Investigating Reaction Rates
In 1989, chemists Martin Fleischmann and Stanley Pons announced cold fusion to the world's press, but not one of 50 international labs could reproduce their result in 6 months.
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Q1 · Before you design an experiment to test "does temperature affect reaction rate?", what are at least three decisions you need to make?
Q2 · Why is it important to write your method in enough detail that someone else could repeat your experiment and get the same results?
● Know
- The definitions of independent, dependent and controlled variables
- How to write a testable hypothesis with a cause-and-effect relationship
- Methods for measuring reaction rate in school laboratory settings
● Understand
- Why fair testing requires controlling all variables except the one being tested
- How to collect, process and represent data reliably
- That repeated trials improve the reliability of conclusions
● Can do
- Design a valid and safe practical investigation into reaction rate
- Collect and tabulate data, calculate averages and identify anomalies
- Draw evidence-based conclusions linked to collision theory
Imagine two students both testing how temperature affects the rate of marble chips dissolving in HCl, one keeps the acid volume constant but forgets to control chip size, the other controls both; their results are completely different, even though they used the same temperature values. Designing a valid experiment to investigate reaction rates requires careful attention to variables and controls.
Independent variable (IV): The factor you deliberately change. In a rate investigation, this might be concentration, temperature, surface area, or the presence of a catalyst. You should have at least 5 different values to establish a pattern.
Dependent variable (DV): The factor you measure. This might be time for a visible change, volume of gas produced per minute, or colour change measured with a colorimeter. The DV must be quantitative and objectively measurable.
Control variables: All other factors that could affect the DV must be kept constant. For a concentration investigation: temperature, total volume, stirring rate, particle size (if solid), and light intensity (for photochemical reactions) must be constant.
Repeats: Each condition should be tested at least 3 times to assess reliability. Calculate means and identify anomalies.
Investigating how temperature affects the rate of reaction between magnesium and hydrochloric acid:
IV: Temperature (20C, 30C, 40C, 50C, 60C)
DV: Time for magnesium to completely dissolve (measured with stopwatch)
Controls: Mass of magnesium (0.5g ribbon), volume and concentration of HCl (50 mL of 1M), same conical flask, no stirring, same observer timing.
Method: Heat water baths to target temperatures. Place flask with acid in water bath until it reaches temperature. Add magnesium, start timer, stop when magnesium fully dissolved. Repeat each temperature 3 times. Calculate mean time and rate = 1/time. Plot rate vs temperature.
Australian science education: The Science by Doing program developed by the Australian Academy of Science provides inquiry-based investigations including reaction rate experiments. These resources are used in schools nationwide and emphasise fair-test design, data analysis, and evidence-based conclusions. Australian science curricula require students to plan, conduct, and evaluate investigations, developing skills that transfer to research and industry careers.
You only need one repeat to check reliability. This is false. A single repeat cannot distinguish between natural variation and a systematic error. With only two data points, you cannot assess whether they agree closely or vary widely. Three repeats are the minimum for calculating a meaningful mean and range. Professional research typically uses many more repeats. In school science, three is a practical minimum that still provides some statistical basis.
Put these steps for investigating concentration and reaction rate into the correct order.
- Pour sodium thiosulfate solution into a conical flask over a marked cross.
- Repeat with different concentrations of sodium thiosulfate.
- Keep temperature, volume, and acid concentration constant.
- Add hydrochloric acid and start the timer immediately.
- Measure the time for the cross to disappear for each concentration.
- Record results in a table and plot a graph of 1/time vs concentration.
Safety is paramount in chemistry experiments. A proper risk assessment identifies hazards and specifies control measures.
Common hazards in rate experiments:
- Acids: Cause chemical burns and eye damage. Wear safety goggles and gloves. Use low concentrations (1M or less) where possible.
- Heat: Hot water baths can cause scalds. Use tongs to handle hot glassware. Never heat sealed containers.
- Gas production: Hydrogen gas is flammable. Never produce hydrogen near flames. Use well-ventilated areas.
- Breakage: Hot glass looks the same as cold glass. Use heat-resistant borosilicate glass. Check for cracks before heating.
Risk assessment format: Identify the hazard, assess the risk (likelihood × severity), identify control measures, and specify emergency procedures.
For the magnesium + acid experiment:
Hazard: Hydrochloric acid (1M) - causes skin and eye irritation.
Risk: Moderate (likely contact possible, injury usually minor with prompt washing).
Control: Wear safety goggles and gloves. Have eye wash station accessible. Use small volumes (25 mL). Wipe spills immediately with damp cloth.
Hazard: Hydrogen gas production - flammable.
Risk: Low (small volumes, well-ventilated lab, no flames nearby).
Control: Ensure room ventilation. No Bunsen burners in use. Small magnesium pieces to limit gas volume.
Emergency: If acid contacts skin, wash with water for 10 minutes. If in eyes, use eye wash for 15 minutes and seek medical attention.
Australian laboratory safety: Australian schools follow state-specific science safety guidelines (e.g., Science ASSIST in Victoria, NSW Department of Education safety guidelines). These documents specify maximum concentrations, permitted equipment, and mandatory safety gear for school experiments. They are developed in consultation with chemists, educators, and safety professionals. Adherence to these guidelines has made school chemistry in Australia remarkably safe despite the inherent hazards of the subject.
If you are careful, you do not need safety equipment. This is dangerously false. Accidents happen to careful people too. Safety equipment is not for expected mishaps but for unexpected ones - a bumped elbow, a cracked flask, a sudden vigorous reaction. Goggles protect against splashes you did not anticipate. Gloves protect against spills you did not plan. The best chemists are not those who avoid safety gear but those who use it habitually so they never need its protection.
Find the error in this experimental plan.
- 5M HCl is strongly acidic and corrosive.
- 80C is very hot and increases the reaction rate.
- A sealed container is dangerous because gas pressure will build up.
- The student should use a lower concentration and an open container.
Recording, analysing, and reporting data are essential scientific skills.
Data tables: Include clear column headings with units. Record all raw data, not just means. Note any anomalies and possible causes. Include control variables and conditions.
Graphs: Choose appropriate scales. Label axes with quantities and units. Use a line of best fit (for continuous relationships) or bars (for discrete categories). Do not force lines through obviously anomalous points.
Calculations: Show working. Calculate rates as change per unit time. Convert units consistently. Calculate percentage differences when comparing conditions.
Conclusions: State whether the data supports or refutes the hypothesis. Refer to specific data points. Discuss limitations (measurement precision, control of variables, sample size). Suggest improvements.
A student investigating temperature and rate obtains these times for magnesium to dissolve in acid:
20C: 120, 118, 125s (mean = 121s, rate = 0.0083 s-1)
30C: 62, 60, 64s (mean = 62s, rate = 0.016 s-1)
40C: 31, 30, 32s (mean = 31s, rate = 0.032 s-1)
Conclusion: The data supports the hypothesis that increasing temperature increases reaction rate. Rate approximately doubles for each 10C increase, consistent with collision theory. The relationship is reliable (small ranges at each temperature). Limitations: timing by eye introduces error (~1s); temperature may have varied slightly during reaction; only one acid concentration tested. Improvements: use a colorimeter for objective timing; maintain temperature with a water bath; test multiple concentrations.
Australian science education standards: The Australian Curriculum: Science requires students to process and analyse data using digital technologies and appropriate mathematical representations. Science teachers assess students on their ability to construct valid conclusions from evidence, evaluate experimental design, and communicate findings. These skills are assessed in practical examinations and extended investigations, ensuring that Australian science graduates can design and evaluate experiments rigorously.
Anomalous results should always be discarded. This is false. Anomalies should be investigated first. If a clear experimental error caused the anomaly (wrong concentration, equipment malfunction, timing mistake), it can be excluded with explanation. But if no error can be identified, the anomaly may represent genuine variability or an unexpected effect. Discarding data simply because it does not fit expectations is cherry-picking and is scientific misconduct. Always report what you did with anomalies and why.
Find the error in this data table from a rate experiment.
- The times vary widely for the same concentration.
- The 87s reading is likely an anomaly and should be investigated.
- The student should calculate the mean of all three values.
- Without knowing the temperature, the data is incomplete.
Wrong: "A hypothesis is just a guess." No � a hypothesis is an informed, testable prediction based on scientific understanding. It should specify the relationship between variables.
Right: A hypothesis is a specific, testable prediction grounded in existing scientific knowledge, not a random guess. A well-formed hypothesis states the expected relationship between the independent and dependent variable and can be proven false by evidence.
Wrong: "One trial is enough if you are careful." No � even careful investigators can have unexpected results. Repeating trials and calculating averages improves reliability and confidence.
Right: Even a careful experimenter cannot prevent random error entirely. Repeating trials at least three times and calculating a mean reduces the influence of random variation and strengthens confidence in your results.
Wrong: "If results don't match the hypothesis, the experiment failed." No � unexpected results are valuable. They may reveal flaws in the method, or they may lead to new scientific understanding.
Right: Results that contradict a hypothesis are scientifically valuable, they show the hypothesis needs revision and prompt further investigation. Some of the most important discoveries in science came from unexpected results, not confirmed predictions.
Investigating Reactions in Australian Industry
Australian industries depend on controlling reaction rates. In aluminium production, the Bayer process uses controlled temperature and concentration to extract alumina from bauxite ore. Engineers carefully monitor reaction conditions to maximise yield and minimise waste.
In agriculture, fertiliser production relies on the Haber process, where nitrogen and hydrogen are combined under controlled temperature and pressure with an iron catalyst. Australian farmers use these fertilisers to improve crop yields, but overuse can lead to unwanted reactions in waterways.
Indigenous fire management also involves controlled reaction rates. By adjusting fuel load (surface area) and temperature, Aboriginal fire practitioners control combustion rate to achieve desired ecological outcomes.
✍ Copy Into Your Books
▾Variables
- Independent: what you change
- Dependent: what you measure
- Controlled: what you keep the same
Hypothesis Structure
- "If [independent] changes, then [dependent] will... because [reasoning]."
- Must be testable and based on scientific understanding
Data and Conclusions
- Repeat trials and calculate averages
- Identify and investigate anomalies
- Use graphs to show trends
- Link conclusions to collision theory
Design Your Investigation
Analyse the Data
| Time (s) | 0.5 mol/L acid (mL) | 1.0 mol/L acid (mL) | 2.0 mol/L acid (mL) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 30 | 12 | 24 | 48 |
| 60 | 22 | 45 | 88 |
| 90 | 30 | 62 | 95 |
| 120 | 35 | 70 | 96 |
At the start of this lesson, the hook told you about the 1989 cold fusion announcement, two scientists went to a press conference before peer review, and no one could reproduce their results. Did you already have an idea of why reproducibility matters before reading that?
Now that you understand how to design a valid experiment and why reproducibility is the most important test a result must pass, how would you evaluate the cold fusion claim using what you've learned today? How has your definition of "good evidence" shifted from your starting point?
Q1. 1. Define independent, dependent and controlled variables. For an investigation into how surface area affects the reaction rate between marble chips and hydrochloric acid, give one example of each variable. 4 MARKS
Q2. 2. A student wants to investigate how the concentration of sodium thiosulfate solution affects the rate of its reaction with hydrochloric acid. Write a hypothesis for this investigation and explain why it is testable. Describe how the student could measure the reaction rate. 4 MARKS
Q3. 3. Using the data table from Activity 2, describe how you would represent this data on a graph. Explain what the shape of each line would tell you about reaction rate, and how you could use the graph to support a conclusion about the effect of concentration. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- How would you now improve your original hypothesis about the antacid tablet?
- What additional controlled variables would you identify?
Model answers (click to reveal)
Answers
▾MCQ 1
B The dependent variable is what you measure. In this case, it is the time taken for the reaction to finish (or another measure of reaction rate).
MCQ 2
C The volume and concentration of the acid must be kept constant so that only surface area differs between trials. The size of the chips is the independent variable, and time and mass of CO2 are dependent variables.
MCQ 3
A28 s is very different from 45 s and 47 s, so it is likely an anomaly caused by an error. The student should investigate what went wrong (e.g., did they start the timer late?) and exclude it if an error is identified.
MCQ 4
D A good hypothesis specifies the variables, predicts a direction of change, and provides a scientific reason. Option D does all three: it identifies concentration and rate, predicts an increase, and explains it using collision theory.
MCQ 5
B Unexpected results are valuable in science. They may reveal problems with the method (such as uncontrolled variables) or they may challenge existing understanding and lead to new discoveries. All results contribute to scientific knowledge.
Short Answer 1
Model answer: The independent variable is the factor deliberately changed: the surface area of the marble chips (e.g., powdered vs large chips). The dependent variable is what is measured: the time taken for the reaction to finish, or the volume of gas produced per minute. A controlled variable is something kept constant: the volume and concentration of hydrochloric acid, the temperature of the acid, or the mass of marble chips used. Keeping these the same ensures that any difference in reaction rate is due to surface area alone.
Short Answer 2
Model answer: Hypothesis: "If the concentration of sodium thiosulfate is increased, then the reaction with hydrochloric acid will be faster, because there are more thiosulfate particles per unit volume, leading to more frequent collisions with acid particles." This is testable because concentration can be precisely measured and changed, and reaction rate can be measured by timing how long it takes for a cross placed under the flask to disappear as sulfur precipitate forms. The student could time this for different concentrations and compare the results.
Short Answer 3
Model answer: I would plot time (seconds) on the x-axis and volume of gas (mL) on the y-axis. Each concentration would have its own line. The steeper the line, the faster the reaction. The 2.0 mol/L line would be steepest at the start and level off first, showing the fastest reaction. The 0.5 mol/L line would be the shallowest and level off last. This supports the conclusion that higher concentration increases reaction rate, because more acid particles per unit volume lead to more frequent successful collisions, as predicted by collision theory.