7.10Equilibrium

Reaction Quotient and Le Chtelier’s Principle — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 17 of 19 1 quiz

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Lesson summary

7.10 Reaction Quotient and Le Châtelier's Principle — AP CHEMISTRY notes: Use Q compared with K to explain the shift after a stress, and apply it to measurable properties.Q — the value of the equilibrium expression calculated from concentrations at any momentAt equilibrium — Q = …

Adding a reactant
Adding a product
Value of K
Sr(OH)2(s) ⇌ Sr2+ + 2OH−
Temperature increase
Temperature decrease

Section 1 of 9Reaction quotient

Q the value of the equilibrium expression calculated from concentrations at any moment

At equilibrium

Q = K and Q no longer changes with time

Section 2 of 9Predicting the shift

Comparison	Direction to equilibrium
Q < K	Forward (right)
Q > K	Reverse (left)
Q = K	No shift

Section 3 of 9Stress and Q

Adding a reactant

Q decreases below K, so the reaction shifts forward

Adding a product

Q increases above K, so the reaction shifts in reverse

Value of K

Value of K returns to the same value after concentration and pressure stresses; only temperature changes K

Section 4 of 9Effect on pH

Sr(OH)₂(s) ⇌ Sr²⁺ + 2OH⁻
- adding SrCl₂ increases Sr²⁺ and shifts reverse
- [OH⁻] decreases, [H⁺] increases, so the pH decreases.
- K does not change.

Section 5 of 9Effect on colour (CoCl₄²⁻ system)

Effect on colour (CoCl₄²⁻ system)

Equation

CoCl₄²⁻ (blue) + 6H₂O ⇌ Co(H₂O)₆²⁺ (pink) + 4Cl⁻, forward exothermic

Temperature increase

Temperature increase shifts to the endothermic reverse direction, so the solution becomes blue

Temperature decrease

Temperature decrease shifts forward, so the solution becomes pink

Dilution

Dilution shifts to the side with more aqueous particles (5 against 1), so the solution becomes pink

Transfer to a larger flask

Transfer to a larger flask no effect because no gases are involved

Section 6 of 9Using K to deduce enthalpy

Using K to deduce enthalpy

K decreases when temperature increases

K decreases when temperature increases the forward reaction is exothermic

K increases when temperature increases

K increases when temperature increases the forward reaction is endothermic

2SO₂ + O₂ ⇌ 2SO₃

K falls from 9.9 × 10²⁵ at 298 K to 1.0 × 10¹² at 500 K, so the forward reaction is exothermic

Section 7 of 9More from the specification

1Equilibrium shifts with changing gas moles

Equilibrium shifts with changing gas moles compressing a chemical equilibrium system shifts composition toward the side containing fewer moles of gas.

2Equilibrium constant

The reaction quotient at equilibrium.

3Volume

The space taken up by an object.

4pH

-log[H+]

What the symbols mean: pH has no unit · [H⁺] = concentration of hydrogen ions (mol dm⁻³); each 1 pH unit is a ten-fold change in [H⁺]

Section 8 of 9Key terms

Term	Meaning
Reaction quotient	Equilibrium expression evaluated at any time.
Q < K	Reaction shifts forward.
Q > K	Reaction shifts in reverse.
Spectator ion	Ion that does not take part in the equilibrium.

Section 9 of 9Exam technique

Avoid these mistakes

Reading Q > K as a forward shift — Q > K favours the reverse reaction.

Saying adding a spectator ion shifts the equilibrium.

Saying K changes whenever the equilibrium shifts.

Last updated: 18 October 2026