Reversible reactions and equilibria

In a nutshell

Reversible reactions include a forward and backward reaction. The Haber process is an example of a reversible reaction. Reversible reactions establish a dynamic equilibrium.

Irreversible reactions

In all equations, reactants are on the left-hand side and products are on the right-hand side. An irreversible reaction is shown by using a forward arrow: $\rightarrow$ and this arrow shows that the product(s) cannot be converted back into the reactant(s). So, the reaction occurs in one direction only to form the product.

Example

$A + B \rightarrow C + D$

In this reaction, $A$ and $B$ (reactants) are converted into $C$ and $D$ (products). The use of this arrow: $\rightarrow$ shows that $C$ and $D$ cannot be converted back to $A$ and $B$ .

Reversible reactions

A reversible reaction is shown by using these arrows: $\rightleftharpoons$ This shows that the product(s) can be converted back into the reactant(s). The reaction occurs in the forward direction to form the product, indicated by this arrow: $\rightharpoonup$ The reaction occurs in the backward direction to form the reactant, indicated by this arrow: $\leftharpoondown$ .

Example

$E + F \rightleftharpoons G + H$

In this reaction, the forward reaction produces $G$ and $H$ and the backward reaction produces $E$ and $F$ .

Dynamic equilibrium

In a reversible reaction, a dynamic equilibrium is established when the rate of the forward reaction is equal to the rate of the backwards reaction. Factors which affect the position of equilibrium are:

Concentration
Temperature
Pressure

Pressure affects reactions in the gaseous state only. If there are no gases or the moles of gas on each side is equal, change in pressure has no effect on the equilibrium.

Catalysts are substances that speed up the rate of a reaction. Catalysts lower the activation energy (minimum energy required for particles to react) of a reaction by providing an alternative mechanism or pathway. Catalysts do not affect the position of equilibrium, they speed up the rate of the forward and backward reaction by an equal amount.

Position of equilibrium

When the equilibrium shifts to the right, more products are formed. Whereas, when the equilibrium shifts to the left, more reactants are formed.

Example

$ammonium ~chloride \rightleftharpoons ammonia + hydrogen~chloride \newline \ \\NH_4Cl \rightleftharpoons NH_3 +HCl$

When the concentration of ammonium chloride increases, the equilibrium shifts to the right to produce more ammonia and hydrogen chloride.

Exothermic and endothermic reactions

If the forward reaction is exothermic, the backward reaction is endothermic. Whereas, if the forward reaction is endothermic, the backward reaction is exothermic.

Example

$hydrated~copper~sulfate \rightleftharpoons anhydrous ~copper ~sulfate + water\newline \ \\CuSO_4\bullet5H_2O \rightleftharpoons CuSO_4 + 5H_2O$

Hydrated copper sulfate crystals are blue and anhydrous copper sulfate powder is white. The forward reaction is endothermic and the backward reaction is exothermic.

When the equilibrium shifts to the right, in the endothermic direction, white anhydrous copper sulfate powder and water are formed.

When the equilibrium shifts to the left, in the exothermic direction, blue hydrated copper sulfate crystals are formed.

The Haber process

Ammonia ( $NH_3$ ) is produced by the Haber process. Ammonia as a variety of uses, particularly the production of fertilisers.

$nitrogen + hydrogen \rightleftharpoons ammonia\\ \ \\ N_2 (g) + 3H_2 (g) ⇌ 2NH_3 (g)$

Chemistry; Extracting metals and equilibria; KS4 Year 10; Reversible reactions and equilibria

1.	Hydrogen and nitrogen, mixed in a $3:1$ ratio
2.	Reaction vessel
3.	Trays of iron catalyst
4.	$400\degree C$ and $250\ atm$
5.	Liquid ammonia
6.	Condenser
7.	Unused nitrogen and hydrogen is recycled

PROCEDURE

1.	Hydrogen from natural gas (methane and other gaseous hydrocarbons) is pumped into a compressor, along with nitrogen from the air.
2.	The gases are subjected to a pressure of $200$ atmospheres, temperature of $450\degree C$ and an iron catalyst to produce ammonia gas.
3.	These reaction conditions balance the cost of production with speed of production and yield of ammonia.
4.	The ammonia gas is condensed into a liquid by cooling and is separated from the rest of the mixture.
5.	Unreacted hydrogen and nitrogen are recycled.