Equilibrium Constants & Reaction Quotients — Practice · Grade 11 Science

Chapter: Rates & Equilibrium

What this practice covers

By the end of Equilibrium Constants & Reaction Quotients you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: reaction quotient Q, ICE table.

Problem 1

For A⇌B, [A]=.20 M and [B]=.60 M at equilibrium. Find K.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

3

K=.60/.20.

Problem 2

Why omit a pure solid from a simple K expression?
Answer and explanation

Its activity is approximately constant while that pure phase is present.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

For A⇌B, equilibrium [A]=.40 M and [B]=.80 M. Find K.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

2

.80/.40=2.

Problem 4

Q=10 while K=2. Predict adjustment.
Answer and explanation

Net reverse change lowers the product-to-reactant expression toward K.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

For A⇌B with K=3 and Q=6, predict direction. Show the reasoning, not just the conclusion.
Hints

Compare Q and K for the same expression and temperature.

Answer and explanation

Reverse toward A, reducing Q toward K.

Problem 6

Why are pure solids omitted from standard equilibrium expressions?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Their activities are treated as unity when the pure phase is present; changing its amount alone does not change that activity.

Problem 7

A quadratic solution must also satisfy material constraintsA quadratic solution must also satisfy material constraints2A ⇌ A₂[A] (M)[A₂] (M)Initial1.000Change−2x+xEquilibrium1.00−2xxK=2: x/(1−2x)² = 2 → x≈0.3048 MChoose the physical root: 0 ≤ x ≤ 0.500 M.
Original calculated hypothetical dimerization using ideal normalized concentrations and no volume change; A and A₂ conserve the same atomic building units.
Interpret the supplied visual. Why must x be no greater than 0.500 M for the selected initial 1.00 M A inventory?
Answer and explanation

Each A₂ requires two A units, so producing more than 0.500 M A₂ would require a negative remaining A amount.

For dimerization, producing x concentration units of A₂ consumes 2x of A. The ICE table therefore differs from a one-to-one isomerization. Substitution into the selected K equation can yield more than one algebraic root; reject any root that makes a species concentration negative or exceeds available material. The displayed K=2 case gives x≈0.3048 M and [A]≈0.3904 M. Substitute the accepted values back into both the equilibrium ratio and atom-equivalent conservation to check the result.

Each A₂ requires two A units, so producing more than 0.500 M A₂ would require a negative remaining A amount.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

For 2A⇌A₂ under the normalized ideal convention, K=2.00, [A]=0.400 M and [A₂]=0.160 M. Compute Q, predict net direction, and state the maximum A₂ concentration if only an initial 0.800 M A inventory were available at fixed volume.
Answer and explanation

Q=0.160/(0.400)²=1.00<2.00, so net forward is favored. From an initial 0.800 M A-only inventory, conservation limits A₂ to at most 0.400 M. The equilibrium value need not reach that maximum, and no rate is given.

Q=0.160/(0.400)²=1.00<2.00, so net forward is favored. From an initial 0.800 M A-only inventory, conservation limits A₂ to at most 0.400 M. The equilibrium value need not reach that maximum, and no rate is given.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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