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Chemistry · 09 / 10

Chemical equilibrium: stable composition, active reactions

Final-year concepts · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Units: numbers have dimensionsThe mole: connect mass and particlesBalance a reaction equation
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

At equilibrium, forward and reverse reactions continue at equal rates, giving stable bulk composition. Reaction quotient Q uses activities and stoichiometric coefficients. Comparing it with K at a given temperature gives net evolution direction. K depends on temperature, not merely introduced amounts.

THE PICTURE TO REMEMBERChemical equilibrium: stable composition, active reactions
  1. 01Express Q
  2. 02Temperature and K
  3. 03Compare
  4. 04Direction then equilibrium

WORKED EXAMPLE

From principle to reasoning

For A ⇌ B, Q=a(B)/a(A). Q<K favours net B formation; Q=K gives no net change despite microscopic exchange.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

Q=2 and K=5 for A ⇌ B. Which direction is favoured?

One hint

Evolution brings Q towards K.

Reveal and understand the solution

Forward reaction forms B and consumes A under the model conditions.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

Do forward and reverse reactions stop at equilibrium?

A hint to get started

Chemical equilibrium is dynamic.

Step-by-step solution
  1. Both transformations continue.
  2. Their rates are equal.
  3. Macroscopic amounts stay constant despite ongoing microscopic reaction.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

For A ⇌ B with activities aB = 0.6 and aA = 0.2, find Qr.

A hint to get started

Use dimensionless activities.

Step-by-step solution
  1. Qr = aB/aA.
  2. Qr = 0.6/0.2 = 3.
  3. In sufficiently dilute solution, activity may be approximated by c/c°.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

For A ⇌ B with Qr = 3 and K = 5, which way does the system evolve?

A hint to get started

Evolution brings Qr toward K.

Step-by-step solution
  1. Qr < K means the product/reactant ratio is too low.
  2. Net evolution is forward, toward B.
  3. At equilibrium Qr = K; K depends on temperature.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Equilibrium does not mean equal concentrations or stopped reactions.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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