IB Chemistry Topic 7/17

What is equilibrium

When the rate of the forward reaction is equal to the rate of the backward reaction

What is dynamic equilibrium

When the reaction has not stopped but both the forwards and backwards reactions are still occurring at the same time.

What is Kc

The equilibrium constant

What is the equilibrium constant expression

for the reaction

aA + bB ⇌ cC +dD

The equilibrium expression is

C^c D^d

---

A^a B^b

What does a high Kc mean

The equilibrium lies to the right

Proportionally more products than reactants

The reaction almost goes to completion

What does a low Kc mean

The equilibrium lies to the left

Proportionally more reactants than products

The reaction has barely taken place

What is the reaction quotient (Q)

When the concentrations of reactants and products are taken at any given time and substituted into the equilibrium constant expression. Kc is used when the concentrations of reactants and products are at equilibrium.

What does Q show us in relation to Kc

Q = Kc the reaction is at equilibrium so there is no net reaction

Q < Kc reaction proceeds to the right in favour of products

Q > Kc reaction proceeds to the left in favour of products

How does changes in concentration affect the equilibrium

If an increase in concentration of one of the reactants occurs, the rate of the forwards reaction increases. According to le chateliers principle the equilibrium will re-establish itself where it is in favour of the products. This opposes the change.

How do changes in pressure affect the equilibrium

Only has an affect if the reactants and products are in a gaseous state.

If a reaction is subjected to increased pressure the equilibrium will shift to oppose the change by moving in favour of the side with the least molecules.

How do changes in temperature affect the equilibrium

If the forward reaction is exothermic, a decrease in temperature will favour the exothermic reaction as it wants to oppose the loss of heat

Increasing the temperature will favour the backwards endothermic reaction.

How does addition of a catalyst affect the equilibrium

It speeds up both the forward and the backwards reaction equally so it has no affect on the position of equilibrium.

What is homogenous equilibria

Where the reactants and products are all in the same phase.

How does Gibbs free energy show the state of equilibrium

ΔG = negative then the reaction proceeds in the forwards direction

ΔG = positive then the reaction proceeds in the backwards direction

ΔG = 0 the reaction is at equilibrium

How are Gibbs free energy and the equilibrium constant related

ΔG° = -RTln(K),

ΔG° is standard Gibbs free energy change,

R is the gas constant

T is temperature,

ln is the natural logarithm,

K is the equilibrium constant."