WJEC AS Chemistry Unit 1.7 - Simple Equilibria and Acid-Base Reactions

Reversible reaction

• a reaction which can go in either direction depending on the conditions

Dynamic Equilibrium

• the forward and reverse reactions are both taking place at the same rate

Equilibrium

• rate of forward reaction=rate of backward reaction

• changes occur on a molecular level only

• concentration of all reactants and products remains constant, therefore no observable overall change

• Conditions; to reach equilibrium, the reaction must be a closed system (all chemicals kept in), therefore rates of reaction equal

Position of Equilibrium

• can be changed by changing the reaction conditions

• Lies to the left = more reactants than products, therefore higher reactant concentration

• Lies to the right = more products than reactants, therefore high product concentration

Le Chatelier’s Principle

If a system at equilibrium is subjected to a change then the position of equilibrium to shift to minimise/oppose that change

Effect of concentration change on equilibrium

• Increase concentration of a reactant;

  • position of equilibrium will move so that the conc of this reactant decreases by reacting with other reactants and turning into products

  • position of equilibrium moves to the right

• Decreasing the concentration of a reactant;

  • position of equilibrium will move so that the conc of this reactant increases again

  • More products will react to replace the reactant that’s been removed

  • The position of equilibrium moves to the left

Effect of temperature on position of equilibrium

• Temperature raised; equilibrium moved to cool down reaction by moving in the endothermic direction (backward reaction = endothermic)

• temperature lowered; equilibrium moves to heat up by moving in the exothermic direction (forward direction = exothermic)

Effect of a catalyst on position of equilibrium

• makes no difference to position of equilibrium

• speeds up both forward + backward reaction