Chemical Equilibrium

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  • Chemical Equilibrium
    • Many chemical reactions are reversible -they can go in either direction
      • Eventually the amounts of the substances involved reach constant values and the reaction seems to have stopped.
        • In reality, the reaction is still taking place in both directions, but the rate at which the product is being formed is the same are the products turning back into the reactants. A state of dynamic equilibrium is said to exist;
          • The rate of the forward reaction is the same as the rate of the reverse reaction
          • The system must be closed. This means that it must not be possible for reactants or products to enter or leave the system
          • Macroscopic properties are constant
    • Effect of changing conditions on equilibria
      • If a chemical reaction has reached equilibrium and the conditions are changed, then the position of equilibrium generally changes
        • The direction in which an equilibrium shifts when conditions are changed can be predicted by Le Chatelier's Principle;
          • 'When one of the conditions affecting the position of a dynamic equilibrium is altered, then then position of equilibrium will shift in the direction that opposes the change'
      • Temperature
        • If an equilibrium mixture is cooled, the equilibrium will shift in the direction that gives out heat, and therefore opposes the cooling
      • Concentration
        • Equilibrium will shift in the direction that reduces the concentration of the molecule or ion of which the concentration has been increased
      • Pressure
        • At a given temperature, the pressure in a gas mixture depends on the total number of moles of gas
          • So, an equilibrium can oppose a pressure increase by shifting in the direction where there are fewer moles of gas
        • If pressure is increased, the equilibrium will shift in the direction which tends to reduce the pressure i.e. in the direction of fewer moles
      • Catalyst
        • The catalyst does not affect the position of an equilibrium. This is because it speeds up the rate of the forward reaction and the rate of the corresponding reverse reaction by the same factor
    • Conditions for industrial processes
      • The factors to discuss are;
        • Effect on equilibrium yield
        • Effect on rate of reaction
        • Cost and safety considerations
    • Example: The Haber Process
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