)

Answer 13

The equilibrium constant decreases with increasing temperature
BECAUSE:
Raising the temp decreases the equilibrium yield of product as the equilibrium position shifts to the left
Therefore the denominator (reactants) becomes larger and the K value will decrease

Question 14

How does K change with increasing temperature if the forward reaction is endothermic?

Answer 14

The equilibrium constant increases with increasing temperature
BECAUSE:
Raising the temp decreases the equilibrium yield of reactant as the equilibrium position shifts to the right
Therefore the numerator (products) becomes larger and the K value will increase

Question 15

How is the equilibrium constant affected by changes in concentration/pressure?

Answer 15

K remains constant

Question 16

What is still affected by changing concentrations and pressure?

Answer 16

Equilibrium position (moves to ensure K stays constant)

Question 17

How does Kp change when pressure increases (and there are less gaseous mol on the product side)?

Answer 17

Equilibrium shifts to the right as there are less gaseous mol on products side
Increased pressure increases the denominator of Kp (reactants)
So the equilibrium shifts to restore the Kp ratio by increasing the numerator

Question 18

How does a catalyst affect equilibrium constants?

Answer 18

They are unaffected

Question 19

Why do catalysts not affect the equilibrium constant?

Answer 19

Catalysts affect the rate of a chemical reaction but not the position of equilibrium
They speed up both the reverse and forward reactions by the same factor.
Equilibrium is therefore reached much quicker, but the constant is not changed

Question 20

What are 2 features of a system that is in dynamic equilibrium?

Answer 20

rate of forward reaction = rate of reverse reaction
concentrations of reactants and products are constant but they are constantly interchanging

Question 21

In the Haber process, why is it easier to increase N2 than H2?

Answer 21

N2 is obtained from the air whereas H2 must be manufactured