Equilibrium constants and calculations
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 3 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- A reaction that never finishes
- Dynamic equilibrium
- The Kc expression
- Units
Read the transcript of the teaching part
Last time
Before we start: last time, in Oxidising agents, reducing agents and disproportionation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A reaction that never finishes
Mix hydrogen and iodine in a sealed flask and they react to make hydrogen iodide. But heat pure hydrogen iodide and it breaks back into hydrogen and iodine. If neither direction wins, the flask ends up with a fixed mixture of all three, and a single number, the equilibrium constant, describes it exactly.
Dynamic equilibrium
A reversible reaction in a closed system reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. The concentrations of reactants and products are then constant. Nothing seems to change, but both reactions are still going on. A closed system is needed, since nothing can enter or leave. The reverse rate starts at zero, rises as product builds up, and meets the falling forward rate.
The Kc expression
For a general reaction a A plus b B in equilibrium with c C plus d D, K c equals the concentration of C to the c, times D to the d, over A to the a, times B to the b. Concentrations are equilibrium concentrations in moles per dm cubed. Powers are the coefficients in the balanced equation. Pure solids and liquids are left out of the expression. K c changes only with temperature. It is not changed by concentration, pressure or a catalyst.
Units
Put the units of each concentration into the expression and cancel. For H two plus I two in equilibrium with 2 HI: concentration squared over concentration squared. No units. For N two plus 3 H two in equilibrium with 2 NH three: K c is NH three squared over N two times H two cubed. The units are dm cubed to the six per mole squared. With concentrations 0.20, 0.50 and 1.0, K c is 0.040 over 0.50, which is 0.080.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Calculations with concentrations.
The whole chapter: Equilibria, Equilibria.
Chapter 7: Equilibria
- Equilibrium constants and calculations 7 min
- Le Chatelier's principle and acid-base titration curves 7 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.