IB Chemistry SLTopic 5 — The Rate of ReactionPaper 1 & 2Practical skill~13 min read
Measuring Reaction Rates
You cannot watch concentration directly. What you can do is find something that changes with concentration — a volume, a mass, a colour — and follow that instead. Choosing the right something is most of the skill.
📚 What you need to know
Any method works by tracking a property proportional to concentration as it changes with time.
Gas volume: a gas syringe works for any gas; collection over water only for gases of low solubility.
Mass loss: needs a gas dense enough to register on the balance — fine for CO2, useless for H2.
Colorimetry: needs a coloured species in solution, and fails with precipitates, which scatter the light.
Titration requires quenching samples, so it is discontinuous.
Clock reactions time one fixed visible change, giving one data point per run, with rate ∝ 1/t.
Continuous or one-off?
Methods divide into two families. Continuous methods let you take readings throughout, giving a full curve you can draw tangents on. Discontinuous methods give you a single number per experiment — usually the time to reach a fixed point — and you compare that across several runs.
Collecting a gas
Both give a volume against time curve. The syringe is more versatile; collection over water is cheaper and copes with larger volumes.
If the reaction produces a gas, measuring its volume is usually the easiest route. Start the clock the moment the reactants meet, and read the volume at fixed intervals.
The limitation to remember is solubility. Hydrogen and carbon dioxide can be collected over water without much loss, but a soluble gas such as ammonia or hydrogen chloride would simply dissolve and never reach the cylinder.
Following the mass
The cotton wool is not decoration. Without it, spray from a vigorous reaction escapes and the mass loss no longer measures only the gas.
When a gas escapes, the flask gets lighter, and a balance reading every few seconds gives you the data directly. The catch is sensitivity: the change in mass has to be big enough for the balance to see.
Compare the two gases. One mole of CO2 weighs about 44 g; one mole of H2 weighs about 2 g. The same amount of reaction produces twenty-two times less mass change with hydrogen, which is why hydrogen reactions are followed by volume and carbonate reactions by mass.
WORKED EXAMPLE
A flask containing calcium carbonate and hydrochloric acid loses 0.44 g in the first 50 s. Calculate the rate of reaction in g s–1 and in mol s–1 of CO2. (Mr CO2 = 44.01)
Step 1 — rate as a mass change0.44 ÷ 50 = 8.8 × 10⁻³ g s⁻¹Step 2 — convert the mass lost to molesn = 0.44 ÷ 44.01 = 0.0100 molStep 3 — divide by time0.0100 ÷ 50 = 2.0 × 10⁻⁴2.0 × 10⁻⁴ mol s⁻¹All the mass lost is carbon dioxide, so the mass change converts straight to moles of gas — and from there, via the equation, to moles of carbonate consumed.
The other continuous methods
Method
What it follows
Use it when
Watch out for
Colorimetry
Light absorbed or transmitted by a coloured species
A reactant or product is coloured, e.g. iodine or a transition metal ion
Useless if a precipitate forms — it scatters light rather than absorbing it
Conductivity
Electrical conductivity of the mixture
The number or charge of the ions present changes
No use if the ion count barely changes
Titration
Concentration of a sample, measured directly
Nothing observable changes during the reaction
Samples must be quenched; discontinuous and laborious
Change in pH
H+ concentration
An acid is used up or produced
pH is logarithmic, so the response is not linear in concentration
Quenching is worth understanding rather than memorising. Taking a sample and titrating it takes minutes, during which the reaction would carry on and spoil the reading — so the sample is frozen in time, usually by rapid cooling or dilution, before it is analysed.
Clock reactions
Judging when the cross has “gone” is subjective, so the same person should judge every run in the same lighting.
Sometimes it is far easier to time one clearly visible event than to monitor anything continuously. In the thiosulfate reaction a precipitate of sulfur gradually clouds the solution until a cross beneath the flask can no longer be seen; you simply time it.
Rate from a clock reaction
rate ∝ 1 / t
The logic is that the same fixed amount of product forms each time, so a shorter time means a faster rate. Plotting 1/t against concentration or temperature gives a graph that behaves like a rate.
WORKED EXAMPLE
In a disappearing-cross experiment, 1.0 mol dm–3 thiosulfate takes 42 s and 2.0 mol dm–3 takes 21 s. Use 1/t to compare the rates and comment.
Step 1 — calculate 1/t1 ÷ 42 = 0.024 s⁻¹1 ÷ 21 = 0.048 s⁻¹Step 2 — comparedoubling the concentration doubles the rateA single run tells you almost nothing. The method only becomes useful when several runs are compared, which is exactly its main limitation.
WORKED EXAMPLE
Suggest, with a reason, the best method of following each reaction: (a) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (b) CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) (c) the fading of a purple manganate(VII) solution as it is reduced
(a) magnesium and acidgas syringeHydrogen is far too light for mass loss, but it has low solubility so any gas collection works.(b) carbonate and acidmass loss on a balanceCO₂ is dense enough to give a measurable change, and no bung or syringe is needed.(c) manganate(VII) fadingcolorimetryNo gas at all, but a strong colour that fades in proportion to concentration — and no precipitate to scatter the light.
💡 Exam tip
Justify a method by naming what changes and why it is measurable — “a gas is produced” alone is not enough.
State the limitation the examiner is fishing for: soluble gases, light gases, precipitates.
Mention controlling the variables: same volumes, same concentrations, same temperature, and start timing on mixing.
For clock reactions, remember rate ∝ 1/t and that judging the endpoint is subjective.
Continuous methods give a curve you can take tangents from; clock methods give one point per run.
⚠️ Common mix-up
Choosing mass loss for a hydrogen reaction. The change is too small to measure.
Collecting a soluble gas over water and wondering where it went.
Using colorimetry on a precipitate. Scattered light is not absorbed light.
Plotting t instead of 1/t for a clock reaction, which gives a curve going the wrong way.
Forgetting to quench a titration sample, so the reaction continues while you analyse it.
Up next: Collision Theory — the model that explains why any of these rates are what they are, and why almost every collision achieves nothing at all.
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