Reversible Reactions & Equilibria

4.13 Recall that chemical reactions are reversible, the use of the symbol in equations and that the direction of some reversible reactions can be altered by changing the reaction conditions
  • In some chemical reactions, the products of the reaction can react to produce the original reactants
    • These are called reversible reactions
    • The direction of the reaction can be changed by changing the conditions

E.g. The Haber Process

NITROGEN + HYDROGEN ⇌ AMMONIA

4.14 Explain what is meant by dynamic equilibrium
  • When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
4.15 Describe the formation of ammonia as a reversible reaction between nitrogen (extracted from the air) and hydrogen (obtained from natural gas) and that it can reach a dynamic equilibrium
  • Used to manufacture ammonia, which is used to produce nitrogen-based fertilisers
  • The raw materials for the Haber process are nitrogen and hydrogen.
  • Nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources.
  • The purified gases are passed over a catalyst of iron at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres).
  • Some of the hydrogen and nitrogen reacts to form ammonia.
  • The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen.
4.16 Recall the conditions for the Haber process as: temperature 450˚C, pressure 200 atmospheres and iron catalyst
4.17 (HT only) Predict how the position of a dynamic equilibrium is affected by changes in: temperature, pressure and concentration
  • The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
  • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
    • Effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
  • Effect of changing concentration
    • An increase in concentration will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.

Assuming you are changing the conc. of one of the products…

If a reaction produces… …more moles of product/large conc. of product you are changing in comparison to reactants …less moles of product/smaller conc. of product you are changing in comparison to reactants
An increase in concentration… Decreases yield (shifts left) Increases yield (shifts right)
A decrease in concentration…

 

Increases yield (shifts right) Decreases yield (shifts left)
  • Effect of changing pressure
    • In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.

 

If a reaction produces a… …larger volume of gas …smaller volume of gas
An increase in pressure… Decreases yield of reaction Increases yield of reaction
A decrease in pressure… Increases yield of reaction Decreases yield of reaction
  • Effect of changing temperature
    • If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases.
    • If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases.
Exothermic Endothermic
An increase in temperature… Decreases yield of reaction Increases yield of reaction
A decrease in temperature… Increases yield of reaction Decreases yield of reaction