IB Chemistry Reactivity 2: How much, how fast and how far?

What Reactivity 2 covers

Reactivity 2 asks three questions about a reaction. How much? R2.1 is stoichiometry: balanced equations and mole ratios, limiting and excess reactants, theoretical and percentage yield, atom economy, and reacting volumes of gases and solutions. How fast? R2.2 is collision theory, the Maxwell–Boltzmann distribution, the factors that change the rate and how a catalyst works; at higher level, rate equations and orders of reaction, the rate constant, reaction mechanisms and the rate-determining step, and the Arrhenius equation. How far? R2.3 is dynamic equilibrium, the equilibrium constant and Le Chatelier's principle; at higher level, the reaction quotient, equilibrium calculations, and the link between ΔG⦵ and K.

Where students slip

• The limiting reactant is not always the one with fewer moles. Divide each amount by its coefficient in the equation first: 3 mol of H₂ runs out before 2 mol of N₂.
• A rise in temperature speeds up a reaction mainly because a larger fraction of collisions have at least the activation energy, not just because the particles collide more often.
• A catalyst gets a system to equilibrium sooner but doesn't move the position of equilibrium. Only a change of temperature changes the value of K.