Five rate factors, each with a direction
Core C6.2: describe (not yet explain) the effect on rate of five factors: (a) concentration of solutions, (b) pressure of gases, (c) surface area of solids, (d) temperature, (e) adding or removing a catalyst. Increasing any of the first four, or adding a catalyst, increases the rate; decreasing them decreases it, and removing a catalyst restores the uncatalysed rate. None of them changes the total amount of product formed from a fixed amount of reactant.
Catalyst — increases rate, itself unchanged
A catalyst increases the rate of a reaction and is chemically unchanged (not used up) at the end — both halves earn the mark. Not being consumed, it never appears as a reactant or product in the balanced symbol equation, and a small amount can be reused indefinitely. (Extended) The mechanism: it decreases the activation energy, Ea, by offering an alternative reaction pathway, so a greater proportion of collisions succeed — particle kinetic energy is unchanged.
(Extended) Collision theory — the four ideas
Collision theory explains rate through four ideas: the number of particles per unit volume, the frequency of collisions, the kinetic energy of the particles, and the activation energy, Ea — the minimum energy colliding particles need in order to react. A collision only reacts if it happens with energy of at least Ea and a suitable orientation — a successful collision. On an energy-profile diagram Ea is the height of the barrier from reactants to the peak.
Drawn from real examiner reports.
Faster rate ≠ more product
A larger particle size (or a lower concentration) decreases the rate but not the amount of product. Powdering a fixed mass of marble chips in excess acid gives the same total volume of , just sooner. On a graph the faster curve is steeper yet levels off at the same height; a different final height means a different amount of reactant.
Jun 2022 P11 Q19
A catalyst is not just "speeds it up"
"A substance that speeds up a reaction" is a half-answer: the mark scheme also requires chemically unchanged (not used up) at the end of the reaction. And do not answer "define a catalyst" with "it lowers the activation energy" — that explains how a catalyst works and is no substitute for either half of the definition.
Catalyst ≠ higher temperature
Both raise the proportion of successful collisions, but by different routes. A catalyst lowers Ea — the energy barrier itself shrinks, and the particles' kinetic energy is unchanged. Raising the temperature raises the particles' kinetic energy, so more of them clear the same, unchanged barrier (and they collide more often as well). Never swap the two mechanisms over.
Rate is change ÷ time, not time ÷ change
Inverting the division turns 45 cm³ in 15 s into 0.33 s/cm³ instead of 3.0 cm³/s. Two more habits cost marks: dropping the unit, and giving the overall average (final volume ÷ final time) when the question named an interval. Write "change in quantity ÷ change in time" with the numbers substituted — that earns the method mark even if the arithmetic slips.
Sealing the flask in the mass-loss method
Mass loss is only measurable if the gas can leave: the conical flask stands directly on the balance, open to the air, with a loose cotton-wool plug in the neck — never an airtight stopper, which traps the gas so the balance reading never falls (and can build dangerous pressure). The gas-collection method is the opposite case: there the bung must be airtight.
Naming a factor without a direction
"Temperature affects the rate", or just "surface area", is not credited. The mark scheme wants the direction of the change and the direction of the rate change: "increasing the temperature increases the rate of reaction", or "decreasing the particle size (increasing the surface area) increases the rate". Never leave either half of that sentence vague.
Jun 2022 P31 Q8biii; Nov 2023 P33 Q8aii
(Extended) "More collisions" is too vague
For the concentration effect, "more particles", "more collisions" and "more crowded" all score nothing on their own. The credited chain: more particles per unit volume -> particles closer together -> collisions happen more frequently (more per second) -> more successful collisions per second -> faster rate. Concentration does not give particles more energy.
Jun 2022 P41 Q11b; Jun 2023 P43 Q2d; Nov 2023 P42 Q8b
Read the shape of a rate graph
Steepest region = fastest rate, usually at the start. Flat and horizontal = the reaction has finished, not "still fast but slowing down". Comparing two runs that start with the same amount of reactant, the faster one is steeper and flattens sooner, at the same final height.
Only straight lines are "proportional"
Use the word "proportional" only for a straight line through the origin — never for a rate curve. Draw a smooth best-fit line rather than joining the points dot-to-dot, and label both axes with the quantity and its unit.
Match the method to what you can measure
Gas from a solid reactant -> mass loss on a balance or gas volume in a syringe. Gas that dissolves in water -> mass loss or a syringe, not collection over water. No gas but a clouding precipitate -> time how long a marked cross takes to disappear; compare rates as 1/time.
Label the gas-collection apparatus fully
Show an airtight bung in the reaction flask, a delivery tube running from inside the flask to inside the collection vessel (a gas syringe, or a measuring cylinder drawn inverted and full of water), and a stopclock started the instant the reactants are mixed.
Cambridge 0654 spec reference: Section C6 "Chemical reactions", sub-topic C6.2 "Rate of reaction". Core: describe the effect of five factors on rate, state the catalyst effect, describe practical rate-investigation methods, interpret rate data/graphs. Extended (Supplement): explain the same five factors using collision theory, state that a catalyst decreases activation energy , describe collision theory.
| Term | Mark-scheme-precise meaning |
|---|---|
| Rate of reaction | How quickly reactants are converted into products; found by measuring how a quantity (mass or volume of gas) changes per unit time |
| Catalyst | A substance that increases the rate of a reaction and is chemically unchanged (not used up) at the end of the reaction |
| Activation energy, | The minimum energy that colliding particles must have in order to react |
| Collision theory | A reaction only occurs when particles collide with energy (and correct orientation); rate depends on the frequency of these successful collisions |
Five factors that increase the rate of reaction (Core: describe with a direction; Extended: explain using collision theory):
| Factor (increasing it...) | Core: effect on rate | Extended: collision-theory reason |
|---|---|---|
| Concentration of a solution | increases the rate | more particles per unit volume -> more frequent collisions |
| Pressure of a gas | increases the rate | gas particles pushed closer together -> more particles per unit volume -> more frequent collisions |
| Surface area of a solid (smaller pieces/powder) | increases the rate | more particles exposed on the surface -> more frequent collisions between reacting particles at the surface |
| Temperature | increases the rate | particles have more kinetic energy -> move faster -> collide more frequently AND a greater proportion of collisions have energy |
| Adding a catalyst | increases the rate | provides an alternative pathway with a lower -> a greater proportion of collisions are successful, without changing particle energy |
Removing a catalyst returns the rate to its original (uncatalysed) value. None of these five factors changes the total amount of product formed from a given amount of reactant -- they change only how quickly that amount is reached.
Define a catalyst.
A student reacts excess marble chips (calcium carbonate) with dilute hydrochloric acid and collects the gas produced in a gas syringe, to investigate the rate of reaction.
Write the balanced symbol equation for this reaction. (2 marks)
