Equilibrium (OCR Gateway GCSE Chemistry: Combined Science)

• Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when at least one reactant has been exhausted
• In reversible reactions, the product molecules can themselves react with each other or decompose and reform the reactant molecules
• It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse, or backward, direction (which forms the reactants)
• When writing chemical equations for reversible reactions, two opposing arrows are used to indicate the forward and reverse reactions occurring at the same time
• Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left
• The direction a reversible reaction takes can be changed by changing the reaction conditions
• The energy changes are also opposite; if the forward reaction is exothermic then the backward reaction is endothermic
• For example heating ammonium chloride produces ammonia and hydrogen chloride gases:

NH₄Cl (s) → NH₃ (g) + HCl (g) 

• As the hot gases cool down they recombine to form solid ammonium chloride

NH₃ (g) + HCl (g) → NH₄Cl (s)  

• So, the reversible reaction is represented like this:

NH₄Cl (s) ⇌ NH₃ (g) + HCl (g)

• We have already seen that a reversible reaction is one that occurs in both directions
• When during the course of reaction, the rate of the forward reaction equals the rate of the backward reaction, then the overall reaction is said to be in a state of equilibrium
• This is called a dynamic equilibrium as there are still forward and backward reactions occurring, but the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
• The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
• Dynamic equilibrium can only occur in a closed system so that none of the participating chemical species are able to leave the reaction vessel and nothing else can enter

Equilibrium can only be reached in a closed container

• An example of a dynamic equilibrium is the reaction between H₂ and N₂ in the Haber process
• When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
• As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease
• However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen)
• Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

Diagram showing when the rates of forward and backward reactions become equal