Changing the position of equilibrium - Higher
This video looks at reversible reactions and dynamic equilibrium
The equilibrium positionA measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. of a reversible reactionA chemical reaction which can go both ways. is a measure of the concentrationThe concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. of the reactantA substance that reacts together with another substance to form products during a chemical reaction. and productA substance formed in a chemical reaction. at equilibriumIn chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.. Using the Haber processThe industrial chemical process that makes ammonia by reacting nitrogen and hydrogen together. as an example:
N2(g) + 3H2(g) â 2NH3(g)
The equilibrium position is:
- to the left if the concentrations of N2 and H2 are greater than the concentration of NH3
- to the right if the concentration of NH3 is greater than the concentrations of N2 and H2
The equilibrium position can be changed by altering the reaction conditions, such as by:
- changing the pressure
- changing the concentration
- changing the temperature
Changing the pressure
In a reaction involving gases, if the pressureForce exerted over an area. The greater the pressure, the greater the force exerted over the same area. is increased, the equilibrium position moves in the direction of the fewest moleculeA collection of two or more atoms held together by chemical bonds. of gas.
There are fewer molecules on the right hand side of the equation for the Haber process:
\(\begin{array}{rcl} N_{2}(g) + 3H_{2}(g) & \rightleftharpoons & 2NH_{3}(g) \\ 1 + 3 = 4~molecules && 2~molecules \end{array}\)
If the pressure is increased, the equilibrium position moves to the right.
Question
Calcium carbonate decomposes when it is heated: CaCO3(s) â CaO(s) + CO2(g)
Predict the effect of increasing the pressure.
The equilibrium position will move to the left, in the direction of the fewest molecules of gas.
Changing the concentration
In a reaction involving solutionMixture formed by a solute and a solvent., if the concentration of a soluteThe dissolved substance in a solution. is increased, the equilibrium position moves in the direction away from this solute. For example, bismuth chloride reacts with water in a reversible reaction:
BiCl3(aq) + H2O(l) â BiOCl(s) + 2HCl(aq)
The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from HCl(aq) in the equation.
Question
Iron(III) ions react with thiocyanate ions, SCN-, in a reversible reaction:
Fe3+(aq) + SCN-(aq) â FeSCN2+(aq)
Predict the effect of adding more iron(III) ions.
The equilibrium position will move to the right, in the direction away from Fe3+ in the equation.
Changing the temperature
In a reversible reaction, if the reaction is exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. in one direction, it is endothermicReaction in which energy is taken in. in the other direction. If the temperatureHow warm or cold something is. is increased, the equilibrium position moves in the direction of the endothermic process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:
2SO2(g) + O2(g) â 2SO2(g) (forward reaction is exothermic)
If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves to the left.
Question
Hydrogen can be manufactured by reacting carbon with steam:
C(s) + H2O(g) â H2(g) + CO(g) (forward reaction is endothermic)
Predict the effect of increasing the temperature, at constant pressure.
The equilibrium position will move to the right, in the direction of the endothermic reaction.
Summary of condition changes to the equilibrium position
| Change | Equilibrium: |
| Pressure increased | Position moves towards the fewest molecules of gas |
| Concentration of a reactant increased | Position moves away from that reactant |
| Temperature increased | Position moves in the direction of the endothermic reaction |
| Change | Pressure increased |
|---|---|
| Equilibrium: | Position moves towards the fewest molecules of gas |
| Change | Concentration of a reactant increased |
|---|---|
| Equilibrium: | Position moves away from that reactant |
| Change | Temperature increased |
|---|---|
| Equilibrium: | Position moves in the direction of the endothermic reaction |