Reversible reactions and equilibria

Reversible reaction

Reversible reaction

• the products of the reaction can react together to produce the original reactants. (depending on the conditions)

• reaction goes both ways

Describe a reversible reaction: the dehydration of hydrated copper (II) sulfate

1. blue hydrated copper sulfate is heated.

2. decomposes to white anhydrous copper sulfate and water vapour.

3. if water is re-added, the change reversed is and hydrate copper sulfate re-forms.

equation for the reversible reaction: dehydration of hydrated copper (II) sulfate

CuSO4 . 5H2O(s) → ← CuSO4(s) + 5H2O

Describe a reversible reaction: effect of heat on ammonium chloride

1. ammonium chloride heated in an evaporating basin.

2. decomposes into ammonium gas and hydrogen chloride gas.

3. the 2 mixed gases rise and hit the cooler glass filter tunnel.

4. reaction reversed → ammonium chloride powder re-forms.

equation for the reversible reaction: the effect of heat on ammonium chloride

NH4Cl(s) → ← HCl(g) + NH3(g)

Describe a reversible reaction: making ammonia using syringes

- forms basis of Haber process

1. using syringes, the mixture of hydrogen and nitrogen is pushed over heated iron wool.

2. gases in syringes ejected onto red litmus paper.

3. litmus paper turns blue (alkaline) - shows ammonia is produced.

   note: experiment repeated, starting with ammonia in syringes.

conditions to maximise yield of ammonia (The Haber Process)

1. high pressure of gas

2. low temperature of gas

equation for reversible reaction: making ammonia

2NH3(g) → ← N2(g) + 3H2(g)

a reversible reaction can reach dynamic equilibrium in a…

• sealed container/closed system

• otherwise, products would escape.

dynamic equilibrium

• forward and reverse reaction occur at same rate.

• the concentration of reactants and products remain constant.

explain why a catalyst doesn’t affect the position of equilibrium in a reversible reaction

• catalyst increases rate both forwards and backwards.

• increases rate of both reactions to same extent.

• therefore, catalyst does not affect position of equilibrium. However, equilibrium will be reached more quickly.

explain ‘Le Chatelier’s Principle’

the position of equilibrium shifts to try to cancel out any changes you make.

effect of temperature on the position of equilibrium in a reversible reaction

increasing temperature shifts equilibrium in direction that takes in heat (because it wants to lower temp. again.)

• temp increase = endothermic direction = yield of product decreases

• temp decrease = exothermic direction = yield of product increases

→

endothermic direction

←

exothermic direction

effect of pressure on position of equilibrium in reversible reaction

• pressure increase = equilibrium shifts in direction of fewest molecules (TO DECREASE PRESSURE)

• pressure decrease = equilibrium shifts in direction of most molecules (TO INCREASE PRESSURE)

effect of concentration on position of equilibrium in reversible reaction

increasing the concentration of a substance shifts the equilibrium in the direction that produced less of that substance.

eg. increase substance A = equilibrium shifts to decrease amount of substance A