Grade by Grade
D. Energy Changes and Rates of Reaction · Energy Changes and Rates of Reaction

Inquiry: Investigating Factors That Affect Reaction Rate

D2.8 D3.5 A1.1 A1.2 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11 A1.13About 75 minutes
Learning goals
  • I can write a testable question and a hypothesis, with a reason, about a factor affecting reaction rate.
  • I can identify independent, dependent and controlled variables and plan a safe procedure with repeated trials.
  • I can calculate rates as 1/time, graph them, and decide whether data support a hypothesis.
  • I can evaluate an investigation by identifying specific sources of error and suggesting improvements.

Words to know

testable question
A question that can be answered by an experiment, naming the variable changed and the variable measured.
hypothesis
A prediction of the result of an experiment, with a reason based on theory.
independent variable
The variable deliberately changed by the investigator.
dependent variable
The variable measured, which responds to changes in the independent variable.
controlled variable
A variable kept the same so it cannot affect the results.
reliability
How consistent results are when an experiment is repeated.

Your turn to be the scientist

You now know the theory: concentration, surface area, temperature, catalysts and the nature of the reactants all affect rate. In this lesson you will plan and carry out an investigation to test one of these factors, analyse the data properly, and write a conclusion that a scientist would accept (expectation D2.8). The investigation can be done safely at home with effervescent antacid tablets, and the lesson also describes the classic school-lab versions.

Step 1: Ask a testable question and make a hypothesis

A testable question names the independent and dependent variables: How does the temperature of water affect the rate at which an effervescent tablet reacts?

A good hypothesis predicts the result and gives a reason based on theory: If the water temperature is increased, then the tablet will react faster (take less time to stop fizzing), because at a higher temperature a larger fraction of collisions between the acid and the hydrogen carbonate particles have energy equal to or greater than the activation energy.

Effervescent antacid tablets contain a solid acid (citric acid) and sodium hydrogen carbonate. When they dissolve, the acid and hydrogen carbonate ions react and release carbon dioxide: H⁺(aq) + HCO₃⁻(aq) → H₂O(l) + CO₂(g).

Step 2: Identify the variables

  • Independent variable (what you change): water temperature (for example 5, 15, 25, 35 and 45 °C).
  • Dependent variable (what you measure): the time for the tablet to stop fizzing, t. The rate is taken as 1/t.
  • Controlled variables (kept the same): the brand and mass of the tablet, whether it is whole or broken, the volume of water (for example 200 mL), the type of glass, whether or not you stir, the person judging the end point.

Plan at least three trials at each temperature, and calculate an average. Repeated trials let you see how reliable your results are.

Step 3: Safety and procedure

Safety: effervescent antacid tablets are a medicine: do not drink the solutions, and keep the tablets away from younger children. A grown-up prepares the hot water; use hot tap water, not boiling water. Wipe up spills so no one slips.

Procedure (home version):

  1. Prepare water baths at five temperatures using ice water, tap water and hot tap water mixed in a large bowl. Measure 200 mL into a glass and record the exact temperature.
  2. Drop in one whole tablet and start the timer at the same moment.
  3. Stop the timer when the fizzing stops (decide your end-point rule in advance, for example "no bubbles rising from the bottom for 3 s").
  4. Repeat three times at each temperature with fresh water and fresh tablets.

School-lab versions of the same inquiry:

  • Sodium thiosulfate and hydrochloric acid: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l). The sulfur makes the solution cloudy; time how long until a cross drawn under the flask disappears. (Done in a well-ventilated lab because sulfur dioxide is an irritant.)
  • Marble chips (CaCO₃) and hydrochloric acid: compare large chips, small chips and powder of equal mass (surface area), measuring mass loss or gas volume.
  • Hydrogen peroxide with different amounts of a catalyst, such as manganese(IV) oxide or the catalase in yeast or potato.
Water temperature (°C)Average time (s)Rate = 1/t (s⁻¹)
5960.0104
15640.0156
25420.0238
35290.0345
45200.0500
Sample class data (averages of three trials each, one whole tablet in 200 mL of water).
010203040500.010.020.030.040.05water temperature (°C)rate = 1/time (s⁻¹)0.01040.01560.02380.03450.0500
Rate (1/time) against temperature for the sample data. The rate increases steadily with temperature: from 5 °C to 45 °C it increases almost five-fold.
Worked example

Example 1: Analysing the temperature data

Use the sample data. (a) By what factor does the rate increase from 25 °C to 35 °C? (b) Does the data support the hypothesis? (c) Compare with the "doubles every 10 °C" rule of thumb.

  1. (a) Rate at 35 °C ÷ rate at 25 °C = 0.0345 ÷ 0.0238 = 1.45. (Or 42 s ÷ 29 s = 1.45.)
  2. (b) Every increase in temperature gave a shorter time and a higher rate, in all five conditions: the data support the hypothesis.
  3. (c) The factor per 10 °C is about 1.4 to 1.5, not 2. The rule of thumb is only rough, and this process is not a single chemical step: the tablet must also dissolve and the gas must escape, and those physical steps are less sensitive to temperature.

(a) about 1.45 times; (b) yes; (c) less than the rule of thumb, which is reasonable for a process that includes dissolving.

one 2 cm cubesurface area = 6 × 2² = 24 cm²cuteight 1 cm cubes (same mass)surface area = 8 × 6 × 1² = 48 cm²
Cutting a cube into eight smaller cubes of the same total mass doubles the surface area. Crushing a tablet into powder increases its surface area far more.
Worked example

Example 2: A surface-area trial

At 25 °C, a whole tablet takes 42 s, a tablet broken into quarters 30 s, and a crushed tablet 18 s. Calculate the rates and write a conclusion.

  1. Rates: whole 1/42 = 0.024 s⁻¹; quarters 1/30 = 0.033 s⁻¹; crushed 1/18 = 0.056 s⁻¹.
  2. Crushed is about 0.056 ÷ 0.024 = 2.3 times faster than whole.
  3. Conclusion: as the surface area of the tablet increased, the rate increased. This supports collision theory: more exposed particles allow more collisions with water per second, so the solid acid and hydrogen carbonate dissolve and react faster.

Rates 0.024, 0.033 and 0.056 s⁻¹. Increasing surface area increased the rate, as collision theory predicts.

Step 4: Analyse, evaluate and communicate

A strong report:

  • Graphs the dependent variable (rate) against the independent variable, with labelled axes and units.
  • States the trend with numbers ("the rate increased from 0.0104 s⁻¹ at 5 °C to 0.0500 s⁻¹ at 45 °C").
  • Says whether the evidence supports or refutes the hypothesis, and explains the result with theory (collision theory and activation energy).
  • Evaluates the method: identifies specific sources of error (judging the end point by eye; the water cooling or warming during a trial; tablets not identical) and says how each could affect the results. It suggests improvements (a water bath to hold the temperature; measuring mass loss on a balance instead of judging the end point; more trials).
  • Notes the precision of the measurements (a stopwatch reads to 0.01 s, but human reaction time is about 0.2 s, so times are reliable only to about the nearest second).
Hands-on activity

Plan and run your own rate inquiry

You need: effervescent antacid tablets (at least 9, same brand), clear glasses, a measuring cup, a kitchen thermometer, a timer, ice and hot tap water, a spoon, paper for a data table, and a grown-up to prepare the hot water. Do not drink the solutions.

  1. Choose one factor: temperature (3 temperatures) or surface area (whole, quarters, crushed). Temperature and surface area are the easiest factors to test fairly with tablets.
  2. Write your testable question, hypothesis (with a reason), and lists of independent, dependent and controlled variables.
  3. Draw a data table before you start, with units in the headings and room for three trials and an average.
  4. Carry out the trials. Calculate rate = 1/t for each average.
  5. Graph rate against your independent variable and write a conclusion and evaluation, as in Step 4.

What to look for: hot water and crushed tablets react fastest. A good evaluation mentions at least two specific errors and improvements. Ask a family member to read your conclusion: can they tell what you found and why, without seeing your data?

Just for fun

Scientists say "the data support the hypothesis" and never "the data prove the hypothesis". The tablets, however, are absolutely certain about one thing: they always fizz faster in hot water.

Your turn

1

Write a testable question for an inquiry on how the surface area of marble affects its rate of reaction with hydrochloric acid.

2

Write a hypothesis for that question.

3

List the independent, dependent and three controlled variables for that inquiry.

4

A tablet takes 50 s to stop fizzing. Calculate the rate as 1/t.

5

Why are three trials done at each temperature?

6

Identify one source of error in judging the end of fizzing by eye and suggest an improvement.

More practice: warm-up

1

What is the independent variable?

2

What is the dependent variable?

3

Why must the other variables be controlled?

4

If the time doubles, what happens to the rate (1/t)?

5

In the thiosulfate reaction, what makes the cross disappear?

More practice: on your own

1

In a thiosulfate experiment, the cross disappears after 80 s at 20 °C and 22 s at 40 °C. Find the rates and the factor per 10 °C (assume the same factor for each 10 °C step).

2

A student's results at 35 °C were 31 s, 29 s and 46 s. What should she do with the 46 s result?

3

Explain why using 1/t as the rate is reasonable in these experiments.

4

Suggest a way to test the effect of a catalyst on the decomposition of hydrogen peroxide safely in a school lab, naming the variables.

5

Write a conclusion for the sample temperature data, in 3 to 4 sentences.

Challenge

1

Design an inquiry to test whether the rate of the tablet reaction depends on the concentration of the acid it contains, using vinegar solutions of different strengths and baking soda instead. Include the variables, a data table outline and one safety note.

2

Using the sample temperature data, a student calculates that the rate increases by about 0.010 s⁻¹ for each 10 °C, and predicts the time at 55 °C will be 1/(0.050 + 0.010) = 17 s. Evaluate this prediction.

3

A student finds that a crushed tablet reacts faster than a whole one, but a powder dropped onto the water clumps and floats, and sometimes reacts more slowly than quarters. Explain and suggest a fix.

Check your understanding

Quiz

1.

In an inquiry on how temperature affects the rate of the tablet reaction, the dependent variable is

2.

Which hypothesis is best?

3.

A tablet takes 25 s at one temperature and 50 s at another. The first rate is

4.

Which is a controlled variable in a surface-area inquiry with marble chips and acid?

5.

Which statement about the sample temperature data is most accurate?

For parents and teachers

Big ideas (D2.8): a rate inquiry tests how one factor (temperature, surface area, concentration or a catalyst) affects reaction rate, with all other variables controlled. Results are analysed with rate = 1/time, graphed, compared with a hypothesis grounded in collision theory, and evaluated for error.

How to help: This is a great family activity. Let your teen lead: they should write the question, hypothesis, variables and data table before starting. Act as a lab partner who follows instructions exactly; if the instructions are unclear, that is useful feedback.

Common difficulties:

  • Hypotheses without a reason ("it will go faster") or not testable.
  • Changing more than one variable at a time (for example, using different water volumes at different temperatures).
  • Recording time but forgetting to convert to rate, or graphing time when the question is about rate.
  • Vague error analysis. Ask "Which way would that error push the result?"

Activity safety: antacid tablets are medicines: do not drink the solutions and store tablets away from young children. A grown-up prepares hot tap water. School-lab versions with acids and thiosulfate are done only at school, with goggles and ventilation.