Edexcel International A Level Chemistry

Topic 13: Chemical Equilibria

The equilibrium constant turns the Unit 2 rule about which way an equilibrium shifts into a single number. You write it, calculate it, and learn that only temperature changes it, because of entropy.

The one idea

In Unit 2 you could say which way an equilibrium shifts when you change something. You could not say how far the reaction goes, or what stays the same. This topic gives every equilibrium one number, the equilibrium constant, $K$.

$K$ comes from the concentrations (or partial pressures) at equilibrium, each raised to the power of its coefficient. At a fixed temperature it is a constant. Squeezing the mixture, adding a reactant or adding a catalyst does not change it. A change in concentration or pressure knocks the mixture away from $K$, and the mixture moves back until it fits $K$ again.

Temperature is the one thing that changes $K$, and the position of equilibrium then moves to match the new value. Entropy explains why: $\Delta S_\text{total} = R \ln K$, and the surroundings term $-\Delta H/T$ is the part that depends on temperature.

The four jobs

What you need first

• Dynamic equilibrium and Le Chatelier's principle from the Unit 2 kinetics and equilibria topic. This topic extends them.
• Amount of substance: moles, concentration in mol dm⁻³, and gas calculations.
• Entropy: $\Delta S_\text{system}$, $\Delta S_\text{surroundings} = -\Delta H/T$ and $\Delta S_\text{total}$ from the entropy topic, for the last two pages.
• Titration, for the experiment on page 6.

Values used in these notes

Reaction data such as $\Delta H$ and $\Delta S_\text{system}$ are given in the worked examples as they would be in an exam question. Where the data are invented, they are labelled illustrative.

How to use these notes

• Read the pages in order the first time. Later pages assume the earlier ones.
• Each page opens with its one idea in a sentence. If you can explain that sentence to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Pages

1. Writing Kc

   At a fixed temperature the equilibrium concentrations of a reaction always fit one expression, and the value of that expression is the equilibrium constant Kc.

   12 min 13.1

2. Partial pressures and Kp

   For gases it is often easier to use pressures than concentrations. Each gas contributes a partial pressure, found from its mole fraction, and Kp has the same shape as Kc with partial pressures in atm.

   13 min 13.2

3. Heterogeneous systems

   When a system has more than one phase, pure solids and pure liquids are left out of Kc and Kp, and Kp contains gases only. Check the state symbols first.

   12 min 13.113.2

4. Units of K

   Kc and Kp have units that depend on the equation. Put the units into the expression and cancel, or count the coefficients. When everything cancels there are no units.

   10 min 13.3

5. Kc from equilibrium amounts

   To calculate Kc, build a table from the starting amounts to the equilibrium amounts, convert to concentrations by dividing by the volume, then substitute. Use equilibrium values only.

   15 min 13.3

6. Kc from a titration

   A classic experiment finds Kc by letting an ester and water reach equilibrium and then titrating the acid that formed. Subtract the blank for the acid catalyst, then use the amounts in the expression.

   14 min 13.3

7. Calculating Kp

   For Kp, extend the table by two rows. Total the moles of gas, turn each amount into a mole fraction, multiply by the total pressure, then substitute the partial pressures.

   15 min 13.3

8. What changes K

   At one temperature K is a fixed number. Changing a concentration, the pressure or adding a catalyst cannot change it. A change in concentration moves the mixture until the expression equals K again.

   13 min 13.413.5

9. Pressure and catalysts

   Pressure moves the mixture when the moles of gas differ on each side, but it leaves Kp alone. A catalyst changes neither the mixture nor K, only how fast equilibrium is reached. Solids make no difference.

   15 min 13.413.5

10. Temperature changes K

    Heating lowers K for an exothermic reaction and raises it for an endothermic one. The position of equilibrium moves because K has changed, so always explain a temperature effect through K.

    14 min 13.413.613.7

11. Entropy and K

    The temperature dependence of K comes from entropy. Only the surroundings term, minus delta H over T, changes with temperature, and ΔS total = R ln K turns that change into a change in K.

    15 min 13.8(i)13.8(ii)

12. How far does it go

    The size of K says how far a reaction goes, not how fast. A large K means mostly products, a small K mostly reactants, and K near 1 a real mixture. The page ends by explaining one tube in three ways.

    15 min 13.413.713.9

Also in this topic: Topic 13 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.