Rate of Reaction and the Factors That Affect It

Rates is the most heavily examined part of Section 6: a structured 6-8 mark question on Paper 3 or 4 most series, plus a strong presence on the alternative-to-practical Paper 6. The question formula barely changes: a graph of gas volume against time, two or three curves, “compare and explain”. Marks are lost not on the chemistry but on graph language: “curve B is faster” without quoting the gradient, or “more collisions” without the per-second.

What rate means and how it is measured (Core)

Rate of reaction measures how quickly reactants are used up or products form. Two standard practical methods, both based on a gas being produced (for example marble chips in hydrochloric acid):

• Gas syringe: record the volume of gas collected every 30 seconds.
• Mass loss: stand the flask on a balance with a cotton-wool plug and record the decreasing mass as CO2 escapes.

For reactions producing a precipitate, the disappearing-cross method works: time how long until a cross under the flask becomes invisible (sodium thiosulfate + acid is the stock example). Shorter time = faster rate.

On a graph of product against time, the rate at any moment is the gradient. Steep at the start (reactants most concentrated), shallowing as reactants are used up, flat when the reaction finishes because one reactant has run out. The flat level marks the amount of product; the slope marks the rate. Separating those two readings is half the marks in any graph question.

The four factors (Core)

Factor	Change that speeds the reaction	Everyday exam example
Concentration	Increase it (pressure, for gases)	More concentrated acid on magnesium
Temperature	Increase it	Warm acid vs cold acid
Surface area	Smaller pieces / powder	Powdered marble vs lumps
Catalyst	Add one	Manganese(IV) oxide decomposing hydrogen peroxide

A catalyst increases the rate without being used up (its mass is unchanged at the end) and it works by providing an alternative pathway with a lower activation energy. Enzymes are biological catalysts. Note what a catalyst does not do: it does not increase the amount of product, so the catalysed curve rises more steeply but flattens at the same final volume.

The same point applies to concentration and surface area when the other reactant is limiting: they change the slope, not necessarily the plateau. Only changing the amount of the limiting reactant raises the plateau, a distinction that decides 2 marks in most graph questions.

Why each factor works (Supplement)

The particle-level explanations belong to Collision Theory, but the headlines: more concentrated solutions have more particles per unit volume, so collisions are more frequent; smaller pieces expose more surface, so more collisions per second; higher temperature makes particles move faster, so collisions are both more frequent and more energetic (two marking points, not one); catalysts lower the activation energy so a greater proportion of collisions succeed. The energy-diagram version of the catalyst story is drawn in Energy Level Diagrams.

Reading and sketching the curves

Asked to sketch the curve for a faster condition, draw it steeper at the start and flattening earlier, at the same final volume if the amounts of reactants are unchanged. Asked to compare curves, use three ticks: initial gradient (rate), time to flatten (duration), final level (amount of product). Quote numbers off the axes wherever the grid allows: “curve A reaches 60 cm³ in 2 minutes; curve B takes 5 minutes.”

Worked exam question

Excess marble chips react with 50 cm³ of dilute hydrochloric acid. The volume of CO2 is recorded against time (curve X). The experiment is repeated with the same mass of powdered marble (curve Y). (a) State two differences between curve Y and curve X. [2] (b) Explain why the final volume of gas is the same in both experiments. [1] (c) Describe how the rate of reaction changes during experiment X, and explain why. [2]

Model answer: (a) Curve Y is steeper at the start (1) and levels off sooner, at the same final volume (1). (b) The same amount of acid is used and the acid is the limiting reactant, so the same amount of CO2 is produced (1). (c) The rate is fastest at the start and decreases over time (1) because the acid is used up, so its concentration falls (1).

Mark-by-mark: (a) needs both the gradient point and the levelling point. “Y is faster” alone is one mark at best. (b) hinges on identifying the limiting reactant; “same amount of chemicals” is too loose. (c) splits description (rate decreases) from explanation (concentration of acid falls); giving only one half is the classic 1-out-of-2.

The mistakes that cost marks

1. Saying a catalyst or powder gives “more gas”. They give the same gas, faster. Only more limiting reactant gives more gas.
2. “The reaction stops because the reaction is finished.” Circular. It stops because a named reactant has been completely used up.
3. Describing graphs without numbers. Quote volumes and times from the axes. Comparative marks usually need data.
4. Treating rate and time as the same direction. A faster rate means a shorter time; in disappearing-cross questions, the fastest run has the smallest time.

How examiners want it phrased

Student wording	Mark-scheme wording
”The line is steeper so it’s quicker"	"The initial gradient is greater, so the initial rate is higher"
"It runs out of stuff"	"The hydrochloric acid is used up; it is the limiting reactant"
"The catalyst helps it along"	"The catalyst provides an alternative pathway with lower activation energy and is not used up"
"Powder reacts better"	"Powder has a larger surface area, so collisions are more frequent”

The full Section 6 map is on the Chemical Reactions pillar. Rates questions reward rehearsed structure more than inspiration (gradient, plateau, limiting reactant, collision phrase), and rehearsing that structure on real Paper 4 and Paper 6 questions is precisely what happens in a free 1-hour trial lesson with one of our Chemistry specialists.

Test yourself

Sketch or write each answer first; the mark scheme is one click away.

Q1 (2 marks). Magnesium ribbon reacts with dilute hydrochloric acid, producing hydrogen. Describe a method to follow the rate of this reaction.

Show answer

• collect the hydrogen in a gas syringe and record the volume at regular time intervals (e.g. every 30 s) [1] • plot volume against time; the steeper the gradient, the faster the rate [1]

Q2 (3 marks). An experiment with marble chips and excess dilute hydrochloric acid is repeated with the acid at a higher temperature, all amounts unchanged. State two differences in the volume–time curve and state the effect on the final volume of gas.

Show answer

• the curve is steeper at the start (higher initial rate) [1] • it levels off sooner (reaction finishes earlier) [1] • the final volume is the same, because the same amount of the limiting reactant is used [1]

Q3 (2 marks). Manganese(IV) oxide is added to hydrogen peroxide solution. State what is meant by the term catalyst.

Show answer

• a substance that increases the rate of a reaction [1] • and is unchanged / not used up at the end of the reaction [1]