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Chemistry LibreTexts

15.10: The Effect of Temperature Changes on Equilibrium

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  • Page ID
    47585
  • Skills to Develop

    • Explain the following laws within the Ideal Gas Law

    When temperature is the stress that affects a system at equilibrium, there are two important consequences:

    • an increase in temperature will favor that reaction direction that absorbs heat (i.e. the endothermic reaction)
    • the value of Keq will change

    Consider the following equilibrium system

    \[\ce{N_2O_4(g) \leftrightarrow 2NO_2(g)}\]

    with \(\ce{\Delta H^{\circ}={58.0}\:kJ}\)

    We see by the sign of ΔH° that the forward reaction is endothermic. Heat is absorbed (required as a reactant) when the reaction proceeds as

    \[\ce{N_2O_4(g) \rightarrow 2NO_2 (g)}\]

    By adding more heat, equilibrium will shift to use up the additional heat, thus favoring this forward direction.

    Why will Keq change, when it did not change when concentration, pressure, and volume were the applied stresses?

    When temperature changes cause an equilibrium to shift, one entire side of the reaction equation is favored over the other side. Mathematically, this will alter the value of Keq as follows:

    \[\ce{K_{eq}=\dfrac{[products]}{[reactants]}}\]

    if the forward reaction is favored

    more products are produced; fewer reactants

    Keq will increase

    if the reverse reaction is favored

    fewer products; more reactants

    Keq will decrease

    So in our example given above, increasing the temperature will favor the forward direction. The value of Keq will increase. Removing heat (making the system colder) will favor the exothermic reaction - the exothermic reaction releases heat to the surroundings, thus "replacing" the heat that has been removed.