Equilibria

Question 1

Define the term dynamic equilibrium

Answer 1

The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentration do not change..

Question 2

Describe what is meant by a closed system

Answer 2

A closed system is a system that is isolated from its surroundings, so temperature, pressure and concentrations of reactants and products are unaffected by outside influences.

Question 3

Define Le Chatelier’s Principle

Answer 3

When a system in dynamic equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of the change and to restore equilibrium.

Question 4

Affect of increasing concentration on position of equilbrium

Answer 4

1. If more reactants are added the equilibrium position shifts to the right to produce more products- use up some of the extra reactant added.
2. If some of the products is taken away- lower concentration- position shifts to right to produce more products.

Question 5

How can you see the effect of changing concentration on position of equilibrium

Answer 5

1. If there is an equilibrium where the reactants and products have different colours.
2. Increase the concentration of one of the reactants that isn’t coloured e.g H+ ions
3. Then increase the concentration of one of the products or something to change the pH e.g alkali
4. The solution will turn different colours as the position of equilibrium shifts.

Question 6

Describe the effect of temperature on the position of equilibrium

Answer 6

1. An increase in temperature will shift the equilibrium position in the endothermic direction.
2. A decrease in temperature will shift the equilibrium position in the exothermic direction.

Question 7

How can you see the effect of changing temperature on position of equilibrium

Answer 7

1. Use reactants and products which are different colours at equilibrium
2. Put in iced water- turns one colour
3. Put in hot water bath- turns the other colour.

Question 8

Describe the effect of changing the pressure on the position of equilibrium

Answer 8

1. Only for gases
2. If there are more gaseous molecules on one side of the equation than the other.
3. If pressure is increases the position of equilibrium shifts to the side where there are fewer gas molecules to minimise the increase in pressure.
4. Decreasing pressure shifts it in the opposite direction.

Question 9

Describe the effect of a catalyst on the position of equilibrium

Answer 9

1. Catalyst speeds up the rate of the forward and backward reaction equally.
2. Resulting in an unchanged position of equilibrium

Question 10

Haber process conditions - what would increase the yield of the products and why are these conditions not used

Answer 10

1. Decreasing the temperature will push the equilibrium to the right. BUT would do so very slowly - less is produced and the equilibrium may not even be reached.
2. A high pressure pushes equilibrium to the right and increases the rate of reaction. BUT a very high pressure requires a very strong container and a large quantity of energy increasing the cost of the process. And making it less safe- hot gases like toxic ammonia could leak under pressure.

Question 11

What do equilibrium constants show

Answer 11

1. Provides the actual position of equilibrium.
2. The magnitude of an equilibrium constant indicates whether there are more reactants or ore products in an equilibrium system.

Question 12

Show the equation for Kc for this reaction.

aA + bB ⇌ cC + dD

Answer 12

Kc= ( [C]^c [D]^d)/ ( [A]^a [B]^b] )

Put in the equilibrium constants and do them to the power of the balancing number.

Question 13

What do the values of Kc mean

Answer 13

1. Kc of 1 indicates position of equilibrium that is halfway between reactants and producs.
2. Kc > 1 indicates position of equilibrium that is towards the products.
3. Kc< 1 indicates position of equilibrium that is towards the reactants

Question 14

Describe how to find units for Kc values

Answer 14

1. Substitute units into expression for Kc

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

Question 15

Describe what is meant by homogeneous equilibrium

Answer 15

An equilibrium in which all the species making p the reactants and products have the same physical state

Question 16

Describe what is meant by a heterogeneous equilibrium

Answer 16

Am equilibrium in which the species making up the reactants and products have different physical states.

Question 17

What shouldn’t be included in Kc calculations

Answer 17

Solids as they don’t have a concentration

Question 18

What affects Kc

Answer 18

Only Temperature

Question 19

Describe how to calculate equilibrium moles form the initial moles of reactants

Answer 19

1. See how many moles of product are formed in equilibrium
2. Minus this amount from the reactants initial moles- Using mole ratios (subtract double if ratio is 2:1)
3. This gives you the equilibrium moles.

Question 20

Describe how to calculate the mole fraction

Answer 20

1. Mole fraction x(A) = Number of moles of A/ Total number of moles in gas mixture.
2. Sum of mole fractions in a gas mixture must equal 1

Question 21

State equation for measuring partial pressure.

Answer 21

1. Partial pressure p(A) = mole fraction of A * Total pressure P
2. p(A) = x(A) * P
3. Sum of partial pressures= Total pressure

Question 22

State the equation for Kp for this reaction.

aA (g) + bB (g) ⇌ cC (g) + dD (g)

Answer 22

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

Question 23

What does Kp include

Answer 23

Only gases because only gases have partial pressures.

Question 24

Describe how to calculate units for Kp

Answer 24

1. Substitute units into expression for Kp

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

Question 25

How is loss of reaction mixture at high temperature avoided

Answer 25

1. Evaporating and condensing continuously by using a condenser- heat under reflux

Question 26

Why should analysis of equilibrium mixture be carried quickly

Answer 26

1. To prevent a new equilibrium from being established

Question 27

Describe the effect of temperature on the value of K

Answer 27

If forward reaction is exothermic;

1. the equilibrium constant decreases with increasing the temperature.
2. Raising the temperature decreases the equilibrium yield of products

Question 28

Explain the equilibrium shift for temperature

2SO2 (g) + O2 (g) ⇌ 2SO3 (g) delta H = -197

Answer 28

1. Forward reaction is exothermic
2. If the temperature increases the K value will decrease
3. The ratio of p(SO3)^2/ p(SO2)^2 * p(O2) is now greater than the value of Kp (as Kp has decreased).
4. So the mixture is not at equilibrium
5. So the p(SO3) must decrease and p(SO2) and P(O2) must increase, so that the ratio is equal to Kp.
6. Position of equilibrium shifts to the left so the ratio is now equal to the new Kp value.

Question 29

How does concentration affect the value of Kc:

N2O4 (g) ⇌ 2NO2 (g)

Answer 29

1. if [N2O4] is increased the ratio [NO2]^2/ [N2O4] is now less than Kc and the system is no longer at equilibrium.
2. Therefore the concentrations must change to return the ration back to the original Kc value.
3. [NO2] must increase and the [N2O4] must decrease.
4. To do this the equilibrium position shifts to the right,
5. Kc controls the relative concentrations of reactants and products present at equilibrium.

Question 30

How does pressure affect the value of Kp:

N2O4 (g) ⇌ 2NO2 (g

Answer 30

1. If the total pressure is doubled then p(NO2) is increased more than the p(N2O4) (because p(NO2) is to the power of 2).
2. This means the ratio is now greater than Kp so the system is not longer at equilibrium,
3. The partial pressure of NO2 must decrease and of N2O4 must increase to return the ratio back to the Kp value.
4. Equilibrium shifts to the left to side with fewer gaseous moles. The shift is directed by the value of Kp being restored.