Dynamic Equilibrium
Dynamic equilibrium
Features:
· Rate of forward reaction = Rate or reverse reaction
· Macro properties remain constant (constant temperature, pressure,
concentration)
· Constant movement on the microscopic scale
· Closed system
Examples
N2 + 3H2
2NH3
CH3COOH + C2H5OH
CH3COOC2H5 + H2O
CaCO3
CaO + CO2
Position of equilibrium
Le Chatelier’s principal
When the conditions affecting the position of dynamic equilibrium change, the position of equilibrium shifts to minimise the change.
Effect of pressure on the position of equilibrium
N2(g) + 3H2(g)
2NH3(g)
An increase in pressure results in the position of equilibrium shifting to the right, due to the smaller number of moles (four on the left, two on the right) resulting in more NH3 being produced.
Effect of temperature on the position of equilibrium
CaCO3
CaO + CO2
H = +178 KJmol-1
If the temperature is increased the position of equilibrium will shift to the right because the forward reaction is exothermic so more CaO + CO2 will be formed.
Effect of catalyst
Catalysts have no effect on the position of equilibrium; it increases
the rate of both the forward and reverse reactions.
Ways to increase the yield of product in a reaction. For example:
CaCO3
CaO + CO2
H = +178 KJmol-1
1. Remove the products as they are formed to increase the amount of reactions
2. Lower the pressure because there are more moles of reactants on the
right
3. Increase the temperature because the forward reaction is endothermic
The Haber process
Ammonia is an important product used in fertilisers in the form of ammonium sulphate (NH4)2SO4 and ammonium nitrate NH4NO3. It is also used in explosives (in particular TNT) and nylon.
N2(g) + 3H2(g)
2NH3(g)
H = -92 KJmol-1
Conditions to produce maximum yield
1. Decrease the temperature because the forward reaction is exothermic
2. Increase the pressure because there are more moles of gas on the left
Problems: Low temperature decreases the rate of reaction
High pressure is expensive and dangerous
Compromise: 200 atm pressure increasing the rate of reaction, whilst
keeping costs down
Remove ammonia as it forms and recycle the unreacted N2 and
H2
Iron catalyst to speed up the rate of reaction
400°C temperature giving a quick rate of reaction, however it will
decrease the yield