The reactant that is completely used up and determines the amount of product.
Theoretical yield
Maximum mass of product calculated from the limiting reactant.
Percentage yield
Actual yield ÷ theoretical yield × 100.
Atom economy
Mass of desired product ÷ total mass of all products × 100.
Titration
A technique to find the concentration of a solution by reacting it with a standard solution.
Method: balanced equation → moles of what you know → mole ratio → moles of what you need → mass, volume or concentration.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Write the mole ratio as a fraction with the coefficients next to the substances, so you multiply the right way round.
R2.2How fast? The rate of chemical change
Key terms
Rate of reaction
Change in concentration of a reactant or product per unit time, in mol dm⁻³ s⁻¹.
Collision theory
Particles must collide with the correct orientation and with energy ≥ Ea to react.
Catalyst
Increases the rate by providing an alternative pathway with lower activation energy, and is not used up.
Maxwell–Boltzmann distribution
The spread of kinetic energies of particles in a sample at a given temperature.
Rate is measured by following mass loss, gas volume, colour change, pH or conductivity over time. The gradient of a tangent gives the instantaneous rate.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: For temperature, always mention energy ≥ Ea; for concentration, pressure and surface area, mention collision frequency.
R2.3How far? The extent of chemical change
Key terms
Dynamic equilibrium
Forward and reverse reactions occur at equal rates, so concentrations stay constant; it needs a closed system.
Equilibrium constant (K)
Ratio of product to reactant concentrations at equilibrium, each raised to its coefficient.
Le Châtelier's principle
If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.
Reaction quotient (Q)
The same expression as K but for any moment, not only at equilibrium.
K ≫ 1: equilibrium lies to the right (mostly products). K ≪ 1: lies to the left. K depends only on temperature.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Only temperature changes the value of K. Say 'position of equilibrium shifts' for other changes.