Chemical Equilibruim – SS2 Chemistry Lesson Note (Second Term)

Equilibrium is the state of a system in which there is no observable or detectable change in its properties with time.

• A balanced see-saw: both sides are level and do not move.
• A saturated solution of NaCl: no more salt dissolves and the concentration stays constant.

Behavioural objectives

By the end of the lesson, students should be able to:

1. distinguish between static and dynamic equilibrium;
2. state the properties of a system in equilibrium;
3. state Le Chatelier’s principle and use it to predict the effect of temperature, concentration, pressure and a catalyst on the position of equilibrium;
4. explain the conditions used in the Haber process;
5. state the law of mass action and write expressions for Kc and Kp.

What are static and dynamic equilibrium?

There are two types of equilibrium: static equilibrium and dynamic equilibrium.

• Static equilibrium: the system is at rest and nothing is moving, as in a balanced see-saw.
• Dynamic equilibrium: the forward and backward processes continue at the same rate, so there is no net change.

What are the types of dynamic equilibrium?

Dynamic equilibrium occurs in a physical change or in a reversible chemical change.

• Physical equilibrium: dynamic equilibrium involving a physical change, such as a saturated solution of salt.
• Chemical equilibrium: dynamic equilibrium involving a reversible chemical change, such as N₂O₄(g) ⇌ 2NO₂(g).

What is equilibrium in a reversible reaction?

A reversible reaction is a reaction that proceeds in both the forward and backward directions under suitable conditions.

• Example: NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
• At equilibrium: the forward and backward reactions occur at the same rate.
• Result: there is no net change in the concentrations of reactants or products.

What are the properties of a system in equilibrium?

A system in equilibrium has three main properties.

1. Dynamic state: the rates of the forward and backward reactions are equal.
2. Constant properties: concentrations, colour and pressure do not change with time.
3. Two-way approach: the equilibrium position can be reached from the reactants alone or from the products alone.
4. Closed system: equilibrium can only be established if nothing enters or leaves the system.

What is Le Chatelier’s principle?

Le Chatelier’s principle states that if a system in equilibrium is disturbed by changing one of the factors involved, the equilibrium position shifts to oppose (neutralise) the change.

• Position of equilibrium: the relative amounts of reactants and products present at equilibrium.
• Shift to the right: the forward reaction is favoured and more product forms.
• Shift to the left: the backward reaction is favoured and more reactant forms.

Why is Le Chatelier’s principle important in industry?

• Optimum conditions: it helps to define the best conditions for a chemical process.
• Reversibility: it helps to reduce undesirable reversibility.
• Prediction: it helps to predict the effect of altered factors on the equilibrium.

What factors affect the position of equilibrium?

The position of equilibrium is affected by temperature, concentration and pressure (for gases), while a catalyst has no effect on it.

What is the effect of temperature?

Raising the temperature favours the endothermic reaction, and lowering it favours the exothermic reaction.

What is the effect of concentration?

The equilibrium shifts to use up any substance whose concentration is increased.

What is the effect of pressure?

Pressure affects only equilibria involving gases, and the shift depends on the number of moles of gas on each side.

• Link to moles: at constant temperature and volume, gas pressure is proportional to the number of moles of gas.
• High pressure: the equilibrium shifts to the side with fewer moles of gas.
• Low pressure: the equilibrium shifts to the side with more moles of gas.
• Equal moles on both sides: a change in pressure does not shift the equilibrium.

Worked examples:

• N₂(g) + 3H₂(g) ⇌ 2NH₃(g): 4 moles on the left and 2 moles on the right, so high pressure favours the forward reaction.
• N₂O₄(g) ⇌ 2NO₂(g): 1 mole on the left and 2 moles on the right, so high pressure favours the backward reaction.

What is the effect of a catalyst?

A catalyst does not change the position of equilibrium.

• Rates: it increases the rates of both the forward and backward reactions equally.
• Mechanism: it lowers the activation energy of the reaction.
• Result: equilibrium is reached faster, but the amounts at equilibrium are the same.

What is the Haber process?

The Haber process is the industrial process for the manufacture of ammonia from nitrogen and hydrogen.

• Use of ammonia: it is an important raw material for making fertilisers.
• Equilibrium established: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = -92 kJ mol^-1.
• Moles of gas: 4 moles on the left and 2 moles on the right.
• Nature of forward reaction: exothermic, with a decrease in the number of moles.

What conditions are used in the Haber process?

Why is this temperature chosen?

• Le Chatelier’s principle: the forward reaction is exothermic, so the greatest yield is at low temperature.
• Problem: at a very low temperature, the time taken to reach equilibrium is very long.
• Compromise: about 500 °C gives a reasonable yield in a reasonable time.

Why is this pressure chosen?

• Le Chatelier’s principle: high pressure favours the side with fewer moles, which is ammonia.
• Limit: very high pressure needs strong, costly equipment, so a high but practical pressure is used.

Why is iron used as the catalyst?

• Effect: it speeds up how fast equilibrium is reached.
• Form: it is used in small pieces or finely divided form, which increases the surface area for contact.
• No change in yield: it does not alter the position of equilibrium.

What are the law of mass action and the equilibrium constant (K)?

The law of mass action states that, at constant temperature, the rate of a reaction is proportional to the active masses of the reactants.

• Active mass: the effective concentration of a substance, in mol dm^-3.
• Relation: active mass = molar concentration × activity coefficient.
• In most reactions: active masses may be taken as molar concentrations.
• Square brackets [ ]: stand for concentration in mol dm^-3.

For aA + bB → products:

1. r ∝ [A]^a and r ∝ [B]^b
2. r ∝ [A]^a × [B]^b
3. r = k[A]^a[B]^b, where k is the rate constant.

What is the equilibrium constant, Kc?

Kc is the equilibrium constant written in terms of the concentrations of the substances at equilibrium.

For aA(aq) + bB(aq) ⇌ cC(aq) + dD(aq):

Kc = ([C]^c [D]^d) / ([A]^a [B]^b)

• Products: go on the top of the expression.
• Reactants: go on the bottom of the expression.
• Powers: each concentration is raised to its coefficient in the balanced equation.
• Example: for N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Kc = [NH₃]² / ([N₂][H₂]³).

What is the equilibrium constant, Kp?

Kp is the equilibrium constant for gaseous reactions, written in terms of partial pressures.

For aA(g) + bB(g) ⇌ cC(g) + dD(g):

Kp = (P_C^c × P_D^d) / (P_A^a × P_B^b)

• P_A, P_B, P_C, P_D: the partial pressures of gases A, B, C and D.
• Example: for N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Kp = P_NH3² / (P_N2 × P_H2³).

What does the value of K tell us?

• Solids: their concentrations are ignored in the expression for K.
• Temperature: the value of K changes only when the temperature changes.

Evaluation

1. Define equilibrium and state the difference between static and dynamic equilibrium.
2. State Le Chatelier’s principle.
3. For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), state the effect of increasing the pressure on the position of equilibrium.
4. What is the effect of a catalyst on the position of equilibrium?
5. Write the expression for Kc for the reaction aA(aq) + bB(aq) ⇌ cC(aq) + dD(aq).

Assignment

Answer all three parts in your exercise book.

1. State three properties of a system in equilibrium.
2. Explain why a temperature of about 500 °C is used in the Haber process instead of a very low temperature.
3. State the law of mass action and write the expression for Kp for the reaction aA(g) + bB(g) ⇌ cC(g) + dD(g).