Equilibrium

Lesson

Part 1 — when forward and reverse changes go on at the same rate, and how to predict and control where that balance lies.

In this lesson you will

Describe physical and chemical equilibria as dynamic processes
Write Kc and Kp, relate them by Kp = Kc(RT)^Δn, and manipulate them
Use Q to predict direction and an ICE table to find equilibrium concentrations
Predict the effect of concentration, pressure, temperature, catalyst and inert gas on equilibrium

Table of everyday equilibria with forward and reverse changes
System	Forward ⇌ reverse	Where it matters
Water in a closed bottle	Evaporation ⇌ condensation	Humidity
Ammonia synthesis	$\ce{N2 + 3H2 <=> 2NH3}$	Fertiliser
Dissolved carbon dioxide	$\ce{CO2(g) <=> CO2(aq)}$	Fizzy drinks
Haemoglobin and oxygen	Binding ⇌ release	Breathing

Concept

Physical equilibria

In a closed vessel a physical change can reach equilibrium when two opposing processes occur at equal rates. Each type has a fixed measurable quantity at a given temperature.

Table of physical equilibria with their conditions and the quantity that stays constant
Equilibrium	Condition	Constant at fixed T
Solid ⇌ liquid	Melting point, 1 atm	Both phases coexist
Liquid ⇌ vapour	Closed vessel	Vapour pressure
Solid ⇌ vapour	Closed vessel	Sublimation pressure of the solid
Solute(s) ⇌ solute(aq)	Saturated solution	Solubility
Gas ⇌ gas in liquid	Closed bottle	$\text{Solubility} \propto p$ (Henry)

Concept

Dynamic nature of chemical equilibrium

At equilibrium the rate of the forward reaction equals the rate of the reverse reaction; the concentrations stay constant but both reactions continue. The same state is reached from either direction.

Table of characteristics of chemical equilibrium
Feature	Meaning
Dynamic	Reactions continue in both directions
Constant macroscopic properties	Concentration, colour, pressure stay fixed
Reached from either side	Same K from reactants or from products
Closed system needed	No matter exchanged with the surroundings
Catalyst	Speeds up arrival; does not change the position

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