Concentration Changes at Equilibrium

Introduction

Adding or removing a species from an equilibrium mixture creates two stages: an immediate concentration change imposed externally, then a slower chemical response. Keeping those stages separate prevents mistaken graphs and claims that the system perfectly cancels the original imposed disturbance.

Core explanation

For A ⇌ B with K ≈ [B]/[A], adding A causes [A] to jump upward instantly while [B] initially stays the same if the addition is quick. Q then falls below K. Net forward reaction consumes some A and forms B until Q again equals K. The final A concentration may still be higher than before addition; the chemical response only partly offsets the imposed increase.

Adding B gives an immediate jump in [B], making Q > K, so net reverse reaction forms A. Removing B lowers Q and promotes net forward formation of B. Removing A raises Q and favors reverse reaction. These results assume the reaction is still able to proceed, temperature stays fixed, and the simple species activities follow their concentrations.

For a balanced equation with unequal coefficients, calculate Q rather than relying on bare words. In N₂ + 3H₂ ⇌ 2NH₃, Qc = [NH₃]²/([N₂][H₂]³). Injecting H₂ increases the denominator strongly, lowering Q and promoting net ammonia formation at fixed temperature. Quantitative final amounts require an ICE table or another conservation calculation; Q alone gives direction.

Dilution is more subtle because adding solvent changes all dissolved concentrations at once. The net direction depends on stoichiometric powers and activities, not simply on “less of everything.” In an acid dissociation HA ⇌ H⁺ + A⁻, dilution often increases fractional ionization because the product side has more dissolved particles under ideal dilute assumptions. Q analysis provides the controlled derivation.

If a gas species is added at fixed volume, its own partial pressure rises immediately. If a gas is added at fixed total pressure, volume may also change, affecting other partial pressures. The physical constraint must be stated before assigning the immediate Q change.

Step-by-step reasoning

1. Write Q for the balanced reaction. 2. Apply only the imposed immediate concentration changes. 3. Compare new Q with the old K at unchanged temperature. 4. Predict net direction, then use conservation for final amounts if requested.

Visual explanation

Draw [A] versus time with a vertical jump when A is added and a gradual decline afterward. Draw [B] without an immediate jump but with a gradual rise to its new plateau.

Real-world analogy

Adding water to a reservoir raises its level instantly. An outlet may then drain some extra water, but the final level need not equal the original level.

Real-world example

A reversible gas reactor may receive a pulse of one feed gas. Sensors can show an immediate increase in that gas followed by a slower composition shift as the reaction readjusts.

Why?

Why does adding product favor net reverse reaction at fixed T? It raises the product activity ratio Q above K, and reverse conversion lowers Q toward equilibrium.

Common misconception

“Adding A causes [A] to finish exactly where it started.” The final concentration generally changes; reaction readjustment only partially counteracts the imposed addition.

Worked example

For A ⇌ B, K = 1 and initial equilibrium [A] = [B] = 1.0 M. Instantly add 1.0 M A, giving [A] = 2.0 M, [B] = 1.0 M and Q = 0.5. Let x M shift forward: final A = 2.0 − x and B = 1.0 + x. Set (1 + x)/(2 − x) = 1, giving x = 0.5. Final concentrations are 1.5 M each, not the original 1.0 M.

Quick check

1. In A ⇌ B, does removing B make Q larger or smaller immediately? Answer: Smaller, favoring net forward production of B at unchanged temperature.

Exam focus

Show immediate and final states separately. Use Q for direction, and an amount balance for numerical final concentrations.

Advanced insight

Activity coefficients can change as composition changes, so raw concentration shifts may not map exactly onto activity shifts in concentrated solutions. Thermodynamic Q is the rigorous direction test.

Summary

Changing one species' concentration changes Q immediately while K stays fixed at constant temperature. The reaction then readjusts, usually only partially offsetting the imposed change.

Practice questions

1. What happens to Q for A ⇌ B when A is suddenly added? Answer: Q decreases because its denominator increases. 2. Does Q alone give exact final concentrations? Answer: No. Combine the K condition with conservation or an ICE table. 3. Why may dilution need more than a simple “shift away” slogan? Answer: It changes several concentrations together, and coefficient exponents determine Q's net change.