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Skill Check 0 / 10 complete

The reaction quotient and K

Both Q and K are the same fraction — products over reactants, coefficients as exponents. Build that expression correctly and you can watch Q chase K to equilibrium.

§1

Building the quotient.

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The equilibrium expression is products over reactants, with each concentration raised to its coefficient from the balanced equation. Pure solids and liquids are excluded (their 'concentration' is constant).

Both Q and K use this same expression. The only difference is the concentrations: Q uses the current values, K the equilibrium values.

As a reaction proceeds, Q moves toward K — rising if it starts below K, falling if it starts above — and when Q reaches K, the concentrations stop changing and the system is at equilibrium.

UNIT 7 TOPIC 7.3 • REACTION QUOTIENT AND EQUILIBRIUM CONSTANT Q TRACKER Q MOVES TOWARD K Q time K Q starts above K Q starts below K both approach K At equilibrium the reaction stops changing: Q = K WHAT CHANGES? Q changes as a reaction proceeds and approaches K. K is FIXED for a given reaction at a given temperature — independent of the starting amounts. Only TEMPERATURE changes the value of K. INTERPRETING A SYSTEM 1 Calculate Q from current amounts 2 Compare Q to K 3 Predict the shift 4 Stop when Q = K CED ANCHOR Q is the current ratio; K is the equilibrium ratio. Q shifts until Q = K — and only a temperature change alters K itself. AP Chemistry · Unit 7 · Equilibrium
Fig. 7.3.1 Over time, Q moves toward K: from below, the system shifts to raise Q; from above, to lower it. When Q reaches K, concentrations stop changing. Both Q and K are products over reactants, each raised to its coefficient.
§2

Writing the expression.

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Products up, reactants down, coefficients as exponents.

  1. Put products in the numerator. Each product concentration goes on top.
  2. Put reactants in the denominator. Each reactant concentration goes on the bottom.
  3. Raise each to its coefficient. Use the balanced-equation coefficients as exponents.
  4. Exclude pure solids and liquids. They do not appear in the expression.
§3

The pieces you'll meet.

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One expression, two uses.

expression
Equilibrium expression
Products over reactants, coefficients as exponents.
exclude
Excluded species
Pure solids and liquids are left out.
Q
Q
Uses current concentrations.
K
K
Uses equilibrium concentrations.
toward K
Q → K
Q moves toward K as the reaction proceeds.
coefficients
Coefficients
Become the exponents in the expression.
§4

Worked example: write the expression.

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Reaction. N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

Numerator. The product NH₃, raised to its coefficient: [NH₃]².

Denominator. The reactants, each raised to its coefficient: [N₂][H₂]³.

Expression. K = [NH₃]² / ([N₂][H₂]³). Q has the identical form, evaluated at current concentrations. If a pure solid or liquid had appeared, it would be omitted.

§5

Mistakes that cost real points.

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Pitfall · 01

"Put reactants on top and products on the bottom."

The equilibrium expression is products over reactants — products in the numerator, reactants in the denominator. Inverting them gives the reciprocal of the true quotient and flips every Q-versus-K conclusion.

Fix. Always write products (numerator) over reactants (denominator).

Pitfall · 02

"Use the coefficients as multipliers, not exponents."

Coefficients become exponents in the equilibrium expression, not multipliers. For 2NH₃ the term is [NH₃]², not 2[NH₃]. Treating coefficients as multipliers gives the wrong quotient.

Fix. Raise each concentration to the power of its coefficient; do not multiply by the coefficient.

Pitfall · 03

"Include pure solids and liquids in the expression."

Pure solids and liquids are excluded from Q and K because their effective concentration is constant. Including them (or a solvent like water in dilute solution) is incorrect.

Fix. Leave out pure solids and pure liquids; include only gases and dissolved species.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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