Equilibrium

Reactions

• Reactions involve reactants and products• Reactions that go to completion– Magnesium + oxygen → magnesium oxide• One way reaction, magnesium oxide cannot turn back

into magnesium oxide

• Reversible reactions– Have a forward and a reverse reaction– Carbon dioxide + water ↔ carbonic acid

Reversible reactions

• The forward and reverse reactions occur at the same time, and never stop.– This means that it is a dynamic situation

• At some point, the rate of the forward reaction is equal to the rate of the reverse reaction– This means that reactants are turning into products just as fast

a products are turning back into reactants• At this point, the reaction is said to have reached

EQUILIBRIUMAt equilibrium, the concentrations of the of the reactants and the products are constant, but are not necessarily equal

Factors that disturb equilibrium

• Change in temperature• Change in pressure• Change in concentration

Le Chatelier’s principle

• Any change made to a reaction which is in equilibrium, will result in the equilibrium position moving to minimise the change made

Concentration• If the concentration of a reactant is increased,

then the reaction will favour the forward reaction to reduce its concentration.

• If the concentration of a reactant is decreased then the reaction will favour the reverse reaction to increase its concentration

• A similar thing happens if you vary the concentration of the products

• This “favouring” of a particular direction continues until the system returns to a point of equilibrium

Concentration continued

• According to particle theory– An increase in concentration will provide more

particles to be involved in the reaction

Pressure

• Only affects reactions where there is at least one gas involved.

• Increasing the pressure of a gas is similar to increasing its concentration, because the molecules of gas are pushed more closely together and are, therefore, involved in more reaction collisions.

• That means the reaction will favour whichever direction decreases the amount of gas

Temperature

• Reactions can be either exothermic (releases heat) or endothermic (absorbs heat). If a reaction is exothermic in one direction, it is endothermic in the other direction.

• Using Le Chatelier’s principle with temperature:– An increase in temperature favours the endothermic

reactions (the increase in temperature results in more energy being available to support the endothermic reaction)

– A decrease in temperature favours the exothermic reaction

Experiment – effect of temperature on equilibrium

Experiment – Effect of concentration on equilibrium

Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process in terms of Le Chateliers principle.

• The reaction between carbon dioxide and water is an example of an equilibrium reaction:

• The forward reaction is exothermic (releases heat)

temperature

• An increase in temperature will cause the reaction to go in the reverse direction– More carbon dioxide will be liberated– Hot soda loses its fizz faster

• A decrease in temperature will cause the reaction to go in the forward direction– More carbonic acid will be formed

concentration

• An increase in CO2 causes the reaction to move in the forward direction– Produces more carbonic acid

• A decrease in CO2 causes the reaction to move in the reverse direction– Produces more CO2

pressure

• An increase in pressure causes the reaction to move in the forward direction– That’s how they get the fizz in the drink in

the first place• A decrease in pressure causes the

reaction to move in the reverse direction – Thus, your drink is bubbly when you open

it.

pH

• As part of the forward reaction, the carbonic acid tends to dissociate in water, producing H+

ions and carbonate ions.• If we increased the acidity of our drink by adding

citric acid – which also produces H+ ions, it would move the reaction in the reverse direction– Giving us more CO2 bubbles

Exercises

• Complete the following questions from Conquering Chemistry 2 page 112– 12 through to 16