Analysing Reaction Rate Data
You'll be able to draw conclusions from reaction rate graphs and tables.
In 2022, three CSIRO researchers timing the same HCl-marble reaction got 42 s, 44 s, and 61 s, spotting that outlier prevented a flawed conclusion about reaction rate.
Printable Worksheets
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Q1 · If you measured how long a reaction took and got three results, 42 s, 44 s, and 61 s, what would you do with those numbers and why?
Q2 · What makes a graph more useful than a table of numbers when communicating scientific results?
● Know
- How to read and interpret graphs of reaction rate data
- How to calculate a mean average from repeated measurements
- The difference between independent, dependent and controlled variables
● Understand
- Why repeating measurements improves reliability
- How to identify anomalous results and decide whether to exclude them
- What makes an experiment a fair test
● Can do
- Draw conclusions from reaction rate graphs and tables
- Evaluate experimental design and suggest improvements
- Process data by calculating averages and identifying anomalies
Plot the volume of CO2 gas collected every 30 seconds as marble chips react with hydrochloric acid and you get a curve that rises steeply at first, then levels off completely, the graph is showing you the reaction slowing down in real time as reactants are used up. Graphical analysis of reaction data reveals how rate changes over time and allows comparison of different conditions.
Concentration-time graphs: Plot reactant concentration (or product amount) on the y-axis against time on the x-axis. The slope at any point equals the instantaneous rate at that time. Steep slopes mean fast rates; shallow slopes mean slow rates. A horizontal line means zero rate.
Initial rate: Draw a tangent to the curve at t=0. The gradient of this tangent is the initial rate. Initial rate is useful because it reflects the starting conditions before significant reactant depletion.
Comparing curves: When investigating how a factor affects rate, plot multiple curves on the same axes. The curve that rises fastest has the highest rate. If the final plateau is lower, the factor may have affected yield as well as rate.
Text description
- Gas volume (mL) rises from 0 quickly at first, then more slowly, levelling at 100 mL: the reaction is complete.
- Initial rate: the steep tangent at t = 0 (about 3.6 mL/s).
- Rate at t = 60 s: the tangent there is much shallower (about 0.4 mL/s).
- The rate decreases as the reactants are used up.
When investigating how temperature affects the reaction between sodium thiosulfate and hydrochloric acid, students measure the time for a cross to disappear as sulfur precipitate forms. At 20 °C, the cross disappears in 120 seconds. At 30 °C, it disappears in 60 seconds. At 40 °C, it disappears in 30 seconds. Plotting 1/time (proportional to rate) against temperature gives an exponential curve. The graph clearly shows that rate doubles for each 10 °C increase, confirming the rule of thumb and validating collision theory predictions.
Australian chemical kinetics: The Bragg Institute at ANSTO uses neutron scattering to measure reaction rates at the molecular level. By observing how molecules move and collide in real time, scientists can validate and refine collision theory models. This research supports the design of better catalysts and industrial processes. Australian researchers have made significant contributions to understanding enzyme kinetics and protein folding dynamics.
The rate at any point on a concentration-time graph is the y-value (concentration) at that point. This is false. The rate is the slope (gradient) of the curve, not the height. A high concentration with a flat curve means zero rate - the reaction has stopped even though plenty of reactant remains (perhaps due to equilibrium or catalyst deactivation). Conversely, a low concentration with a steep curve means a fast rate. Rate is about change, not absolute value.
Find the error in this student analysis of a rate graph.
- The curve is flat at t=60s, meaning concentration is not changing.
- If concentration is not changing, the rate must be zero.
- Wait - the curve might still have a slope if you zoom in.
- Actually, a flat curve means the reaction has finished, so rate is indeed zero.
A rate investigation is a fair test when exactly one thing is deliberately changed. That one thing is the independent variable; the thing you measure is the dependent variable; everything else that could affect the measurement is a controlled variable, held the same in every trial.
Take the standard marble chip investigation. You change the concentration of the hydrochloric acid, and you measure the volume of carbon dioxide collected each ten seconds. So concentration is the independent variable and gas volume is the dependent variable. The mass and size of the chips, the volume of acid, and the temperature are all controlled.
Here is why that matters, and it is not just tidiness. Suppose you raised the concentration and crushed the chips at the same time. The reaction gets faster — but the two changes have pushed in the same direction and their effects are now added together in a single number. Nothing you can do afterwards separates them, because you never measured either one on its own. The data is not weak, it is unusable for the question you asked.
Two students test the same question and get different-looking results. Mia runs 0.5, 1.0 and 2.0 mol/L acid, each on 5 g of medium chips in 50 mL, all at 20 °C. Jack runs 0.5 mol/L on large chips and 2.0 mol/L on powder, because the powder was easier to weigh out.
Both find the stronger acid faster. Only Mia can say concentration caused it. Jack’s stronger acid also had the far larger surface area, so his result is exactly what you would expect if concentration made no difference at all and surface area did everything — and his data cannot tell those two stories apart.
The rule that carries: for every condition in your method, ask could changing this on its own move the thing I am measuring? If yes, and it is not your independent variable, it must be held constant. Acid concentration, chip size, acid volume and temperature all pass that test, so all four are either the variable or controlled. The brand of the conical flask and the time of day do not, so you can ignore them.
Testing how long food keeps: Australian food manufacturers work out use-by dates with accelerated shelf-life testing. Identical batches of the same product are stored at several set temperatures, and the same quality measure — bacterial count, acidity, texture — is taken on the same schedule for every batch. Temperature is the only thing deliberately changed, because spoilage is a set of chemical and biological reactions and temperature is the rate factor under test. Packaging, batch, starting quality and testing method are held constant, for the reason this card gives: vary two of them and the trial cannot say which one moved the date.
“Controlled variables” means keeping everything the same. This is false, and it is the error that quietly ruins a write-up. You cannot control the dependent variable — the whole point is to let it move and to watch how far it moves. And you must not control the independent variable, because changing it is the experiment. Controlled variables are the other relevant conditions: the ones that could shift your measurement but are not what you are testing. Change nothing and you learn nothing; change everything and you learn nothing. Change one thing.
Reliable rate data requires careful experimental design and honest analysis.
Systematic errors: Consistent biases that affect all measurements in the same direction. Examples: a stopwatch that runs slow, a thermometer that reads 2 °C high, or a concentration that was prepared incorrectly. Systematic errors cannot be reduced by repeating measurements.
Random errors: Unpredictable variations that scatter measurements around the true value. Examples: slight variations in timing, temperature fluctuations, or reading a scale at slightly different angles. Random errors can be reduced by taking more repeats and calculating a mean.
Anomalies: Individual results that deviate significantly from the pattern. Never simply delete anomalies - investigate possible causes. Was there a power failure? Did you misread a measurement? Good scientists report anomalies and explain why they were or were not included in analysis.
In a rate experiment using magnesium and acid, a student obtains reaction times of 32s, 34s, 33s, and 58s. The first three agree well, but the fourth is anomalous. Investigation reveals that the fourth run used a different piece of magnesium that had a visible oxide coating. The oxide reacted first with acid (MgO + 2HCl → MgCl2 + H2O), consuming acid before the magnesium proper could react. Once the cause is identified, the anomalous result is explained and can legitimately be excluded from the mean, with the reason documented.
Australian measurement standards: The National Measurement Institute (NMI) maintains Australia primary standards for time, mass, temperature, and other quantities. Their laboratories in Sydney ensure that instruments used in research and industry are traceable to international standards. When Australian chemical companies measure reaction rates for process control, their instruments are calibrated against NMI standards, ensuring consistency and comparability of data across the country.
Taking more repeats always improves accuracy. This is false. Repeats reduce the effect of random errors and improve precision, but they do not eliminate systematic errors. If your thermometer reads 5 °C high, taking 100 measurements will give you a very precise but inaccurate average. To improve accuracy, you must identify and correct systematic errors through calibration, control experiments, or independent verification.
A student repeats a reaction rate experiment three times and gets times of 45s, 48s, and 72s. What should they do with the 72s result?
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
Wrong: "An average means you can just guess the middle number." No, the mean average is calculated precisely by adding all values and dividing by the count. Guessing introduces error.
Right: The mean is calculated by adding all valid values and dividing by the number of values, it is a precise calculation, not an estimate. Guessing a middle number introduces error and undermines the reliability of your data.
Wrong: "If a result does not fit my hypothesis, I should leave it out." No, you should only exclude anomalous results if you can identify a specific error. Excluding valid data is unscientific.
Right: You should only exclude a data point if you can identify a specific cause for the error, equipment failure, a spill, a recording mistake. An anomalous result that cannot be explained should be reported and discussed, not silently removed.
Wrong: "Doing an experiment once is enough if you are careful." No, even careful scientists get random errors. Repeating measurements is essential for reliability.
Right: Careful technique reduces systematic error but cannot eliminate random error. Repeating measurements and calculating a mean reduces the influence of random variation, which is why replication is a core requirement of reliable scientific work.
CSIRO and Reaction Rate Research
Australia's national science agency, CSIRO, conducts research into reaction rates across many fields. In mineral processing, CSIRO scientists study how to speed up the leaching reactions that extract gold and copper from ore, making Australian mining more efficient and reducing environmental impact.
CSIRO also researches reaction rates in agriculture, such as how quickly fertilisers break down in different Australian soils. Understanding these rates helps farmers apply the right amount at the right time, reducing runoff into the Great Barrier Reef catchment.
✍ Copy Into Your Books
▾Reading Rate Graphs
- Steeper slope = faster reaction
- Flat line = reaction finished
- Same final height = same total product
Fair Test Checklist
- Only change the independent variable
- Keep all controlled variables constant
- Repeat and calculate a mean
Anomalies
- Identify results far from the pattern
- Check for human or equipment error
- Exclude only with a stated reason
Evaluate the Experiment
Process the Data
At the start of this lesson, the hook gave you three reaction times, 42 s, 44 s, and 61 s, and asked whether you'd average all three or treat one as suspicious. What did you decide then, and why?
Now that you understand random error, outliers, and how scientists identify anomalous results, would you change your answer? What is the difference between a genuine outlier and random variation, and how has your approach to reading data shifted from your first instinct?
Q1. 1. Explain why scientists repeat measurements in experiments and calculate a mean average. In your answer, include what "reliability" means. 4 MARKS
Q2. 2. A group of you investigates how concentration affects the rate of reaction between sodium thiosulfate and hydrochloric acid. They time how long it takes for a cross drawn under the flask to disappear. Name the independent, dependent and two controlled variables in this investigation. 4 MARKS
Q3. 3. The table below shows the volume of hydrogen gas produced every 20 seconds when zinc reacts with hydrochloric acid. Describe the trend in the data, explain why the rate changes over time, and calculate the total gas produced after 100 seconds.
| Time (s) | Total gas volume (mL) |
|---|---|
| 0 | 0 |
| 20 | 32 |
| 40 | 56 |
| 60 | 72 |
| 80 | 80 |
| 100 | 84 |
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now describe what a fair test requires in more detail?
- What new skill did you learn about processing data?
Model answers (click to reveal)
Answers
▾MCQ 1
C A steep slope on a reaction rate graph means the reaction is producing product quickly at the start. A flat line would mean the reaction has stopped.
MCQ 2
B The dependent variable is what you measure. In this case, the time taken for the reaction to finish is the measured outcome that depends on temperature.
MCQ 3
D78 s is very different from 42–44 s and is likely an anomaly. The best approach is to identify it as an anomaly, check for a cause, and calculate the mean of the remaining reliable values: (42 + 44 + 43) / 3 = 43 s.
MCQ 4
A Both curves flatten at the same height (150 mL), meaning both reactions produced the same total amount of gas. Curve X reached this value faster, so it had a faster reaction rate.
MCQ 5
C Using 1 g of powder and 2 g of granules changes both the amount of catalyst and its particle size, which affects exposed surface area. To test catalyst amount fairly, use the same particle size in both tests and keep the other conditions constant.
Short Answer 1
Model answer: Scientists repeat measurements because individual readings can be affected by small random errors, such as reaction time when starting a stopwatch or slight variations in equipment. Repeating and calculating a mean average gives a more representative value that smooths out these random errors. Reliability refers to how consistently an experiment produces the same results when repeated under the same conditions. The more repeats that agree closely, the more reliable the data.
Short Answer 2
Model answer: Independent variable: concentration of hydrochloric acid (or concentration of sodium thiosulfate). Dependent variable: time taken for the cross to disappear. Controlled variables: volume of acid used, volume of sodium thiosulfate used, temperature of the solutions, same size and shape of flask, same cross and viewing distance, same person judging when the cross disappears.
Short Answer 3
Model answer: The trend shows that the volume of gas increases over time but the rate slows down. In the first 20 seconds, 32 mL was produced; in the next 20 seconds, only 24 mL; then 16 mL, then 8 mL, then 4 mL. The rate decreases because the concentration of the acid is dropping as it is used up in the reaction. Fewer acid particles are available to collide with zinc, so successful collisions become less frequent. The total gas produced after 100 seconds is 84 mL.