Chemical Equilibria

What is meant when chemical reactions go to completion

The reaction will continue until one of the reagents has been used up. Magnesium burning in oxygen is a good example

Mg(s) +1/2 O₂(g) → MgO(s). Once MgO has been formed in a reaction you cannot reform Mg and ½ O₂ in a chemical reaction from MgO only

What is a reversible reaction

A reversible reaction is a chemical process where reactants form products, which can then revert back to the original reactants. E.g CH₃CH₂OH + CH₃COOH = CH₃COOCH₂CH₃ + H₂O

What exactly is equilibrium

Equilibrium is the state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.

What is dynamic equilibrium?

Dynamic equilibrium is the condition in a reversible reaction where the rates of forward and reverse reactions are equal and are happening at the same time keeping concentrations of reactants and products constant.

Yellow chromate ions reversible reaction example

2CrO₄^2- + 2H⁺ = Cr₂O₇^2- + H₂O

/ ← Add alkali

Add acid →

Yellow chromate = CrO₄^2-

Orange dichromate = Cr₂O₇^2-

Effect of change in concentration on equilibrium

If the concentrations of reactants are increased, equilibrium would move to the right ( 1 mark). To oppose the change (increase in the concentration of reactants) so more of the reactants react to reduce its concentration (1 mark). So yield of product increases (1 mark) and vice versa

Effect of partial pressure on equilibrium

If the partial pressure of the reactants are increase (effectively the concentration of reactants) Equilibrium move left to right (1 mark) to oppose the change (increase in partial pressure of reactants) and reduce the partial pressure of the reactants (1 mark). So the yield of product increases (1 mark)

What is the effect in total pressure on equilibrium

(Example: 2 moles of GAS on reactant side and 3 moles of GAS on product side)

If we decrease the pressure equilibrium noves to the side with more moles of gas to oppose the change an increase preussre (1 mark) There are 2 moles the left and 3 moles of gas on the right (1 mark) so equilibrium moves from Left to right to oppose the decreased pressure and the yield of product

Effect of changing temperature on equilibrium

The Endothermic/exothermic direction is determined by the overall energy change. If + ve it is endothermic (forward direction) and if -ve it is exothermic (forward direction). Say that the overall energy change is +ve. Increase in temperature moves equilibrium the endothermic direction so equilibrium moves from right to left to make more reactants and decrease the temperature

What happens when the volume of a container is reduced during a chemical reaction

The equilibrium will shift to the side with more moles of gas (lower pressure)