Factors Affecting Rate & the Required Practical
Five factors can change the rate of a chemical reaction. You need to recall all five here, and explain them with collision theory in section 3:
| Factor | Increase it… |
|---|---|
| Concentration (of solutions) | …and the rate increases |
| Pressure (of reacting gases) | …and the rate increases |
| Surface area (of solid reactants) | …and the rate increases |
| Temperature | …and the rate increases |
| Catalyst present | Rate increases — and the catalyst is not used up (section 4) |
Each factor belongs to a particular state of matter: concentration is for solutions, pressure is for gases, and surface area is for solids. Writing “increase the concentration of the magnesium ribbon” tells the examiner the factors have been memorised without being understood — a solid doesn’t have a concentration.
Required Practical 5 — the effect of concentration
The required practical investigates how concentration affects rate, using three different methods of following a reaction. It is also the AQA specification’s named opportunity for developing a hypothesis — so before the method, write down what you expect and why: “increasing the concentration will increase the rate, because there are more particles in the same volume, so collisions are more frequent.”
Sodium thiosulfate solution reacts with dilute hydrochloric acid to make a precipitate of sulfur (a precipitate is an insoluble solid that forms when two solutions react), which slowly turns the mixture cloudy (the technical word is turbidity):
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l)
Read the state symbols: two clear solutions — thiosulfate (aq) and acid (aq) — react, and one of the products, the sulfur, comes out as a solid (s). It’s that fine solid sulfur, spread through the liquid, that clouds the mixture and hides the cross.
- Measure thiosulfate solution into a conical flask standing on a black cross drawn on paper.
- Add the acid, swirl, and start the stop clock.
- Look down through the solution; stop the clock the moment the cross is no longer visible.
- Repeat with different concentrations of thiosulfate, keeping everything else the same.
A shorter time means a faster rate — so rate is proportional to 1time.
React magnesium (or marble chips) with dilute hydrochloric acid and collect the gas in a gas syringe (or an upside-down measuring cylinder full of water). Record the volume every 10 seconds, repeat with a different acid concentration, and plot volume against time — exactly the curves from section 1. The steeper the initial gradient, the faster the rate.
The gas syringe is the neatest way to read a volume, but you can also collect the gas over water in an upturned measuring cylinder — the gas bubbles up and pushes the water down, and you read the volume off the cylinder’s scale:
Collecting a gas over water — an alternative to the gas syringe.
Two of the ways of following the rate in required practical 5. Method 1: time how long the black cross takes to vanish as the solution clouds. Method 2: read the gas volume off a syringe at regular times and plot the curve.
If a reaction gives off a gas, you can follow it on a balance. Stand the flask of reactants on the balance, plug the neck with cotton wool, and record the mass every few seconds. As gas escapes into the air the mass of the flask falls — and a faster fall means a faster reaction.
- The usual reaction is a carbonate with acid, e.g. CaCO3 + 2HCl → CaCl2 + H2O + CO2. The carbon dioxide leaves the flask, so the reading drops.
- The cotton wool plug earns marks of its own: it lets the gas escape (so the mass can change) while stopping any acid spray being thrown out as the mixture fizzes — spray leaving would make the mass fall too far.
- Mass only appears to be lost — no atoms are destroyed; the gas has simply left the flask. Seal the flask and the mass wouldn’t change at all.
Following a reaction by loss of mass: a carbonate reacts with acid in a flask plugged with cotton wool. The CO₂ escapes into the air so the balance reading falls — while the cotton wool stops acid spray being lost with it.
Loss of mass works well for carbon dioxide (a fairly heavy gas), but badly for reactions that give off hydrogen (magnesium + acid). H2 is so light that even a lot of it barely moves the balance, so the change is lost in the noise — collect it in a gas syringe instead. Matching the method to the gas is itself a common exam point.
Variables — and the steps that make it valid
Aim: investigate how the concentration of a reactant affects the rate of reaction.
Method: react sodium thiosulfate with dilute hydrochloric acid and follow the rate by timing how long the black cross takes to disappear (turbidity), or react a metal/carbonate with acid and follow it by the volume of gas collected (or the loss of mass on a balance). Repeat at several different concentrations of the reactant, keeping everything else the same.
- Independent variable (you change it) — the concentration of the reactant solution (the sodium thiosulfate, or the acid).
- Dependent variable (you measure it) — the time for the reaction (the cross to disappear), or the volume of gas at set times.
- Control variables (keep them the same) — the total volume (and so depth) of solution, the temperature (hold it steady with a water bath if the room is warm), the amount of the other reactant, and the same flask, same cross and same observer judging when the cross has gone.
Result: a higher concentration gives a shorter time / a steeper rate — rate is proportional to 1/time, so plotting 1/time against concentration shows the relationship.
“Amount” is too vague to earn marks. Say exactly what you are changing, measuring or keeping the same — the mass, volume, concentration or number of moles. For example, write “the same volume of acid”, not “the same amount of acid”.
Examiner reports repeatedly show students naming the right experiment but missing the step that makes it work. For the disappearing cross, the decisive details are:
- Same depth of solution every time — you are looking down through the liquid, so a deeper mixture would hide the cross sooner and ruin the comparison. Keeping the total volume the same (topping up with water when diluting) keeps the depth the same.
- Measure the volumes accurately — use a measuring cylinder (not a beaker) to measure out the thiosulfate, acid and water. A measuring cylinder has a much finer scale — a better resolution — so each “same total volume” really is the same and the depth stays fair.
- State the observation, not an inference — timing stops when “the cross is no longer visible”. That is the observation. “The reaction finished” is an inference — and in fact it’s false: the reaction is still going; there’s just enough sulfur to block your view.
- Judging the end-point is the weakness — different people (or the same person on different runs) judge “no longer visible” differently. Improve it with the same observer every time, or remove the judgement entirely with a light sensor and data logger measuring the light transmitted through the flask.
- Safety — the reaction makes sulfur dioxide, a toxic gas: keep the room well ventilated (or use a fume cupboard at higher concentrations).
Try it below. Run all four concentrations and watch what happens to the time — then look at the pattern in 1/time.
🧪 Run required practical 5 — the disappearing cross
Choose a concentration of sodium thiosulfate, add the acid, and time how long the cross takes to vanish. The total volume is the same in every run — the diluted solutions are topped up with water — so the depth stays fair. Run all four and watch the pattern build.
| Conc / g/dm³ | Time / s | 1/t / s⁻¹ |
|---|
Time is not the rate — a shorter time means a faster reaction. 1/time behaves like the rate: watch it double when the concentration doubles.
1/t against concentration: a straight line through the origin — rate is directly proportional to concentration.
In the disappearing-cross experiment you measure a time, but the question usually asks about rate. They run opposite ways: the faster the reaction, the shorter the time. To turn times into something that behaves like a rate, calculate 1time — then a graph of 1/t against concentration gives a straight line through the origin, showing rate is directly proportional to concentration. (Doubling the concentration halves the time.)
Predict the curve — steepness vs plateau
This trainer drills the skill from section 1: for each change, decide separately what happens to the rate (how steep the curve is) and to the amount of product (how high the plateau sits). They don’t always move together.
📈 Predict the curve
A reaction is followed by the volume of gas it makes (the grey dashed curve). For each change, predict what happens to the steepness of the curve and the height of the plateau, then press Check.
Describing your measurements — the words examiners use
A handful of “working scientifically” words turn up again and again on rates papers, usually as a one-mark “which word describes…?” question with the others offered as distractors. They are easy marks once you can tell them apart.
- Repeatable — the same person, using the same method and equipment, gets similar results on repeating.
- Reproducible — different people, often with different equipment, get similar results. (The tougher test — it’s how science checks a claim.)
- Accurate — a measurement that is close to the true value.
- Precise — repeated measurements that are close together (a small spread). Not the same as accurate — tight readings can still all be off the true value.
- Valid — the experiment measures what it’s meant to and answers the question, because the other variables were controlled (a fair test).
- Resolution — the smallest change an instrument can detect — its finest scale division (a balance reading to 0.01 g has a higher resolution than one reading to 1 g).
- Anomalous result — a result that doesn’t fit the pattern. Examine it for a cause, and if it came from a poor measurement, leave it out of the mean.
🔗 Match the term to its meaning
Click a term, then click the definition that matches it. Correct pairs lock in green; a wrong pick flashes red.
Worked example — a mean that ignores the anomaly
A student repeats one concentration of the disappearing-cross run five times and records the times: 42 s, 44 s, 43 s, 58 s, 42 s. Calculate the mean time, to a sensible number of significant figures.
- Spot the anomaly first: four results cluster around 42–44 s, but 58 s is well out of line. It doesn’t fit the pattern, so it’s left out of the mean.
- Mean of the four that agree = 42 + 44 + 43 + 424 = 1714 = 42.75 s → 43 s (2 s.f.).
- Include the 58 s by mistake and the mean climbs to 45.8 s — the anomaly drags it up and makes the result less accurate. Always state which value you ignored, and why.
Spot the mistake in the apparatus
Examiners often draw a set-up with one thing wrong and ask why it won’t give good results. Find the fault in each diagram.
🔍 Spot the mistake
Each set-up has exactly one fault that would spoil the results. Tap the reason it won’t work.
Rates questions often ask you to identify the gas a reaction gives off. Three quick tests cover almost all of them:
- Carbon dioxide (carbonate + acid) — bubble it through limewater, which turns cloudy / milky.
- Hydrogen (metal + acid) — hold a lit splint to it for a squeaky pop.
- Oxygen (e.g. hydrogen peroxide decomposing) — it relights a glowing splint.
The full set of gas tests is C8 Chemical Analysis content — here you just need to recognise the gas your apparatus is collecting.
🧪 Exam-style questions
In the disappearing-cross investigation, which of these is the independent variable? Tick (✓) one box.
At one concentration, the cross took 80 seconds to disappear. Calculate 1/t for this run, in s⁻¹.
Show answer
- 1/t = 180 1 mark
- = 0.0125 s⁻¹ (= 1.25 × 10⁻² s⁻¹). 1 mark
- 1/t is used because it is proportional to the rate — a shorter time gives a bigger 1/t, matching a faster reaction.
Why must the total volume of solution be the same in every run of the disappearing-cross experiment? Tick (✓) one box.
Which change would remove the biggest source of error in the disappearing-cross method? Tick (✓) one box.
Three students, in different labs and using their own equipment, each measured the time for the cross to disappear at the same concentration and got very similar results. Which word best describes these results? Tick (✓) one box.
The rate of the reaction between marble chips and dilute hydrochloric acid was followed by measuring the volume of carbon dioxide with a gas syringe. Describe another method that could be used to follow the rate of this reaction.
Show a model answer
- Stand the flask on a balance (neck plugged with cotton wool) and record the mass at regular time intervals. 1 mark
- The mass falls as carbon dioxide escapes; a faster fall means a faster rate. 1 mark
- Allow: collect the gas over water in an inverted measuring cylinder and record the volume at set times. (The disappearing-cross method does not work here — this reaction doesn’t form a precipitate that clouds the solution.)
The diagram shows the gas syringe at one moment during the experiment. What volume of gas has been collected? Tick (✓) one box.
Magnesium reacts with hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). A student investigated how the rate of this reaction changed when the concentration of hydrochloric acid was changed. Write a plan the student could use. In your plan you should describe how you would carry out the investigation and make it a fair test, and describe the measurements you would make. This is a levels-of-response question — you are also assessed on clear written communication, so organise your plan logically and use specialist terms.
Show a model answer
How it is marked — the marks depend on the quality of written communication as well as the science:
- Level 3 (5–6): a workable plan that includes changing the concentration AND measuring the rate AND fair testing.
- Level 2 (3–4): a plan including a change of concentration / ‘volume’ of acid, AND an attempt at measuring rate and/or at fair testing.
- Level 1 (1–2): a simple plan with no change of variable, but an attempt at measuring rate OR at fair testing.
Example chemistry points — the plan:
- add the magnesium to the acid
- time the reaction / ‘count the bubbles’ / measure the volume of gas given off
- change the concentration (‘volume’) of the acid and repeat
Control variables (to make it a fair test):
- same amount / mass / length / ‘size’ of magnesium
- same volume / amount of acid
- (keep the temperature the same)
Source: AQA GCSE Chemistry.