[reversible reactions] equilibria

[ BBC Bitesize 'The rate and extent of chemical change ⇢ Reversible Reactions' page 1-2 ]

Reversible Reactions

Reactions where the products of the reaction can react to produce the original reactants.

Example of Reversible Reaction: Ammonium Chloride

• ammonium chloride ⇌ ammonia + hydrogen chloride

• NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

• forward reaction: thermal decomposition (endothermic)

• backward reaction: neutralisation (exothermic)

Example of Reversible Reaction: Copper Sulfate (II)

• hydrated copper sulfate ⇌ anhydrous copper sulfate + water

• CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

• forward reaction: crystals go from blue to white and give off steam (endothermic)

• backward reaction: exothermic

Anhydrous

A substance, especially a crystalline compound, containing no water. Opposite of hydrated.

Example of Reversible Reaction: Cobalt (II) Chloride Paper

• used to test for water

• hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

• CoCl₂.6H₂O (s) ⇌ CoCl₂ (s) + 6H₂O (l)

• if paper is blue, no water is present; if paper is pink, water is present

• forward reaction: endothermic

• backward reaction: exothermic

Dynamic Equilibrium

Occurs when a reversible reaction occurs in a closed system.

• the rate of the forward reaction equals the rate of the backward reaction

• concentration of the reactant and the products remain constant

Closed Systems

Group of apparatus which prevents the escape of reactants and products.

Equilibrium Position

• if the concentration of the reactants is greatest, the equilibrium lies to the left of the reaction

• if the concentration of the products is greatest, the equilibrium lies to the right of the reaction

La Chatelier’s Principle

When a system at equilibrium is subject to change, the position of equilibrium will shift to minimise the change.