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3.2.3 Chemical equilibrium

Definitions

Term Definition
Dynamic equilibrium When the rates of forward and backward reactions are equal in a closed system

Dynamic equilibrium and Le Chatelier's Principle

Dynamic equilibrium conditions

  • In a closed system for reversible reactions
  • The rate of the forward reaction is equal to the rate of the reverse reaction
  • The amount of reactants and products are constant

Effect of concentration changes on position of equilibrium

  • Increase in concentration of reactants / decrease in concentration of products
    • Rate of forward reaction increases (favoured)
    • Equilibrium shifts to the right, more products formed
  • Increase in concentration of products / decrease in concentration of reactants
    • Rate of backward reaction increases
    • Equilibrium shifts to the left, more reactants formed

Effect of changes in temperature on position of equilibrium

  • Increase in temperature
    • Favours the endothermic reaction
  • Decrease in temperature
    • Favours the exothermic reaction
  • (Equilibrium shifts to ..., there are more ..., less ..., the yield ...)
  • Forward and backward reactions have the same magnitude of enthalpy change but opposite signs

Effect of changes in pressure on position of equilibrium

  • Increasing the pressure
    • ... is favoured because it reduces the number of moles of gas in the mixture
    • Shift the position of equilibrium to side with fewer moles of gas
    • Reduces the pressure of the system
  • Decreasing the pressure
    • ... is favoured because it increases the number of moles of gas in the mixture
    • Shift the position of equilibrium to side with more moles of gas
    • Increases the pressure of the system

Effect of catalyst on equilibrium

  • Increases the rate of both forward and reverse reactions in an equilibrium by the same amount
  • Do not change the position of equilibrium
  • Allow equilibrium to be achieved faster

Investigating changes in position of equilibrium

  • Observe colour change
  • Change in concentration
    • Add more reactants / products to the mixture
  • Change in temperature
    • Heat using boiling water bath
    • Cool using iced water

Industrial processes

Haber process

  • \(N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\) (Forward = exothermic)
  • Lower temperature
    • Higher yield of product
    • Rate may be too slow that equilibrium may not be established
    • Not used
  • High pressure
    • Increase yield & rate
    • Requires very strong container + large quantity of energy \(\rightarrow\) higher cost
    • Failure of steelwork / seals could lead to hot gases (including toxic ammonia) leaking \(\rightarrow\) endangering the workforce and the surrounding area \(\rightarrow\) safety concerns
  • Operate under compromise conditions of 400-500°C, 100-200 atm and iron catalysts
    • Gives a reasonable rate without shifting the equilibrium position too far away from ammonia and back to the reactants
    • Iron catalyst increases the rate so lower temperatures can be used and operating cost is lowered + higher yield can be pursued
    • Only about 15% of the reactants is converted to ammonia, but \(H_2\) and \(N_2\) are recycled repeatedly so nearly all reactants are eventually converted

Industrial process conditions

  • Lower temperature / pressure: rate might be too slow
  • Higher temperature / pressure: safety risk / high cost / high energy use

The equilibrium constant

The equilibrium constant / \(K_{c}\)

  • For reaction \(aA + bB \rightleftharpoons cC + dD\)
  • \(K_{c} = \frac{[C]^{c} [D]^{d}}{[A]^{a} [B]^{b}}\)
  • [] = concentration of ...
  • [A], [B], [C], [D] = equilibrium concentration of the reactants and products of this equilibrium
  • Only solutions should appear in the equation for \(K_{c}\)
    • Include liquid if they have a similar amount to the solutions

Value of \(K_{c}\)

  • \(K_c < 1\)
    • Position of equilibrium is towards the LHS
    • Greater concentration of reactants
  • \(K_c = 1\)
    • Position of equilibrium is halfway between reactants and products
  • \(K_c > 1\)
    • Position of equilibrium is towards the RHS
    • Greater concentration of products

Effect of temperature change on \(K_{c}\)

  • Forward reaction is exothermic
    • \(K_{c}\) decreases when temperature increases
  • Forward reaction is endothermic
    • \(K_{c}\) increases when temperature increases
  • If the direction of reaction is not specified take it as the forward reaction by default