Reversible reactions and equilibria

Describe reversible reactions such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride.

Thermal decomposition of ammonium chloride

• Heating ammonium chloride produces ammonia and hydrogen chloride gases:

NH₄Cl (s) → NH₃ (g) + HCl (g)

• As the hot gases cool down they recombine to form solid ammonium chloride

NH₃ (g) + HCl (g) → NH₄Cl (s)  

• So, the reversible reaction is represented like this:

NH₄Cl (s) ⇌ NH₃ (g) + HCl (g)

Dehydration of hydrated copper(II) sulfate

• Reversible reactions can be seen in some hydrated salts

• These are salts that contain water of crystallisation which affects their shape and colour

  • Water of crystallisation is the water that is included in the structure of some salts during the crystallisation process

• One example is copper(II) sulfate:

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O

• The hydrated salt is copper(II) sulfate pentahydrate, CuSO₄•5H₂O

  • These are usually seen as blue crystals

  • The hydrated calt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO₄ 

  • This reaction is endothermic as energy is taken in to remove the water

• The anhydrous salt is copper(II) sulfate

  • This is usually seen as white crystals / powder

  • Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO₄•5H₂O

  • This reaction is highly exothermic

The dehydration of hydrated copper(II) sulfate

What is a dynamic equilibrium?

Dynamic equilibrium

• When the rate of the forward reaction equals the rate of the reverse reaction, the overall reaction is said to be in a state of equilibrium

  • Equilibrium is dynamic i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate

• The concentration of reactants and products remains constant

  • This is true if there is no other change to the system, such as temperature and pressure

• It only occurs in a closed system

  • This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter

The difference between an open and closed system

Equilibrium can only be reached in a closed container

• An example of a reaction reaching equilibrium is the reaction between H₂ and N₂ in the Haber process:

  • At the start of the reaction, only nitrogen and hydrogen are present

    • This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest

  • As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease

    • So, the rate of the forward reaction will decrease

  • However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase

    • Ammonia will decompose to reform hydrogen and nitrogen

  • In a closed system, the two reactions are interlinked and none of the gases can escape

  • So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

The rate of the forward and reverse reaction during the progress of a reaction

At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction

What are the effects of a catalyst on a equilibrium?

The effect of a catalyst on equilibrium position

• The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached

• This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)

• As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

 

Diagram showing the effect of catalyst on equilibrium position

A catalyst increases the rate of the forwards and reverse reaction but does not alter the position of equilibrium

What is the effect of changing either temperature or pressure on the position of equilibrium in a reversible reaction?

Temperature

• To make this prediction it is necessary to know whether the reaction is exothermic or endothermic

• If the temperature of the reaction increases:

  • The equilibrium will shift in the direction of the endothermic reaction

• If the temperature of a reaction decreases:

  • The equilibrium will shift in the direction of the exothermic reaction

Worked example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride and the forward reaction is exothermic:

ICl (l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown            yellow

What colour will the mixture turn when heated? Explain your answer.

Answer:

• The system will oppose the increase in temperature

  • Increasing the temperature of an equilibrium reaction favours the endothermic reaction

• If the forward reaction is exothermic, then the backward reaction must be endothermic

• Therefore, the equilibrium will move to the left and produce more reactants

• This means that the colour of the mixture will become increasingly brown as the temperature increases

Pressure

• We can predict the effect of changes in pressure on systems in equilibrium

• Changes in pressure only affects gases

• In gaseous reactions:

  • An increase in pressure will favour the reaction that produces the least number of molecules

  • A decrease in pressure will favour the reaction that produces the greatest number of molecules

• If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed

  • Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged

Worked example

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:

2NO₂ (g)   ⇌   N₂O₄ (g)

dark brown     colourless

What will the colour change be if the pressure is increased? Explain your answer.

Answer:

• The number of gas molecules produced by the forward reaction = 1

• The number of gas molecules produced by the reverse reaction = 2

• An increase in the pressure will favour the reaction that produces the least number of molecules

  • This is the forward reaction

• So, the equilibrium shifts to the right

  • This means that the mixture will appear increasingly colourless as the concentration of N₂O₄ increases