Temperature Effects on Chemical Equilibrium

Concept Overview – Temperature & Chemical Equilibrium

  • Le Châtelier’s Principle: A system at equilibrium will counteract any imposed change (temperature, pressure, concentration) to re-establish equilibrium.

  • Temperature acts as a reagent:

    • Endothermic reaction (Delta H > 0): heat is absorbed
      => treat heat as a reactant.

    • Exothermic reaction (Delta H < 0): heat is released => treat heat as a product.

  • Changing temperature therefore changes both (i) the position of equilibrium and (ii) the numerical value of the equilibrium constant K_eq.

General Rules

  • Adding heat (raising T):

    • Shifts equilibrium toward the endothermic direction (heat-absorbing side).

    • K_eq increases for endothermic reactions; decreases for exothermic reactions.

  • Removing heat (lowering T):

    • Shifts equilibrium toward the exothermic direction (heat-releasing side).

    • K_eq decreases for endothermic reactions; increases for exothermic reactions.

Illustrative Example 1 — Industrial SO3 Synthesis

Reaction (overall): 2SO2(g) + O2(g) <=> 2SO3(g)

  • In industry the forward formation of SO3 is exothermic (Delta H approx -198 kJ/mol).

  • Consequences of T-change (exothermic system):

    • Raise T
      => treat heat as added product
      => equilibrium shifts left (toward reactants SO2, O2); K_eq falls.

    • Lower T
      => removes product-side heat
      => equilibrium shifts right (toward SO3); K_eq rises.

  • Industrial compromise: Moderate temperatures (~700 K) maximize yield yet keep rate acceptable; excessive cooling slows the reaction.

Illustrative Example 2 — Example 15.16 (Endothermic)

Reaction: CaCO3(s) <=> CaO(s) + CO2(g) (Delta H > 0) (endothermic)

  • Add heat (raise T):

    • Heat behaves as an extra reactant.

    • Equilibrium shifts right (toward CaO + CO2).

    • K_eq increases.

  • Remove heat (lower T):

    • Equivalent to removing a reactant (heat).

    • Equilibrium shifts left (toward solid CaCO3).

    • K_eq decreases.

  • Practical relevance: Thermal decomposition (calcination) of limestone in cement manufacture requires high T to drive reaction rightward and evolve CO2.

Key Takeaways & Quick-Reference Bullets

  • Memorize verbal cue: “Adding heat favors endothermic; removing heat favors exothermic.”

  • Heat behaves like any other component in the equilibrium expression conceptually, even though it is not written in K_eq.

  • Unlike concentration or pressure changes, **temperature change actually alters *Keq*, because Delta H is built into the van’t Hoff relationship: ln(K2/K1) = -(Delta Hdeg / R) * (1/T2 - 1/T1)

  • Design of industrial reactors, environmental considerations, and geological processes (e.g., metamorphism) rely on these temperature-equilibrium principles.

Mnemonic to Remember

“HEAT on LEFT for ENDO; HEAT on RIGHT for EXO.” — push away from where the heat is added.