Chemistry - 3.1.6: Chemical Equilibria, Le Chatelier's Principle and Kc

Equilibrium mixture

The mixture of reactants and products formed when a reversible reaction is allowed to proceed in a closed container until no further change occurs. The forward and backward reactions are still proceeding but at the same rate.

Dynamic equilibrium

A situation in which the composition of a constant concentration reaction mixture does not change because both forward and backward reactions are proceeding at the same rate.

4 conditions for equilibrium

- Must be a closed system
- The final equilibrium position will be the same no matter the direction equilibrium is approached from
- The rates of the forward and reverse reactions are the same
- The macroscopic (visible) properties of the system do not change with time (e.g. density, colour, pressure)

What is a closed system?

A system where the reactants and products cannot escape

2 features of a reaction at equilibrium

- Forward and reverse reactions proceed at equal rates
- The concentrations of reactants and products remain constant

Le Chatelier's principle

If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance - the position of equilibrium shifts to oppose the change.

3 factors that affect the position of equilibrium

1. The concentration (of reactants / products)
2. The overall pressure
3. The temperature

Effect of changing the concentration on equilibrium?

Increase concentration of reactants -> equilibrium shifts to oppose change and reduce concentration of reactants -> position of equilibrium moves to the right, greater proportion of products
(vice versa for products)

Effect of changing the overall pressure on equilibrium?

Increase the pressure -> equilibrium shifts to oppose change and reduce the pressure -> position of equilibrium moves to side with fewer gas molecules
(vice versa for decrease in pressure)

Changing the pressure of a reaction affects...

Reactions involving gases ONLY.

Effect of changing temperature on equilibrium

Increase temperature -> equilibrium shifts to oppose change and reduce the temperature -> position of equilibrium favours and moves towards the endothermic reaction
(vice versa for decrease in temperature - favours exothermic reaction instead)

Do catalysts affect the position of equilibrium?

No - they affect the forward and reverse reactions equally

How do catalysts affect a reversible reaction?

Produce an alternative route for the reaction, lowering the activation energy - affects both forward and reverse reactions equally so has no effect on the position of equilibrium or the value of Kc

What does [ ] mean?

Concentration

Equilibrium constant symbol and meaning

Kc - the ratio of products' concentration to reactants' concentration for a reaction at equilibrium has a constant value, given that the temperature is constant

How to calculate Kc?

([conc product 1 ]x [conc product 2]) / ([conc reactant 1] x [conc reactant 2])

When does Kc have no units?

When all the units have cancelled out (units = 1)

Effect of changing temperature on Kc

Increase temperature: Kc increases if endothermic (more product) but decreases if exothermic (less product)
Decrease temperature: Kc increases if exothermic (more product) but decreases if endothermic (less product)

What does it mean if Kc is...
1. Greater than 1?
2. Less than 1?

1. Products predominate over reactants - equilibrium position is on the right.
2. Reactants predominate over products - equilibrium position is on the left.

What does it mean if Kc is....
1. Greater than 10^10?
2. Less than 10^-10?

1. Reaction is going to completion
2. Reaction is not taking place at all

Factors considered when using reversible reactions in industry?

1. Highest yield
2. Lower costs - costs needed to maintain high temperatures (energy) + for equipment to deal with high pressures
3. Highest rate of reaction
4. Increasing temperature may increase RoR but reduce yield
5. Use of a catalyst to lower activation energy (can use lower temperatures)
6. Concentration of reactants to increase concentration of products

Why might you not need the volume to calculate Kc?

- The volumes cancel out (because)
- Equal number of moles/molecules on each side of the equation