3.2.3 (a,b,c) Dynamic equilibrium and le Chatelier’s principle

Syllabus

(a) explanation that a dynamic equilibrium exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change

(b) le Chatelier’s principle and its application for homogeneous equilibria to deduce qualitatively the effect of a change in temperature, pressure or concentration on the position of equilibrium

{Definition for le Chatelier's principle not required.}

{Use of le Chatelier’s principle to explain effect of factors on the position of equilibrium.}

(c) explanation that a catalyst increases the rate of both forward and reverse reactions in an equilibrium by the same amount resulting in an unchanged position of equilibrium

What does this mean?

Dynamic Equilibrium

Almost everyone studies the Haber Process at some point.

At which point they learn the meaning of the reversible reaction arrow.

And are meant to understand what is meant by a Dynamic Equilibrium.

This does not always happen.

If you wished to make some Ammonia you could fill a container with a mixture of Nitrogen and Hydrogen in a 1:3 ratio.

Initially the forwards reaction would be quite quick as there would be lots of reactant molecules, leading to a high collision frequency.

But this collision frequency would drop as the reaction progressed because reactant molecules would be used up.

There couldn't be a backwards reaction at the start because there would be no Ammonia molecules to react.

But as the forward reaction produces more Ammonia the backward reaction could begin and would get faster as the concentration of Ammonia increased.

Eventually there would come a time when the speed of the forward reaction and backward reactions would be the same.

This is Dynamic Equilibrium.

Dynamic means in motion - telling you that both reactions carry on.

It is in Equilibrium because the concentrations of Reactant and Product no longer change.

It does not mean that there is 50% product and 50% Reactant - that is possible but unlikely.

le Chatalier's Principle

This is another definition no longer required.

However, to be able to use it you'll still need to know it.

When a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, the system readjusts to counteract the effect of the applied change and a new equilibrium is established.

Which in simpler terms means that if you increase the temperature the system will adjust to lower the temperature at least partially.

If you increase the pressure the system will adjust to lower the pressure at least partially.

If you increase the volume the system will adjust to lower the volume at least partially.

And if you increase the concentration of one of the substances in the system the adjustment will lower the concentration at least partially.

Examiners have never been keen on asking you why le Chatalier's Principle should work -so there's no need to worry about that convoluted argument.

Using le Chatalier's Principle

Pressure

In the Haber Process there are 4 moles of gas on the reactant side.

And only 2 moles of gas on the product side.

This means moving left increases pressure and moving right decreases pressure.

Since le Chatalier's Principle says that the system will oppose change if you increase the pressure the system will move right towards the products to decrease the pressure.

And if you decrease the pressure the system will move left towards the reactants to increase the pressure.

Enthalpy

In the Haber Process the enthalpy of the reaction is -92 kJ/mol.

This refers to the forward reaction.

The backward enthalpy of reaction would be +92 kJ/mol.

If you increase the temperature the system will try to lower it.

This will favour the endothermic (positive enthalpy) reaction.

So the equilibrium shifts towards the left, and the yield of the reaction decreases.

To increase the yield we would need to force the equilibrium to favour the exothermic direction - which increases the temperature.

So, we should decrease the temperature.

Volume

Changing the volume is much the same as changing pressure.

So a smaller volume has the same effect as raising the pressure.

And a higher volume has the effect of lowering the pressure.

Other conditions

The other change that is commonly mentioned is adding more reactant - which pushes the equilibrium to remove it and move to the right, raising the yield of product.

Or removing one of the products - which forces the equilibrium to replace it and again increases the yield.

Finding the sign of ΔH

A favourite examination question involves an unknown equilibrium and the effects of changing temperature.

For example: The yield of the following reaction is decreased when the temperature is increased from 30 to 80 ^oC. What can be deduced about ΔH_Reaction?

A+ B ⇌ C + D

If the yield decreases it means the equilibrium must have moved to the left.

If this happened when the temperature was increased then the equilibrium would have opposed this by favouring the endothermic direction.

ΔH_Reactionrefers to the forward reaction.

So if we know that moving left is endothermic we also know the forward reaction must be exothermic and the sign of the enthalpy change must be negative.

The effect of Catalysts on Equilibrium.

Catalysts lower the Activation Energy - the minimum energy needed for molecules to react.

But lowering the Activation Energy for the forward reaction also lowers the Activation Energy for the backward reaction.

So just as the forward reaction proceeds more quickly, so does the backward reaction.

And the effect cancels out.

So the position of the equilibrium remains unchanged.

Although adding a catalyst allows the reaction to get to equilibrium faster.

So, if the conditions for Haber Process produce 35% Ammonia and 65% reactants at equilibrium they will still only produce 35% Ammonia when the Iron catalyst is added but you simply won't have to wait so long.

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Exam-style Questions

1. Nitrogen, Hydrogen and Oxygen undergo the reactions shown below.

Reaction 1 1/2 N2(g) + 1/2 O2(g) ⇌ NO(g) DH_f = +91 kJ mol–1

Reaction 2 1/2 N2(g) + 1.5 H2(g) ⇌ NH3(g) DH_f = –46 kJ mol–1

Reaction 3 H2(g) + 1/2 O2(g) ⇌ H2O(g) DH_f = –242 kj mol–1

Use this information in answering the questions that follow.

(a) In which, if any, of the reactions above would the percentage yield of products at equilibrium increase if the temperature were to be raised? Explain your reasoning.

Yield increased with increased temperature. ...............................................................................................................

Explanation. ..........................................................................................................................................................

.............................................................................................................................................................................

.........................................................................................................................................................................(3)

(b) In which, if any, of the reactions above would the percentage yield of products at equilibrium not increase if the pressure were to be raised? Explain your reasoning.

Yield not increased with increased pressure. ...............................................................................................................

Explanation. ...........................................................................................................................................................

.........................................................................................................................................................................(2)

(c) (i)Which direction, if any, would any equilibrium reaction move if DH = 0 and the temperature was decreased?

.............................................................................................................................................................................

(ii) Predict for which of the reactions above the percentage yield of products at equilibrium would be most affected by a change in temperature. Explain your answer.

Reaction most affected by a temperature change. ......................................................................................................

Explanation. ..........................................................................................................................................................

.........................................................................................................................................................................(3)

(Total 8 marks)

2. In the Haber Process for the manufacture of Ammonia, Nitrogen and Hydrogen react as shown in the equation.

N_2(g) + 3H_2(g) ⇌ 2NH_3(g) ∆Hο = –92 kJ mol–1

The table shows the percentage yield of Ammonia, under different conditions of pressure and temperature, when the reaction has reached dynamic equilibrium.

(a) Explain the meaning of the term dynamic equilibrium.

.............................................................................................................................................................................

.........................................................................................................................................................................(2)

(b) Use Le Chatelier’s principle to explain why, at a given temperature, the percentage yield of ammonia increases with an increase in overall pressure.

.............................................................................................................................................................................

.........................................................................................................................................................................(3)

(c) Give a reason why a high pressure of 50 MPa is not normally used in the Haber Process.

.........................................................................................................................................................................(1)

(d) Many industrial ammonia plants operate at a compromise temperature of about 800 K.

(i) State and explain, by using Le Chatelier’s principle, one advantage, other than cost, of using a temperature lower than 800 K.

Advantage .............................................................................................................................................................

Explanation ...........................................................................................................................................................

.............................................................................................................................................................................

(ii) State the major advantage of using a temperature higher than 800 K.

.............................................................................................................................................................................

(iii) Hence explain why 800 K is referred to as a compromise temperature.

.........................................................................................................................................................................(5)

(Total 11 marks)

Answers

1. (a) Reaction 1 (1)

must be correct to score any marks in (c)

The reaction is endothermic (1)

system moves in endothermic direction absorbing heat (1)

or reducing temperature

(b) Reaction 1 (1)

must be correct to score any marks in (d)

Dn = 0 (1)

(c) (i) neither – unchanged or “none” (1)

(ii) Reaction 3 (1)

must be correct to score any marks in (e)(ii)

DH has the largest value (1)

or very exothermic

allow lowest (= most –ve) but not smallest [8]

2. (a) Rate forward reaction = rate backward reaction 1

Concentrations of reactants and products are constant 1

(b) System opposes change 1

Moves to the side with fewer moles 1

In this case NH3 (2moles) on right side < N2 + H2 together

(4 moles) on left side of equation 1

(c) Too expensive to generate etc 1

(d) (i) Yield of ammonia increases 1

Exothermic reaction favoured 1

System moves to raise temp / or oppose decrease in temp 1

(ii) Faster reaction 1

(iii) Balance between rate and yield 1 [11]

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