React it.
Speed a reaction up and watch the gas curve bend, drip alkali into acid and walk the pH ladder, then bend a reaction profile until the catalyst earns its keep.
Marble chips, and a stopwatch
FIG. 01 · RATES · 4.6.1.1CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) · 10.0 cm³ acid · chips in excess
More particles in the same volume, so more collisions each second — and more gas in the end, because the acid runs out last.
Roughly doubles the rate for every 10 °C.
The dashed line is the chord from the origin — its gradient is the mean rate.
Why it is marked: mean rate = change in volume ÷ time taken, which is the gradient of that dashed chord. Concentration, temperature and surface area all make the curve steeper at the start; only concentration changes where it flattens out, because only concentration changes how much acid there is to react. Getting that distinction right is usually worth the last mark.
Drip the alkali in
FIG. 02 · CHEMICAL CHANGES · 4.4.2.4pH ladder · the lit box is where the flask is right now
Same concentration, different pH. A weak acid only partly ionises, so it releases fewer H⁺ ions.
H⁺(aq) + OH⁻(aq) → H₂O(l)
Why it is marked: neutralisation is always the same ionic equation — H⁺ meets OH⁻ and makes water. The pH scale is a ×10 scale: drop one pH unit and the hydrogen-ion concentration goes up ten times, which is why the curve barely moves for the first 20 cm³ and then falls off a cliff. The salt you would name here is sodium chloride: acid + alkali → salt + water.
Over the hill
FIG. 03 · ENERGY CHANGES · 4.5.1.2Products end up lower than the reactants — energy leaves the chemicals, so the surroundings heat up.
The minimum energy a colliding pair needs before anything happens.
Why it is marked: label the profile properly and the marks are free — activation energy is measured from the reactants up to the peak, the overall energy change from reactants to products. A catalyst gives a different route with a lower hill, so the dashed peak drops; the reactant and product levels do not move, so ΔH is unchanged. Breaking bonds takes energy in, making bonds gives energy out — which way ΔH points just says which of those two won.