5.4.3

Temperature & Le Châtelier’s Principle

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Temperature & Le Châtelier’s Principle

When an equilibrium shifts in response to a temperature change, it is re-established with a different relative composition that exhibits a different value for the equilibrium constant.

Temperature changes & $$K$$

Temperature changes & KK

  • Changes in the concentration of reactants or products change the value of QQ, whereas changes in temperature affect KK.
    • Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in KK.
    • When an equilibrium shifts in response to a temperature change, however, it is re-established with a different relative composition that exhibits a different value for the KK.
Endothermic vs exothermic

Endothermic vs exothermic

  • Predicting the shift an equilibrium will experience in response to a change in temperature is most conveniently accomplished by considering the enthalpy change of the reaction.
  • For example, the decomposition of dinitrogen tetroxide is an endothermic (heat-consuming) process:

    • N2O4(g)⇌2NO2(g)   Δ𝐻=+57.20 kJ
Heat as a reactant

Heat as a reactant

  • For purposes of applying Le Chatelier’s principle, heat (q) may be viewed as a reactant:
    • Heat + N2O4(g)⇌2NO2(g)
  • Raising the temperature of the system is akin to increasing the amount of a reactant, and so the equilibrium will shift to the right.
  • Lowering the system temperature will likewise cause the equilibrium to shift left.
    • For exothermic processes, heat is viewed as a product of the reaction and so the opposite temperature dependence is observed.
Catalysts

Catalysts

  • Catalysts increase the rate of forward and reverse reactions equally, so have no impact on equilibrium position.
Jump to other topics
1

Structure - Models of the Particulate of Matter

2

Structure - Models of Bonding & Structure

3

Structure - Classification of Matter

3.1

The Periodic Table: Classification of Elements

3.2

Periodic Trends

3.3

Group 1 Alkali Metals

3.4

Halogens

3.5

Noble gases, group 18

3.6

Functional Groups: Classification of Organic

3.7

Functional Group Chemistry

3.8

Alkanes

3.9

Alcohols

3.10

Halogenoalkanes

4

Reactivity - What Drives Chemical Reaction?

5

Reactivity - How Much, How Fast & How Far?

6

Reactivity - The Mechanisms of Chemical Change

7

Measurement, Data Processing & Analysis

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