Equilibrium in physical and chemical processes

Many processes do not go to completion. They reach a point where the amounts stop changing: equilibrium. At equilibrium, the forward and backward processes are still going on, but at equal rates, so measurable properties (temperature, pressure, concentration, colour) stay constant.

Physical equilibria (no chemical change):

• Solid ⇌ liquid: ice and water together at 273 K and 1 atm. Ice keeps melting and water keeps freezing, at the same rate, so the amounts stay the same.
• Liquid ⇌ vapour: water in a closed container. Molecules keep evaporating and condensing; once the rates are equal, the pressure of the vapour stays constant. This equilibrium vapour pressure depends only on the temperature, not on the amount of liquid or the size of the container.
• Solid ⇌ solution: in a saturated solution, solute keeps dissolving and coming back out at equal rates, so there is always undissolved solute in contact with the solution.
• Gas ⇌ solution: the amount of gas that dissolves in a liquid is proportional to the pressure of the gas above it (Henry's law). Soda drinks are bottled under high pressure of CO₂. Open the bottle, the pressure falls, and the gas bubbles out.

Important features of equilibrium:

• It can be reached only in a closed system; in an open container, vapour or gas would escape.
• It is dynamic, not static: molecules keep changing, but the opposite changes balance. Experiments with radioactive isotopes prove this: labelled atoms end up spread through both reactants and products.
• The same equilibrium is reached whether you start from the reactants or from the products. For H₂ + I₂ ⇌ 2HI, starting with H₂ and I₂, or with pure HI, gives the same final mixture.