3.2.3 - Chemical Equilibrium

Question 1

When does dynamic equilibrium occur?

Answer 1

• closed system.
• rate of forward reaction is equal to rate of reverse reaction.
• concentrations of the reactants and products remain the same.

Question 2

What happens when there is an increase in temperature?

Answer 2

Equilibrium shifts in the endothermic direction to absorb the heat.

Question 3

What happens when concentration of products is increased?

Answer 3

The reaction shifts to the left to help remove the extra product.

Question 4

What happens when pressure is increased?

Answer 4

Shifts the equilibrium to the side with fewer gas molecules.

Question 5

What effect do catalysts have on dynamic equilibrium?

Answer 5

No effect on the position as they speed up both the forward and reverse reactions by the same amount. Don’t increase yield but the do mean the equilibrium is reached faster.

Question 6

Why is 300 degrees C used for a reaction when the forward reaction is exothermic?

Answer 6

A lower temperature slows the rate of reaction therefore it is a compromise between maximum yield and a faster reaction.

Question 7

Why is only 60-70 atm used for a reaction when the forward reaction is favoured by by high pressure?

Answer 7

High pressures are expensive to produce so 60-70 atm is a compromise between max yield and expense.

Question 8

What is the equation for Kc?

Answer 8

Products / reactants

Question 9

If Kc is bigger than 1…

Answer 9

There are more products than reactants, equilibrium lies to the right.

Question 10

If the Kc is smaller than 1…

Answer 10

There are more reactants than products, equilibrium lies to the left.

Question 11

Kc for N2 + 3H2 -> 2NH3

Answer 11

[NH3]^2 / [N2]^2 [H2]^3

Question 12

What reversible reaction can be used to investigate the effect of changing temperature on equilibrium position and why?

Answer 12

The brown gas NO2 exists in equilibrium with the colourless gas N2O4.

Question 13

Describe the experiment for investigating changing temperature and equilibrium position.

Answer 13

1. Place two sealed tubes containing the equilibrium mixture in water baths - one in a warm bath and one in a cool water bath.
2. The tube in the warm water will change to a darker brown as the reaction will be pushed to the left.
3. The tube in the cool water will lose colour as the equilibrium will shift to the right.

Question 14

What reaction can be used to investigate the effect of concentration in dynamic equilibrium?

Answer 14

Mixing iron nitrate (yellow) and potassium thiocynate (colourless) to produce iron thiocynate (blood red).

Question 15

Describe the experiment to investigate the effect of concentration on dynamic equilibrium.

Answer 15

1. In three different test tubes increase the concentration of the three substances.
2. The test tubes with an increase in either iron nitrate or potassium thiocynate will turn blood red as the reaction will shift to the right.
3. In the test tube with an increased concentration of iron thiocynate, it will turn yellow as the reversible reaction is increased as the equilibrium moves to the left.