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Factors Affecting Kp

01Factors Affecting Kp

Temperature

This section covers how changing temperature affects the position of equilibrium and the value of Kp.

Temperature changes Kp

Changing temperature can alter both the position of equilibrium and the value of KpK_p. The effect depends on whether the forward reaction is exothermic or endothermic.

Le Chatelier's principle predicts the direction in which the equilibrium position moves when the temperature is changed.

Exothermic forward reaction

Increasing the temperature favours the reverse, endothermic direction, so the equilibrium position moves towards the reactants.

  • More reactants are present at equilibrium.
  • Less product is present at equilibrium.
  • The product-to-reactant relationship in the KpK_p expression decreases.
Increase temperature ⟶\longrightarrow KpK_p decreases

Endothermic forward reaction

Increasing the temperature favours the forward, endothermic direction, so the equilibrium position moves towards the products.

  • More products are present at equilibrium.
  • Less reactant is present at equilibrium.
  • The product-to-reactant relationship in the KpK_p expression increases.
Increase temperature ⟶\longrightarrow KpK_p increases

Example: an exothermic equilibrium

For an equilibrium that is exothermic in the forward direction, such as:

NX2(g)+3 HX2(g)⇌2 NHX3(g)\ce{N2(g) + 3H2(g) <=> 2NH3(g)}

an increase in temperature moves the equilibrium to the left. The equilibrium mixture therefore contains relatively less ammonia and relatively more nitrogen and hydrogen, so KpK_p becomes smaller.

Exam Tip: When explaining a temperature change, state the direction favoured first, link it to the endothermic or exothermic direction, then state the resulting change in KpK_p. Do not say that KpK_p changes simply because the equilibrium position changes.

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