Reaction Quotient & Le Châtelier’s Principle

Chemistry: IB Diploma Higher Level, topic 5.4.2

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

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

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

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.

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.

Test yourself on Reaction Quotient & Le Châtelier’s Principle

What would be the impact on the value of $K$ if N₂ was added to the reaction?

N₂ + 3H₂ ⇌ 2NH₃
What would be the impact on the value of $Q$ if N₂ was added to the reaction?

N₂ + 3H₂ ⇌ 2NH₃
What would be the impact on the value of $Q$ if NH₃ was added to the reaction?

N₂ + 3H₂ ⇌ 2NH₃