Forward rate = reverse rate

Macroscopic properties (concentration, pressure, color) remain constant over time

Any condition change that alters the equilibrium state

Common stresses: concentration, pressure/volume, temperature, inert gas, dilution, catalysts

Shift right: favors products; shift left: favors reactants

K changes only with temperature (for a fixed reaction definition)

Q: describes current state; K: equilibrium at a given temperature

Predict direction

Adding a reactant: shifts toward products (consume added reactant)

Removing a reactant: shifts toward reactants (replace removed reactant)

Adding a product: shifts toward reactants (consume added product)

Removing a product: shifts toward products (replace removed product)

Notes and common caveats

Decreasing volume (increasing pressure): shifts to fewer moles of gas

Increasing volume (decreasing pressure): shifts to more moles of gas

When there is no effect: equal gaseous moles on both sides → no shift

Partial pressures viewpoint: compression raises all partial pressures; system reduces total gas moles on the side with more gas

Treat heat as a reactant or product

Increasing temperature

Decreasing temperature

Practical implication: temperature changes alter equilibrium more fundamentally because K changes

Constant volume: partial pressures unchanged → typically no shift

Constant pressure (volume changes): partial pressures decrease → shifts to more moles of gas

Adding solvent reduces concentrations of dissolved species

Shifts toward the side with more dissolved particles (more moles of solute species)

No change to equilibrium position or K

Speeds forward and reverse equally; reaches equilibrium faster, not differently

Direction of shift after a stress

Qualitative predictions for many equilibrium systems

“Counteracting” tendency toward a new equilibrium state

Exact new equilibrium amounts (requires K + mass balance calculations)

Any shift if Q is not changed (or effect is negligible under constraints)

Complete reversal of the imposed change (only partial counteraction)

Consume what was added; replace what was removed

For gases: compare moles of gas on each side

For temperature: use endothermic/exothermic heat placement

Update Q after the change; compare to K

K changes only with temperature

Solids/pure liquids usually excluded from Q and K

Catalysts don’t change equilibrium composition

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Add H₂: shifts right → more NH₃

Remove NH₃: shifts right → replace NH₃

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Decrease volume: shifts right (3 moles gas → 2 moles gas)

Exothermic forward: A ⇌ B + heat

Increase temperature: shifts left; K decreases (for forward direction)