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Q vs K : Predicting Direction of Reaction

Key Concepts

For a given reaction aA + bB cC + dD
Q, the mass-action expression (concentration fraction or reaction quotient), is calculated using:
Q = [C]^c[D]^d

[A]^a[B]^b
At equilibrium, Q = K (the equilibrium constant for the reaction)
Q = K is referred to as the equilibrium condition.
If Q > K, the reverse reaction is favoured, the reaction moves from right to left, until equilibrium is established.
If Q < K, the forward reaction is favoured, the reaction moves from left to right, until equilibrium is established.

Example: Q > K

For the reaction: N_2(g) + O_2(g)

2NO_(g)
the equilibrium constant, K, is 1.0 x 10^-5 at 1500K.
Predict the direction the reaction will move in if the reactants and products have the following concentrations:
[N₂] = 0.05M
[O₂] = 0.02M
[NO] = 0.30M

Write the mass-action (reaction quotient) expression
Q = [NO]²

[N₂][O₂]
Substitute in the vales for the concentrations of each reactant and product:
Q = [0.30]²

[0.05][0.02]

Q = 90
Compare Q to K: Q (90) > K (1.0 x 10^-5)
Predict direction of reaction: Q > K
The reverse reaction is favoured.
The reaction moves from right to left until equilibrium in established.

Example: Q < K

For the reaction: H_2(g) + I_2(g)

2HI_(g)
the equilibrium constant, K, is 1.59 x 10² at 500K.
Predict the direction the reaction will move in if the reactants and products have the following concentrations:
[H₂] = 0.15M
[I₂] = 0.75M
[HI] = 1.75M

Write the mass-action (reaction quotient) expression
Q = [HI]²

[H₂][I₂]
Substitute in the vales for the concentrations of each reactant and product:
Q = [1.75]²

[0.15][0.75]

Q = 27.22
Compare Q to K: Q (27.22) < K (1.59 x 10²)
Predict direction of reaction: Q < K
The forward reaction is favoured.
The reaction moves from left to right until equilibrium in established.

Example: Q = K

For the reaction: N₂O_4(g)

2NO_2(g)
the equilibrium constant, K, is 5.0 x 10^-1 at 100^oC.
Predict the direction the reaction will move in if the concentration of N₂O₄ is 0.02M and the concentration of NO₂ is 0.10M.

Write the mass-action (reaction quotient) expression
Q = [NO₂]²

[N₂O₄]
Substitute in the vales for the concentrations of each reactant and product:
Q = [0.10]²

[0.02]

Q = 0.50
Compare Q to K: Q (0.50) = K (5.0 x 10^-1)
Predict direction of reaction: Q = K
The reaction is at equilibrium, neither the forward nor reverse reactions are favoured.

Practice Questions

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Q vs K drill
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