Reversible reactions and equilibria

Welcome to Reversible Reactions and Equilibria!

Hello future chemist! This chapter is one of the most exciting parts of Physical Chemistry because we stop thinking about reactions that just finish and start thinking about reactions that find a dynamic, constant balance.

Understanding reversible reactions and chemical equilibrium is vital. It is how chemical engineers maximize the production of important materials (like ammonia in the Haber process) and how chemical systems reach a steady state.

Don't worry if the term "equilibrium" sounds complicated—we will break it down using simple analogies that make perfect sense!

1. Understanding Reversible Reactions

So far, most of the reactions you have studied are irreversible. This means they go one way: reactants turn into products, and the reaction stops when a limiting reactant is completely used up.

Example of an irreversible reaction: Burning magnesium ribbon in air to form magnesium oxide. You cannot reform magnesium ribbon simply by cooling it down!

What is a Reversible Reaction?

A reversible reaction is a reaction in which the products can react together to reform the original reactants. The reaction can proceed in both the forward direction (left to right) and the reverse direction (right to left).

Key Symbol:

We use a reversible-reaction arrow ( \(\rightleftharpoons\) ) to represent a reversible process:

\(\text{Reactants} \rightleftharpoons \text{Products}\)

Classic IGCSE Examples

1. Dehydration of Hydrated Copper(II) Sulfate

When blue hydrated copper(II) sulfate is heated, it loses water of crystallisation (dehydration) to form white anhydrous copper(II) sulfate and water (the forward endothermic reaction).

When water is added back to the white anhydrous copper(II) sulfate, heat is released and the blue colour returns (the reverse exothermic reaction).

\(\text{CuSO}_4\cdot5\text{H}_2\text{O(s)} \rightleftharpoons \text{CuSO}_4\text{(s)} + 5\text{H}_2\text{O(l)}\)

2. Effect of Heat on Ammonium Chloride

When solid white ammonium chloride is heated, it undergoes thermal decomposition into colourless ammonia gas and hydrogen chloride gas (forward reaction):

\(\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}\)

As the gaseous products drift up towards the cooler part of the test tube, they recombine to form a white solid crust of ammonium chloride on the tube walls (reverse reaction).

2. Dynamic Equilibrium: The Perfect Balance (Paper 2)

If a reversible reaction occurs in a sealed container or closed system (where no reactants or products can escape), it will eventually reach a state of dynamic equilibrium.

What does "Dynamic" mean?

The word dynamic means active and continuous. At dynamic equilibrium, the overall macroscopic properties appear unchanging, but molecular reactions are still taking place in both directions at equal speed!

Analogy: The Crowded Dance Floor

Imagine a venue where the main hall is the "Reactants" side and the balcony is the "Products" side:

• If 10 people move from the main hall to the balcony every minute (forward rate)...
• ...and 10 people move from the balcony back to the main hall every minute (reverse rate)...

The number of people in the main hall and on the balcony stays completely constant over time, even though individuals never stop moving back and forth!

Characteristics of Dynamic Equilibrium

A system in dynamic equilibrium exhibits key characteristics:

1. Equal Rates: The rate of the forward reaction equals the rate of the reverse reaction.
2. Constant Concentrations: The concentrations of reactants and products remain constant (though not necessarily equal to each other).
3. Sealed Container: Dynamic equilibrium can only be reached and maintained in a closed system so that no matter is lost.

3. Changing Conditions and Equilibrium Position (Paper 2)

When conditions such as temperature or pressure change in a system at dynamic equilibrium, the position of equilibrium shifts to oppose that change.

A. Effect of Changing Temperature

To predict the effect of temperature, you must know whether the forward reaction is exothermic (releases heat, \(\Delta H\) is negative) or endothermic (takes in heat, \(\Delta H\) is positive).

If the forward reaction is exothermic, the reverse reaction must be endothermic by the same energy value, and vice versa.

• Increasing the temperature: Shifts the equilibrium position towards the endothermic reaction (in the direction that absorbs heat to cool the system).
• Decreasing the temperature: Shifts the equilibrium position towards the exothermic reaction (in the direction that releases heat to warm the system).

B. Effect of Changing Pressure (Gaseous Systems)

Changes in pressure only affect equilibria involving gases with unequal numbers of gas moles on either side of the balanced equation.

Pressure is caused by gas particles colliding with the walls of the container. More moles of gas produce higher pressure.

\(\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}\)

Left side = \(1 + 3 = 4\text{ moles of gas}\).
Right side = \(2\text{ moles of gas}\).

• Increasing the pressure: Shifts the equilibrium position towards the side with fewer moles of gas (reducing overall pressure).
• Decreasing the pressure: Shifts the equilibrium position towards the side with more moles of gas (increasing overall pressure).
• Equal moles of gas: If both sides have the same number of moles of gas, changing pressure has no effect on the position of equilibrium.

4. The Role of Catalysts (Paper 2)

A catalyst increases the rate of a chemical reaction without being chemically changed at the end of the reaction. It provides an alternative reaction pathway with a lower activation energy.

Why Catalysts Do Not Shift Equilibrium

A catalyst speeds up the forward reaction and the reverse reaction by the exact same amount.

Therefore:

• A catalyst does not shift the position of equilibrium.
• A catalyst does not change the yield of products.
• A catalyst allows the system to reach dynamic equilibrium much faster.