Chemical Equilibrium | Industrial Chemistry | HSC Chemistry

Chemical Equilibrium

Joshua explains about Chemical Equilibrium.

  • Chemical Equilibrium

    When the rate of the forward reaction is equal to the rate of the backward reaction, the reaction is at equilibrium. This is a dynamic equilibrium because the microscopic changes continuously occur even if the macroscopic properties of the system remain unchanged.

    The Haber process is a well known equilibrium system:

    \( N_{2}(g) + 3H_{2}(g) \rightleftharpoons 2NH_{3}(g) \text{  } \triangle H = -92 \text{ kJ mol}^{-1} \)

    The factors that affect the position of the equilibrium include temperature, pressure, volume, and concentration for Chemical Equilibrium.

  • Pressure and volume

    According to Boyles Law (PV = k), the pressure becomes inversely proportional to the volume. Thus, an increase in volume of a reaction container would result in a decrease in gas pressure. Change in pressure only affect gaseous reactions where the number of moles of reactants is different from the number of moles of products.

    For example, the oxidation of nitric oxide is an equilibrium process.

    \( 2NO(g) + O_{2}{g} \rightleftharpoons 2NO_{2}(g) \)

    If the volume of reaction container is decreased, the pressure will be increased that would lead the equilibrium to shift to the right hand side, since the number of moles of product (2) is smaller than the number of moles of reactants (3). The reduction in particle number will minimise the imposed pressure increase in Chemical Equilibrium.

  • Changes in concentration

    Increase in reactant concentration increase the rate of the forward reaction. Hence, the position of the equilibrium shifts to the right to form more products.

  • Changing temperature

    The effect of changes to temperature depends on whether the reaction is endothermic or exothermic. An exothermic reaction is one where heat is a product of the reaction.

    In an endothermic reaction, such as photosynthesis,

    \( 6CO_{2}(g) + 6H_{2}(g) \rightleftharpoons C_{6}H_{12}O_{6}(aq) + 6CO_{2}(g) \)

    \( \triangle H = + 2803 \text{ kJ mol}^{-1} \)

    Increasing the temperature will cause the forward reaction to be favoured and more glucose and oxygen to be formed.

    In an exothermic reaction, such as the Haber process,

    \( N_{2}(g) + 3H_{2}(g) \rightleftharpoons 2NH_{3}(g) \)

    \( \triangle H = – 92 \text{ kJ mol}^{-1} \)

    Increasing the temperature would cause the backward reaction to occur, since the imposed temperature increase would be reduced by shifting the equilibrium to the left.

    In both endothermic and exothermic reactions, increasing the temperature results in an increase in the rate of reaction. This in turn results in equilibrium being reached more rapidly for both types of reaction in Chemical Equilibrium.

  • Effects of changing factors on the equilibrium

    Effects of changing factors on the equilibrium
    ChangeNew Equilibrium PositionWhy?
    Pressure UP, Volume DOWNside with fewer gas particlesmore collisions between gas
    particles, therefore more reactions
    Pressure DOWN, Volume UPside with more gas particlesless collisions between gas particles
    Concentration UPfavours reaction with less of that substance formed
    Increasing the concentration of one reactant pushes the reaction away from that side.
    Concentration DOWNfavours reaction with more of the substance formed
    Decreasing the concentration of one reactant pulls the reaction toward that side.
    Temperature UPendothermic reactionReaches equilibrium faster and temperature needs to be reduced by moving equilibrium
    Temperature DOWNexothermic reactionReaches equilibrium slower and temperature needs to be increased by moving equilibrium

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