Collision theory
The rate of reaction is how fast reactants turn into products. Particles react only when they collide with at least the activation energy (collision theory). Making collisions more frequent or more energetic speeds a reaction up: higher concentration, higher gas pressure and larger surface area all raise the collision frequency; higher temperature does BOTH — more frequent collisions AND more particles reaching the activation energy (the larger effect).
Catalyst — lower activation energy
A catalyst increases the rate but is chemically unchanged at the end. It works by providing an alternative reaction pathway with a lower activation energy, so a greater proportion of colliding particles have enough energy to react — more collisions are successful. Two precisions: it lowers the activation energy, not the temperature, and it is not used up, so it can be recovered and reused (e.g. manganese(IV) oxide on hydrogen peroxide).
Measuring rate — gradient and tangents
Rate is measured by following a change over time — the volume of gas produced (gas syringe) or the loss of mass as gas escapes (balance). On a graph of product against time the gradient is the rate: steepest at the start and flat once a reactant is used up. Draw a tangent for the rate at a chosen instant. Faster conditions give a steeper initial gradient but, for the same amount of reactant, reach the same final plateau value.
Drawn from real examiner reports.
"Increasing" vs greatest at the start
When a rate curve is flattening, do not say the "rate is increasing" — say the rate is greatest at the start and then falls as reactants are used up. And keep answers comparative: "faster/slower", "more gas per second", not a vague "it fizzes" or "it reacts". Do not confuse "the rate is high" with "the rate is increasing" — they describe different things.
Nov24 Q8b
Missing "more energy" or "successful"
"Explain" answers must use collision theory: more frequent collisions and/or collisions with more energy than the activation energy, giving more successful collisions per second. The recurring dropped marks: omitting that particles have more energy (writing only "move faster"), omitting the word successful, and never stating that the rate increases.
Nov24 Q8d; Jun23 Q10bi
Catalyst: give the mechanism
Saying a catalyst "speeds up the reaction" is the claim, not the mechanism, and scores nothing on an "explain". State the mechanism: it provides an alternative pathway with a lower activation energy, so more colliding particles have enough energy to react. Also, a catalyst is chemically unchanged / not used up — "used up" or "lowers the temperature needed" is wrong.
Nov24 Q5b; Jun24 1CR Q10a; Jun23 Q10c
Mass loss = CO₂ leaves the flask
In a mass-loss experiment the balance reading falls because carbon dioxide gas is produced and leaves the open flask. "CO₂ given off" without saying it left the flask does not score. The ideas that the chips shrinking lower the reading, or that "HCl escaped", are wrong and score zero — it is the escaping gas that lowers the total mass.
Nov24 Q8a
"Most energy at the start" is wrong
The rate is greatest at the start because the concentration is highest, so collisions are most frequent then — NOT because "the most energy is available at the start". The particles' energy (temperature) does not change during the run; it is the collision frequency that falls as reactant is used up. The energy misconception loses both marks.
Nov24 Q8b
Rate (gradient) vs amount (plateau)
A faster reaction has a steeper gradient, but if the same amount of reactant is used it still reaches the same final volume of gas — just sooner. Changing the amount is what changes the final volume: halving the acid halves the final gas volume. Do not confuse how FAST (gradient) with how MUCH (plateau).
Jun23 Q10bii
Draw the tangent at the right time
To find the rate at a given time, draw a tangent to the curve at exactly that time and take its slope (change in ÷ change in ). Do not divide a total reading by the total time — that gives an average, not the instantaneous rate, and a tangent at the wrong time earns nothing.
Give the rate a unit
Always attach a unit to a rate, e.g. or . Abbreviating minutes as "m" is not accepted — write "min" or "s". An unlabelled number, or "cm³/m", loses the unit mark.
Name the apparatus precisely
Name equipment exactly: a gas syringe collects and measures the gas volume; the reaction happens in a conical flask, not a "beaker". Spelling and the correct vessel both matter — a mislabelled flask loses the apparatus mark.
Average rate over a time interval:
What each change does, in collision-theory terms:
| Factor increased | Why collisions change | Result |
|---|---|---|
| Concentration (solution) | More particles per unit volume → more frequent collisions | Faster |
| Pressure (gases) | Particles squeezed closer → more frequent collisions | Faster |
| Surface area (smaller solid pieces) | More particles exposed at the surface → more frequent collisions | Faster |
| Temperature | Particles move faster (more frequent) and more particles have energy activation energy (more energetic) | Faster |
| Catalyst added | Lower activation energy → a greater proportion of collisions are successful | Faster |
Define activation energy.
In an experiment, magnesium reacts with dilute hydrochloric acid. A gas syringe collects of hydrogen in the first seconds.
Calculate the average rate of reaction over this interval, in .
