Chemical equilibria
Learn H1 Chemistry dynamic equilibrium, predict shifts with Le Chatelier’s principle and practise Kc calculations using the Haber process as a case study.
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Before you begin
Many reactions do not go to completion: they reach a dynamic equilibrium in which the forward and reverse reactions continue at the same rate. This topic explains what equilibrium is, predicts how it responds to changes with Le Chatelier’s principle, describes the equilibrium mixture with Kc, calculates equilibrium concentrations and applies the ideas to the Haber process.
Be comfortable with: solution concentration, enthalpy changes and energy profiles, activation energy and the Boltzmann distribution, and rearranging simple equations.
Learning goals
- Explain reversible reactions and dynamic equilibrium in terms of forward and reverse reaction rates
- State Le Chatelier's Principle and use it to predict how concentration, pressure or temperature changes affect a system at equilibrium
- Deduce whether concentration, pressure, temperature or a catalyst changes the value of an equilibrium constant
- Deduce the expression for the equilibrium constant Kc
- Calculate Kc from equilibrium data
- Calculate the amounts present at equilibrium from given data
- Describe and explain the conditions of the Haber process as an example of equilibrium in industry
Syllabus statements covered
- explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium
- state Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium
- deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction
- deduce expressions for equilibrium constants in terms of concentrations, Kc
- calculate the values of equilibrium constants in terms of concentrations from appropriate data
- calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations)
- describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry
Lessons
Work through them in order.
- Dynamic equilibriumExplain dynamic equilibrium for H1 Chemistry using equal reaction rates, concentration changes and Le Chatelier’s principle.
- Le Chatelier’s principlePredict a controlled equilibrium response and distinguish composition changes from changes in Kc.
- Equilibrium constant KcWrite Kc expressions with the correct units, omit pure solids and liquids, and use Kc to interpret equilibrium position and temperature effects.
- Kc equilibrium calculationsUse ICE tables with the equation's coefficients to calculate Kc and equilibrium amounts.
- The Haber processBalance equilibrium yield, reaction rate, safety and industrial cost.
Practise and check
Or choose
Topic reference
Quick reference
| Question | First move | What to show |
|---|---|---|
| Effect of a concentration change | decide which reaction uses up the added substance or replaces the removed one | direction of shift and the effect on the named species; Kc unchanged |
| Effect of a pressure change | count the moles of gas on each side | higher pressure favours fewer moles of gas; no shift if equal |
| Effect of a temperature change | find the endothermic direction from Δ H | higher temperature favours the endothermic direction; Kc changes |
| Effect of a catalyst | both reactions speed up equally | equilibrium reached sooner; position and Kc unchanged |
| Write Kc | products over reactants, coefficients as powers | leave out pure solids and liquids; work out the units |
| Find an equilibrium composition | ICE table with changes in the ratio of the coefficients | substitute the equilibrium row only; check the answer in Kc |
Core knowledge to remember
- Reversible reaction: a reaction that can go in both directions under the same conditions.
- Dynamic equilibrium: the rates of the forward and reverse reactions are equal, so the concentrations stay constant; both reactions continue. It needs a closed system.
- Le Chatelier’s principle: if a change is made to a system in dynamic equilibrium, the position of equilibrium moves to oppose the change.
- Equilibrium constant: for aA + bB < = > cC + dD, Kc = [C]^c[D]^d/[A]^a[B]^b using equilibrium concentrations.
- For a given equation, Kc depends only on temperature. Concentration, pressure and catalysts do not change it.
- The Haber process runs at about 200 atm and 450 °C with an iron catalyst: a compromise between yield, rate and cost.
Common mistakes
- Saying the reactions stop at equilibrium, or that the concentrations of reactants and products are equal.
- Counting solids, liquids or aqueous species when predicting the effect of pressure.
- Saying that a catalyst increases the yield, or that changing a concentration changes Kc.
- Taking the powers in Kc from formula subscripts instead of equation coefficients.
- Substituting initial values instead of equilibrium values into Kc.
- Using the same change for every species in an ICE table when the coefficients differ.
- Leaving out the units of Kc.