Objectives:

Reminder of AS work on equilibrium

The equilibrium law

The equilibrium constant.

Can you remember the definition of dynamic equilibrium

Dynamic equilibrium is the equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction

What factors affect the position of the equilibrium?

Concentrations of reactant or products.Pressure in reactions involving gases.Temperature

Definition of le Chatelier’s principal?

When a system in dynamic equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the change.

Consider this reaction:

CH3CH2OH(l) + CH3CH2COOH(l) CH3CH2COOCH2CH3(l) + H2O(l)

ethanol propanoic acid ethyl propanoate

What will happen to the position of the equilibrium if we:

increase the concentration of ethanol?

Increase the concentration of propanoic acid?

Increase the concentration of the ester?

Increase the concentration of water?

Consider this reaction:

N2(g) + 3H2(g) 2NH3(g)

What will happen if we increase the pressure of the whole system?

What will happen is we decrease the pressure of the whole system?

What will happen if we increase the pressure of nitrogen?

What will happen if we increase the pressure of ammonia?

Consider this reaction again:

N2(g) + 3H2(g) 2NH3(g) H = -92 kJ mol-1

What will happen if we:

Increase the temperature of the system?

Decrease the temperature of the system?

Add a catalyst?

We need to think about the progress of a reaction.

N2O4(g) can decompose into 2 molecules of NO2(g)

Two molecules of NO2(g) can combine to form a molecule of N2O4(g)

If we place some N2O4(g) into a sealed container what will happen?

Use collision theory to describe what will happen to the particles inside the container – it may help you to draw some diagrams.

Beginning:

Decomposotion startsLots of particles of N2O4reaction rate is fastVery few particles of NO2 reaction rate is slow as not there will be few collisions between NO2 molecules.

A bit later:

Less particles of N2O4 reaction rate starts to slow downMore particles of NO2 reaction rate starts to increase

..and even later…

Rate of forward reaction equals rate of reverse reaction.The number of particles of N2O4 and NO2 remain constant.

i.e. dynamic equilibrium has been reached.

We can use the equilibrium law to determine the concentrations at equilibrium.

We can find out whether a particular reaction favours reactants or products.

We can quantitatively find out what affect changing concentration will have.

The equilibrium law tells us the relative proportions of reactants and products present at equilibrium.

Consider this general reaction:

aA + bB cC dD

[A], [B], [C], [D] are the equilibrium concentrations of the reactants and products.

a, b, c and d are the balancing numbers.

Kc = [C]c[D]d

[A]a[B]b

Kc is the equilibrium constant in terms of concentrations.

REMEMBER Kc = PRODUCTS / REACTANTS

So what do you need to be able to do?

(a) calculate, given appropriate data, the concentration or quantities present at equilibrium;

(b) deduce, for homogeneous reactions, expressions for the equilibrium constant Kc;

(c) calculate the values of the equilibrium constant Kc including determination of units;

(d) explain the effect of changing temperature on the value of Kc for exothermic and endothermic reactions;

(e) state that the value of Kc is unaffected by changes in concentration or pressure or by the presence of a catalyst.

Write an expression for Kc for the following reactions:

N2O4(g) 2NO2(g)

2SO2(g) + O2(g) 2SO3(g)

H2(g) + I2(g) 2HI(g)

Units of Kc have to be worked out each time:

Remember that concentration has the units mol dm-3

Work out the units for Kc for each of the expressions you have written.

More practice:

Questions from p127 of the OCR A2 book.

For each of the following equilibria, write down the expression for Kc and state the units.

2NO(g) + O2(g) 2NO2(g)

H2(g) + Br2(g) 2HBr(g)

N2(g) + 3H2(g) 2NH3(g)