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Effect of Concentration on the position of equilibrium

Answer Text:
Effect of Concentration on the position of equilibrium
- Consider the equilibrium in bromine water system:
`Br_2(aq) +H_2O(l) overset(leftarrow)(rightarrow) OBr^(-)+Br^(-)+2H^(+)`
yelloworange colourless
-Addition of sodium hydroxide to the equilibrium:
- When NaOH(aq) is added to the equilibrium, the concentration of `H^(+)` decrease and the rate of forward reaction are favoured (equilibrium shifts to the right); the reaction of bromine with water increases and there is loss of colour of bromine water.
Explanation:
- Addition of sodium hydroxide provides hydroxyl ions into the equilibrium.
- The hydroxyl ions react with the hydrogen ions (on the right of equilibrium) to form water.
Equation:
`OH^(-)(aq)+H^(+)(aq) overset(leftarrow)(rightarrow)H_2O(l)`
- This process removes hydrogen ions from the equilibrium mixture.
- This shifts the equilibrium to the right hence formation of more products;
- This leads to a change in colour from yellow orange to colourless;
-Addition of hydrochloric acid:
- Addition of HCl(aq) is added concentration of H+ increases; more bromine is formed and the orangeyellow colour of bromine water becomes more intense;
Explanation:
- Addition of hydrochloric acid introduces more hydrogen ions into the equilibrium;
- The hydrogen ions react with the colourless bromide and hypobromite ions to form yellow-orange aqueous bromine;
- This shifts the equilibrium to the left hence the increase in the intensity of the yellow-orange colour of bromine water;
Further examples:
(i). Given the equilibrium:
`SO_2(g)+O_2(g) to 2SO_3(g)`
Removal of sulphur (VI) oxide:
- Reducing the concentration of `SO_3` by removing it causes more sulphur (IV) oxide to be converted to sulphur (IV) oxide.
-Addition of either oxygen or sulphur (IV) oxide.
- Addition of either sulphur (IV) oxide or oxygen to the equilibrium shifts the equilibrium to the right; \due to increase of concentration of products
hence more yield of sulphur (VI oxide;
-Addition of pyrogallic acid:
- Addition of pyrogallic acid into the equilibrium shifts the equilibrium to the left; since pyrogallic acid dissolves oxygen gas reducing concentration of
reactants on the left hence less yield of sulphur (VI) oxide;
(ii). Given the equilibrium:
`2CrO_4^(2-)(aq) +2H^(+)(aq) to 2Cr_2O_7^(2-)(aq) H_2O(l)`
-Addition of sodium hydroxide to the equilibrium:
- When NaOH(aq) is added to the equilibrium, the concentration of `H^(+)` decrease and the rate of backward reaction are favoured (equilibrium shifts)
to the left); the reaction of dichromate ions with water to form chromate ions and hydrogen ions increases and there is change in colour to yellow.
Explanation:
- Addition of sodium hydroxide provides hydroxyl ions into the equilibrium.
- The hydroxyl ions react with the hydrogen ions (on the left of equilibrium) to form water.
Equation:
`OH^(-)(aq)+H^(+)(aq) overset(leftarrow)(rightarrow)H_2O(l)`
- This process removes hydrogen ions from the equilibrium mixture.
- This shifts the equilibrium to the left hence formation of more reactants (chromate and hydrogen ions);
- This leads to a change in colour from orange to yellow;
-Addition of hydrochloric acid:
- Addition of HCl(aq) is added concentration of `H^(+)` increases; more dichromate solution is formed and the orange colour of dichromate ions become
more intense;
Explanation:
- Addition of hydrochloric acid introduces more hydrogen ions into the equilibrium;
- The hydrogen ions react with the yellow chromate solution to form orange dichromate solution (and water);
- This shifts the equilibrium to the right hence the increase in the intensity of the orange colour of dichromate solution;