IB Chemistry SL & HLTopic 5 — The Rate of Chemical ChangePaper 1 & 2Practical skill~11 min read
Measuring Reaction Rates
You cannot see molecules colliding, so you never measure rate directly. Instead you pick something about the mixture that changes as the reaction runs — a volume, a mass, a colour — and track that. The skill being tested is choosing the right thing to track.
📘 What you need to know
Every method works by following a property that changes over time and is proportional to how far the reaction has gone.
Gas volume (gas syringe or inverted measuring cylinder) — for reactions that give off a gas.
Mass loss (open flask on a balance) — only if the escaping gas is heavy enough to register.
Colorimetry — if the mixture changes colour. Will not work with precipitates.
Conductivity — if the number or type of ions changes.
Titration with quenching — take samples, stop them reacting, then titrate.
Clock reactions give one time for a fixed visible change, so rate is measured as 1 ÷ time.
Continuous or one-off?
There are two very different styles of experiment, and knowing which one you are looking at tells you what kind of answer is expected.
Continuous methods let you take readings all the way through — every 10 seconds, say. You end up with a full set of data, so you can plot a curve and take tangents. Gas volume, mass loss, colorimetry and conductivity all work this way.
One-off methods give you a single number: the time taken to reach some obvious point. A clock reaction is the classic example. You get one piece of data per run, so you compare runs rather than plotting a curve from one.
Neither style is “better”. Continuous methods give richer data; one-off methods are quick, cheap and often more reliable when the reaction is fast. Examiners like asking you to weigh up both.
Following a gas
If a reaction fizzes, you are in luck — gases are easy to follow. You have two obvious options, and they measure opposite things.
The syringe flask must be sealed or the gas escapes and you measure nothing. The balance flask must be open, or the mass never changes. Opposite requirements — do not mix them up.
Why the cotton wool? It lets the gas out but stops droplets of the mixture spraying out as the reaction fizzes. Lose liquid and your mass drop is too big, so your rate comes out too high.
The inverted cylinder alternative
Instead of a syringe you can push the gas under water into an upturned measuring cylinder and read the volume of water pushed out. Cheap and easy, but it only works for gases that do not dissolve much in water. Hydrogen and oxygen are fine. Carbon dioxide and ammonia will partly dissolve, so your readings come out low.
Following a colour
A colorimeter shines light of one colour through the mixture and measures how much gets through. If a coloured reactant is being used up, more light gets through as time passes. If a coloured product is forming, less does.
The filter matters. You choose the colour of light that the reacting species absorbs most strongly, which makes the signal as large as possible.
Following the ions
Ions carry current, so if a reaction changes the number of ions in solution, its conductivity changes too. Track that with a probe and you have another continuous method.
Watch out for a subtlety here: it is not only about how many ions there are. Different ions conduct differently. H+ in particular is unusually good at carrying charge, so a reaction that uses up H+ shows a big conductivity drop even if the total ion count barely moves.
Taking samples: quenching
Sometimes the only way to know a concentration is to titrate it. The problem is obvious — while you are busy titrating, the reaction in your sample keeps going, so by the time you finish the answer is wrong.
The fix is quenching: freeze the reaction in the sample the moment you take it, so the concentration stops changing.
🧩 Sampling with quenching
Start the reaction and start the clock.
Withdraw a small sample with a pipette at a known time.
Quench it straight away — plunge it into ice-cold water, or add something that removes one reactant (for example a base to neutralise the acid catalyst).
Titrate the quenched sample at your leisure. The concentration is now locked.
Repeat at several times and plot concentration against time.
Notice the pattern: quenching works by removing one of the things the reaction needs, or by dropping the temperature so far that the rate is effectively zero. Same logic as everything else in this topic.
Clock reactions and the 1 ÷ time trick
A clock reaction gives you one clear, visible finish line. The best known is sodium thiosulfate with hydrochloric acid, which slowly goes cloudy with sulfur:
You stand the flask on a paper cross and time how long the cross takes to disappear. Each run gives one number: a time.
Here is the bit students find odd. A long time means a slow reaction, so time is not a measure of rate — it is the opposite of one. To turn it into something that behaves like rate, take the reciprocal:
Rate from a clock reaction
rate ∝ 1 ÷ t
Why the reciprocal genuinely works. Every run finishes at the same amount of sulfur, because the cross always disappears at the same cloudiness. So the amount formed is fixed and only the time changes. Since rate = amount ÷ time, and the amount is the same every run, rate must be proportional to 1 ÷ time.
Choosing a method
Method
What you actually measure
Good when
Falls apart when
Gas syringe
Volume of gas collected
A gas is produced and the flask can be sealed
The gas dissolves, or more than about 100 cm3 is made
Inverted cylinder over water
Volume of water pushed out
The gas has low solubility, e.g. H2 or O2
The gas dissolves, e.g. CO2, SO2, NH3
Mass loss on a balance
Falling mass of the flask
A dense gas escapes, e.g. CO2
The gas is light, e.g. H2, so the change is too small to read
Colorimetry
Light absorbed by the mixture
A reactant or product is coloured
Nothing is coloured, or a precipitate forms
Conductivity
Ability of the solution to carry current
The number or type of ions changes
Ion count and type barely change
Sampling and titration
Concentration in each quenched sample
Nothing convenient changes physically
The reaction is too fast to sample and quench cleanly
Clock reaction
Time to reach a fixed visible point
You want a quick comparison between runs
You need a full curve from a single run
Worked examples
WORKED EXAMPLE
Choosing between gas volume and mass loss
0.0486 g of magnesium (0.00200 mol) reacts with excess hydrochloric acid to give hydrogen. Decide whether mass loss or a gas syringe is the better way to follow this reaction. (Mr of H2 = 2.02; molar volume of a gas = 22.7 dm3 mol−1)
Step 1: how much H₂ is made?
Mg + 2HCl → MgCl₂ + H₂ so n(H₂) = 0.00200 mol
Step 2: what mass is that?0.00200 × 2.02 = 0.00404 ga 2 decimal place balance would read 0.00 g — hopelessStep 3: what volume is that?0.00200 × 22.7 = 0.0454 dm³ = 45.4 cm³that fills about half a 100 cm³ syringe — idealUse a gas syringehydrogen is far too light for mass loss to work
WORKED EXAMPLE
Comparing two clock reaction runs
In a disappearing-cross experiment the cross vanishes after 45 s in run A and after 90 s in run B, which used a more dilute thiosulfate solution. Compare the rates of the two runs.
Step 1: convert each time to a rate measurerun A: 1 ÷ 45 = 0.0222 s⁻¹run B: 1 ÷ 90 = 0.0111 s⁻¹Step 2: compare them0.0222 ÷ 0.0111 = 2Run A is twice as fast as run Bhalf the time means double the rate — never say A took “half the rate”
WORKED EXAMPLE
A rate from colorimeter data
A coloured reactant is followed with a colorimeter. Its absorbance falls from 0.860 to 0.500 over 120 s. Calculate the average rate of change of absorbance, and state one advantage of this method over sampling and titration.
Step 1: change in absorbance0.860 − 0.500 = 0.360Step 2: divide by the time0.360 ÷ 120 = 3.0 × 10⁻³3.0 × 10⁻³ absorbance units per secondAdvantagenothing is removed from the mixture, so the reaction is not disturbed — and readings can be taken continuously
💡 Exam tip
Justify with a number where you can. “Hydrogen is too light” is worth less than “only 0.004 g would be lost, which a 2 dp balance cannot detect”.
If asked to improve a method, aim at the biggest source of error: a bigger syringe, more frequent readings, a water bath to hold the temperature steady, or a data logger instead of a stopwatch.
Say what you would keep constant. Temperature is almost always the answer, because rate is so sensitive to it.
For clock reactions, plot 1 ÷ t on the y-axis, not t. Plotting t gives a curve that is much harder to read.
Name real apparatus. “Gas syringe”, “cotton wool plug”, “burette”, “colorimeter” — vague words like “a container” score nothing.
Remember the trade-off question: continuous methods give more data, one-off methods are faster and often more precise.
⚠ Common mix-up
Sealing the flask for the mass loss method. If nothing escapes, the mass cannot change.
Leaving the syringe flask open. Then the gas goes into the room and the syringe stays empty.
Using an inverted cylinder for CO2. It dissolves, so the volume reads low and the rate looks too slow.
Trying colorimetry on a precipitate. A cloudy mixture scatters light rather than absorbing it, so the reading is meaningless.
Treating time as rate. A longer time is a slower reaction. Take the reciprocal.
Forgetting to quench. Titrating an unquenched sample measures the concentration when you finished the titration, not when you took the sample.
Assuming conductivity always rises. It depends on which ions appear and disappear — it can easily fall.
Up next: Collision Theory — why any of this happens at all. Two things have to go right for a collision to turn into a reaction, and one of them fails far more often than the other.
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