Reactivity: How Much, How Fast, and How Far?

Every reactivity question in IB Chemistry falls into one of three boxes: extent (Kc, Qc, Le Chatelier), amount (stoichiometry, yield, atom economy) or rate (rate law, order, Arrhenius). These are independent — a reaction can be fast with a tiny equilibrium constant, or thermodynamically favourable yet kinetically dead slow, like diamond turning into graphite. HL adds the numerical machinery to all three: ICE tables for Kc, integrated rate laws, and the linear Arrhenius equation, while SL stays qualitative throughout.

In a closed system, dynamic equilibrium means the forward and reverse reactions run at equal rates, so macroscopic concentrations stop changing — but reactants and products keep interconverting. $K_c$ uses equilibrium concentrations in the general expression, while $Q_c$ uses the same expression at any point in the reaction, so comparing $Q_c$ to $K_c$ tells you which way the system still needs to shift. Solids and pure liquids never appear in the expression because their effective concentration is constant.

This is the HL distinction examiners hunt for: only a change in temperature alters the actual value of $K_c$. Concentration, pressure/volume changes and catalysts can shift the position of equilibrium — which side has more stuff — and change how quickly equilibrium is reached, but the ratio $K_c$ itself stays fixed at constant temperature. Compression or addition questions always have two stages worth separate marks: the instant-after concentrations (before any reaction occurs) and the new equilibrium concentrations after the shift.

Every stoichiometry question follows the same three moves: convert what you're given into moles, use the balanced equation's ratio to jump between species, then convert back into mass, volume or concentration as the question asks. The limiting reagent is whichever reactant runs out first and fixes the maximum possible product — you find it by comparing moles divided by coefficient for each reactant, never by comparing raw starting masses.

Percentage yield compares actual product obtained to the theoretical yield calculated from the limiting reagent, capturing real-world losses like side reactions or purification. Atom economy is a purely theoretical measure comparing the desired product's molar mass to the total molar mass of all reactants, showing how little mass is wasted regardless of practical losses — a reaction can have 100% atom economy and still give a low percentage yield, or vice versa. At STP, the IB data booklet uses $273\text{ K}$ and $10^5\text{ Pa}$, giving a molar volume of $22.7{\text{ dm}}^3{\text{mol}}^{-1}$, not the older 22.4 value.