6: Equilibrium

Question 1

When is a reversible reaction at equilibrium

Answer 1

When the forward and backward rates are equal

Question 2

when does Kc change

Answer 2

When temperature is changed

Question 3

what is the Kc expression for this reaction

N₂ + 3H₂ ⇌ 2NH₃

Answer 3

Kc = [NH₃]² / [N₂][H₂]³

Question 4

What is the general Kc expression for a reaction

aA + bB ⇌ cC +dD

Answer 4

Kc = [C]^c [D]^d / [A]ª [B]^b

Question 5

How to work out the units of Kc

Answer 5

Add up the powers in the top line of the expression and subtract the powers on the bottom line = x

mol^x dm^-3x

Question 6

Examples of ways to follow change in concentration

Answer 6

Change in absorption of light, if one of the substances is coloured.

Change in pH, if one of the reactants is acid or alkaline

Titration of one of the reactants

Question 6

How do you know if equilibrium is reached

Answer 6

Whatever parameter you are measuring to follow the reaction will stop changing

Question 7

How to calculate Kc from experimental data

Answer 7

• Write the moles of each substance at the start of the reaction.
• Write the moles of each substance at equilibrium (you can find this by comparing how one substance changes when equilibrium is reached with the other substances : use mole ratio).
• Calculate the concentration of each substance at equilibrium (by dividing mol by vol).
• Put values into Kc expression

Question 8

What can you say about the equilibrium if the value for Kc is high

Answer 8

It lies further to the right, or forward direction

Question 9

What happens to the equilibrium constant for an exothermic reaction if the temperature is increased

Answer 9

It decreases

Question 10

If the equilibrium constant for an endothermic reaction increases, has the temperature risen or dropped

Answer 10

the temperature would have increased

Question 11

Does change in pressure affect Kc

Answer 11

No

Question 12

What is homogeneous equilibria

Answer 12

All of the reactants are in the same phase

Question 13

What is heterogeneous equilibria

Answer 13

Not all reactants are in the same phase

Question 14

What phases (states of matter) are included in the Kc expression

Answer 14

gases and aqueous solutions

NOT solids and pure liquids

Question 15

How to calculate the mol fraction of a substance, A

Answer 15

Mole fraction of A (xA) = moles of A / total moles of all gaseous substances

Question 16

How to calculate partial pressure of a substance, A

Answer 16

Partial pressure of A = mole fraction of A × total pressure

Question 17

How to write a partial pressure of substance A

Answer 17

p(A)^mol ratio value in equation

Question 18

Explain in terms of Kc what happens to the amount of N₂O₄ if the pressure increases in the reaction:

2NO₂(g) ⇌ N₂O₄(g)

Kc = [N₂O₄] / [NO₂]²

Answer 18

• Kc stays the same.
• All the concentrations initially increase.
• The bottom line increases more than the top line because there is a higher power of concentration on the bottom line.
• This makes the value smaller and therefore > Kc
• To get back to Kc, the top must increase and/or bottom decrease
• Shift in forward direction
• Amount of N₂O₄ increases