5.4.2

Reaction Quotient & Le Châtelier’s Principle

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Reaction Quotient & Le Châtelier’s Principle

The effect of changes in concentration on equilibria can be rationalized in terms of reaction quotients.

Changes in concentration

Changes in concentration

  • If an equilibrium system is subjected to a change in the concentration of a reactant or product species, the rate of either the forward or the reverse reaction will change.
  • As an example, consider the equilibrium reaction:

    • H2(g)+I2(g)⇌2HI(g)     𝐾𝑐= 50.0 at 400°C
  • The equilibrium expression for this reaction is given below:
    • 𝑄𝑐= =[HI]2[H2]  [I2]= \large\frac{[HI]^2}{[H_2]\; [I_2]} =𝐾𝑐
Shifting the equilibrium

Shifting the equilibrium

  • If a reactant is added (increasing the denominator of the reaction quotient) or product is removed (decreasing the numerator), then QcQc < KcKc.
    • This will cause the equilibrium to shift right.
  • This will restore the system to equilibrium such that QcQc = KcKc again.
    • The same logic may be applied for stresses involving removing reactants or adding product, in which case QcQc > KcKc and the equilibrium will shift left.
Adding products & reactants

Adding products & reactants

  • If H2 is added, the right shift will consume I2 and produce HI as equilibrium is re-established.
    • This will yield a mixture with a greater concentrations of H2 and HI and a lesser concentration of I2 than was present before.
  • If I2 is added, the new equilibrium mixture will have greater concentrations of I2 and HI and a lesser concentration of H2.
  • Finally, if HI is removed, the new equilibrium mixture will have greater concentrations of H2 and I2 and a lesser concentration of HI.
Impact on value of $$Kc$$

Impact on value of KcKc

  • Despite these differences in composition of these different mixtures, the value of the equilibrium constant will be the same after the stress as it was before.
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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