Equilibrium and Kc

Reversible Reactions Many chemical reactions continue until one of the reactants is completely used up. Such reactions are said to ‘go to completion’. Combustion reactions are a good example of this. e.g. The combustion of methane. CH4 + 2O2  CO2 + 2H2O

Many other reactions do not go to completion and are reversible. reactants ⇌ products When the reactants and products have different colours, the reversibility of a reaction can easily be demonstrated. For example adding acid or alkali to the reversible reaction involving the chromate(VI) and dichromate(VI) ions. 2CrO42- + 2H+ ⇌ Cr2O72- + H2O (yellow) (orange)

By convention, the reaction shown going from left to right is called the forward reaction and the reaction occurring in the opposite direction is called the backward or reverse reaction.

Chemical Equilibrium The forward and reverse reactions continuously occur at the same time, so this is known as a dynamic situation. When the forward and reverse reactions occurring at the same rate, the concentrations of reactants and products no longer change, and a chemical equilibrium has been established. A chemical equilibrium can only be established in a closed system, which means that no reagents can be added to or removed from the mixture.

Chemical equilibrium is a dynamic situation which is characterised by: • The forward and reverse reactions occurring at the same rate. • The concentrations of all reactants and products remaining constant. On a graph of concentration against time, equilibrium is reached at the time when the concentration of products and reactants levels off (the gradient becomes zero).

What Happens when a System at Equilibrium is Disturbed?   Le Chatelier’s principle states that: “A system at equilibrium will respond to oppose any change imposed upon it.”

Change in Concentration   If the concentration of one species in a reaction at equilibrium is increased, then the position of equilibrium will shift to reduce the increased concentration.

Change in Concentration   For example in the dichromate(VI)/chromate(VI) reaction below: 2CrO42- + 2H+ ⇌ Cr2O72- + H2O If acid is added to disturb the equilibrium, then the reaction will shift to the right. This produces more dichromate(VI) and the colour of the mixture will become more orange.

Change in Concentration  For example in the dichromate(VI)/chromate(VI) reaction below: 2CrO42- + 2H+ ⇌ Cr2O72- + H2O Conversely, if alkali is added, this will disturb the equilibrium by reducing the concentration of H+ ions. The system will respond by shifting to the left and increase the concentration of both H+ and CrO42- ions and the colour of the mixture will become more yellow.

Change in Total Pressure   Changes in total pressure will only affect chemical equilibria that involve gases. The changes observed are a result of changes in concentration. If the total pressure is increased, the equilibrium position will oppose the increased pressure by shifting to the side with the fewest moles of gas.

Change in Total Pressure   Conversely, if the total pressure is decreased the equilibrium opposes the decreased pressure by shifting to the side with the most moles of gas.

Change in Total Pressure   This is often demonstrated with NO2/N2O4 equilibrium mixtures. 2NO2 (g) ⇌ N2O4(g) (brown) (colourless) This reaction has 2 moles of gas on the left hand side and 1 mole of gas on the right hand side. If the mixture is contained in a gas syringe, the pressure can be changed by moving the plunger. When the plunger is pushed in to increase the pressure, the mixture goes darker initially, but after a few seconds the colour becomes paler. Initially the colour goes darker as the brown NO2 molecules are contained in a smaller volume, but the equilibrium has been disturbed, so the system reacts to oppose the increase in pressure, by shifting to the side with the fewest moles of gas. This is the right hand side, so equilibrium is re-established with a lower concentration of brown NO2, resulting in a paler mixture.

Change in Total Pressure   Conversely, if the total pressure is decreased the equilibrium opposes the decreased pressure by shifting to the side with the most moles of gas. This is often demonstrated with NO2/N2O4 equilibrium mixtures. 2NO2 (g) ⇌ N2O4(g) (brown) (colourless) This reaction has 2 moles of gas on the left hand side and 1 mole of gas on the right hand side.

Change in Total Pressure   2NO2 (g) ⇌ N2O4(g) (brown) (colourless) If the mixture is contained in a gas syringe, the pressure can be changed by moving the plunger. When the plunger is pushed in to increase the pressure, the mixture goes darker initially, but after a few seconds the colour becomes paler.

Change in Total Pressure  2NO2 (g) ⇌ N2O4(g) (brown) (colourless) Initially the colour goes darker as the brown NO2 molecules are contained in a smaller volume, but the equilibrium has been disturbed, so the system reacts to oppose the increase in pressure, by shifting to the side with the fewest moles of gas. This is the right hand side, so equilibrium is re-established with a lower concentration of brown NO2, resulting in a paler mixture.