3.1.6: Chemical Equilibria

Question 1

What is equilibrium?

Answer 1

In a reversible reaction when both the foward and backwards reaction remain constant at the same rate so equal quantities of reactants and products are made (don’t change, reaction appears to stop). This occurs in a closed system.

Question 2

What is Le Chatelier’s principle?

Answer 2

the idea that if a condition is changed, the system will counteract the change (e.g. concentration, temperature and pressure).

Question 3

Le Chatelier’s principle

What does the enthalpy change tell you?

Answer 3

+ΔH: endothermic forward reaction
-ΔH: exothermic forward reaction

Question 4

Le Chatelier’s principle

What happens to equilibrium if the temperature changes?

Answer 4

ALL REACTIONS OCCUR EXOTHERMIC IN ONE DIRECTION BUT ENDOTHERMIC IN THE OTHER
- if the temperature increases, equilibrium will shift in the endothermic direction to lower the temperature.
- if the temperature decreases, equilibrium will shift in the exothermic direction to increase the temperature.

Question 5

Le Chatelier’s principle

What happens to equilibrium if the pressure changes?

GASES ONLY

Answer 5

• if pressure increases, equilibrium will shift in the direction where there’s fewer gas molecules/moles to make more and decrease pressure.
• if pressure decreases, equilibrium will shift in the direction where there’s more gas molecules/moles to raise the pressure.
  GASES ONLY/ BALANCED EQUATION

BALANCE EQUATION

Question 6

Le Chatelier’s principle

What happens to equilibrium if the concentration changes?

Answer 6

• if the concentration of reactants increases, the forward reaction is favoured, making more products so equilibrium shifts to the right.
• if the concentration of reactants decreases, the backward reaction is favoured, making more products so equilibrium shifts to the left.

Question 7

What is the equation for equilibrium’s constant

BALANCE EQUATION/ (g) ONLY

Answer 7

BALANCE EQUATION/ GASES ONLY
Kc= [products]^(no./[reactants]^no.×[reactants]^no.)
[]= means conc. of on moldm-3

n=cv

Question 8

What is the difference between homogeneous and heterogeneous equilibria?

Answer 8

homogeneous equilibria- all species in the same phase/ state
heterogeneous equilibria- species are in different phases/ states

Question 9

What is the significance of the Kc value?

Answer 9

The magnitude (size) of Kc shows how far a chemical reaction has gone. It
indicates the position of equilibrium.

Question 10

What does Kc value show?

Answer 10

• A Kc of 1 means the position of equilibrium is half way between reactants and products.
• When Kc > 1 the reaction is product favoured.
• When Kc&raquo_space; 1 equilibrium lies to the right
• When Kc < 1 the reaction is reactant favoured.
• When Kc &laquo_space;1 equilibrium lies to the left

Question 11

How does temperature changes affect the Kc value?

Answer 11

• Increasing temperature shifts the position of equilibrium in the endothermic direction (ΔH +ve)
• Decreasing temperature shifts the position of equilibrium in the exothermic direction (ΔH -ve)
• The ΔH values for the forward and reverse reactions in an equilibrium have the same magnitude but have opposite signs.

Question 12

Why is the Kc value only affected by temperature?

Answer 12

If the forward reaction is endothermic ΔH +ve:
When the forward reaction is endothermic; as temperature increases, Kc increases

If the forward reaction is exothermic ΔH –ve:
When the forward reaction is exothermic; as temperature increases, Kc decreases

Question 13

Why does pressure, concentration and catalysts not affect Kc value?

Answer 13

• The value of Kc changes with temperature but is unaffected by concentration and pressure as equlibrium shifts to counteract the changes.
• Catalysts effect the rate of reaction not position of equilibrium. A catalyst will speed up the rate of the forward and backwards reaction equation Increases the speed that a reaction reaches equilibrium but not the equilibrium conc.