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Chemistry · 5. Rates and equilibrium

Reaction rates and dynamic equilibrium

Explain rates with collisions and predict equilibrium shifts with proper limits.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How do conditions affect rate and equilibrium composition differently?

Curriculum coverage

Vietnam Chemistry 10–11; IGCSE reactions; AS/A Level equilibrium.

Model scope and limitations

Le Châtelier predicts direction, not quantitative extent, and does not replace equilibrium constants. The simulated Fe³⁺–SCN⁻ colour uses a simplified dominant-complex model; real solution speciation also depends on concentration and ionic strength.

Learning objectives

  • Explain concentration, temperature, surface area and catalyst effects.
  • Distinguish equilibrium from a stopped reaction.
  • Predict shifts from concentration, pressure or temperature changes.

Prerequisite knowledge

  • Equations and activation energy.

Core knowledge

Successful collisions

Particles must collide with enough energy and suitable orientation.

Catalyst

A catalyst lowers Ea for both directions and does not change equilibrium position.

Dynamic equilibrium

In a closed system, forward and reverse rates are equal; concentrations stay constant while reactions continue.

Worked example

For exothermic A ⇌ B + heat, heating favours the endothermic reverse direction, shifting toward A; both rates initially increase.

Misconceptions to avoid

  • A catalyst does not increase equilibrium yield.
  • Equilibrium does not mean equal reactant and product concentrations.

Virtual experiment procedure: Fe³⁺–SCN⁻ equilibrium

  1. Establish equilibrium and record the initial colour.
  2. Change one condition at a time: add Fe³⁺, SCN⁻ or dilute.
  3. Use a control for colour comparison and explain using complex concentration.

Safety and cautions

  • Visual colour is not precise quantitative measurement; real metal-salt waste needs proper handling.

Evidence to collect

  • A change–observation–shift–explanation table.
Open the virtual experiment

Diagnostic check

1. At dynamic equilibrium, which is true?

  1. Forward and reverse rates are equal
  2. Reaction stops
  3. Both concentrations are equal
Show answer and explanation

Correct answer: Forward and reverse rates are equal

Both reactions continue at equal rates.

2. What does a catalyst do to equilibrium position?

  1. No change
  2. Always shifts right
  3. Always shifts left
Show answer and explanation

Correct answer: No change

It speeds attainment but does not change K.

3. Increasing solid surface area usually does what to rate?

  1. Increase
  2. Decrease
  3. Never changes
Show answer and explanation

Correct answer: Increase

More exposed sites give more collisions per second.

Application practice

1. Heating lets more particles exceed Ea, so?

  1. Successful collision frequency rises
  2. Ea rises
  3. Every equilibrium shifts right
Show answer and explanation

Correct answer: Successful collision frequency rises

The energy distribution extends to higher energies.

2. For a gas equilibrium with unequal gas mole counts, which side does reduced volume favour?

  1. Fewer gas moles
  2. More gas moles
  3. Always reactants
Show answer and explanation

Correct answer: Fewer gas moles

At constant temperature, the pressure increase from reduced volume shifts equilibrium toward fewer gas moles.

3. For a fair temperature-rate test, what should be controlled?

  1. Concentration and volume
  2. Temperature
  3. Reaction time zero
Show answer and explanation

Correct answer: Concentration and volume

Temperature should be the only independent variable.

Academic references

  1. OpenStax Chemistry 2e — peer-reviewed general chemistry
  2. IUPAC Gold Book — authoritative chemical terminology
  3. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer