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D2.8 · Investigate how conditions affect reaction rate

Learn to investigate how conditions affect reaction rate through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Energy Changes and Rates of Reaction

Observing, explaining, and fairly testing changes in reaction speed

An effervescent tablet can fizz in water. The bubbles are an observable sign that gas is forming. If the tablet fizzes to a defined endpoint sooner in warm water than in cold water, the reaction reached that endpoint in less time. This raises a useful question: how do conditions affect reaction rate? Reaction rate describes how quickly reactants are used or products are formed. To investigate a condition fairly, choose a clear measurement, change one condition at a time, and keep other relevant conditions the same.

What you will learn

Describe reaction rate using a measurable change over time.
Explain how temperature, concentration, gas pressure, solid surface area, and catalysts can affect reaction rate.
Plan a fair investigation by changing one condition and controlling other relevant conditions.
Compare measured rates and report suitable units and significant digits.

1. From an observable change to a rate

A chemical reaction changes reactants into products. Atoms are rearranged, not created or destroyed. A balanced chemical equation represents this conservation. Reaction rate describes how quickly a measurable amount of reactant disappears or product appears.

A reaction can be followed by observing a change such as gas bubbles, a colour change, or a solid forming. Choose an observation that can be recorded consistently. For example, a student might time how long it takes until a marked cross beneath a cloudy mixture can no longer be seen. That moment is the chosen endpoint.

When two trials use the same starting amounts and endpoint, the trial that reaches the endpoint in less time has the faster average rate over that interval. This comparison works because the trials are being compared at the same measured change. If the starting amounts or endpoint differ, time alone may not give a fair comparison.

A rate can also be calculated when a measured amount of product forms over a measured time. For example, gas volume divided by time gives a gas-production rate. The units must describe both measurements, such as millilitres per second. Use the same measurement method and units in all trials being compared.

Keep observations separate from explanations. “Bubbles appeared sooner” is an observation. “Higher temperature increased particle motion” is an explanation using a particle model. A good investigation records what happened before interpreting why.

Reaction rate describes how quickly a measurable change occurs.
A shorter time to the same endpoint indicates a faster average rate when starting conditions are matched.
Record observations with units and a clearly defined measurement.

2. How conditions affect particle encounters

Matter is made of particles. During a reaction, particles must meet in a way that can lead to products. An effective encounter is an encounter that can result in reaction. Not every encounter is effective. The particle model helps explain why changing conditions can change the rate.

Concentration tells how much of a substance is present in a given volume of solution. If the concentration of a reactant increases, more of its particles are present in the same volume. Reactant particles can encounter one another more often, so the reaction rate can increase. When comparing concentrations, keep other relevant conditions, such as temperature and the amounts of other reactants, consistent.

For reacting gases, increasing pressure by compressing the gas places particles in less space. Their encounters can become more frequent, which can increase reaction rate. This explanation concerns reacting gases; it should not be applied automatically to every reaction mixture.

Raising temperature makes particles move faster on average. They can encounter one another more often in a given time. Also, a greater share of their encounters can lead to reaction. The overall result is that raising temperature usually increases reaction rate. This is a particle-level explanation, not a claim that every encounter produces a reaction.

For a solid reactant, crushing it into smaller pieces exposes more surface area. Surface area is the part of a solid that is in contact with its surroundings. More exposed surface lets more encounters with the other reactant occur at once, so the reaction can be faster. To test particle size fairly, keep the total amount of solid the same.

Higher solution concentration can increase how often reactant particles encounter one another.
Higher pressure can increase encounters when the reacting substances are gases.
Higher temperature increases average particle motion and the share of encounters able to react.
Smaller pieces of a solid expose more surface area.

3. Catalysts and fair investigations

A catalyst is a substance that increases reaction rate and is not used up overall in the reaction. It provides a different route for the reaction that requires less energy to get started. The energy needed to get a reaction started is called activation energy. A catalyst changes the rate, but it does not change the substances shown as reactants and products in the balanced equation.

An investigation should be designed so that the effect of the condition being tested is clear. The independent variable is the condition deliberately changed. The dependent variable is the result measured or observed. Controlled variables are other relevant conditions kept the same.

For example, a student could ask how water temperature affects the time for an effervescent tablet to reach a defined fizzing endpoint. Temperature is the independent variable, and time is the dependent variable. The tablet type and amount, water volume, container, and mixing method should be kept consistent. A thermometer can record temperature, and a timer can record the endpoint time. This is a plan; it does not claim that results have been collected.

A stronger comparison tests several values of the changed condition and repeats each trial where possible. Repeats help reveal whether one measurement was unusual. Define a subjective endpoint carefully, and use the same rule in each trial. Record the results before explaining the pattern. Do not present an expected trend as an observation.

Use appropriate safety practices and follow teacher instructions. Wear required protective equipment and handle warm water carefully. Never seal a gas-producing reaction in a closed container, because gas pressure can build up.

A catalyst increases rate and is not used up overall.
Change one independent variable and measure a defined dependent variable.
Control other relevant conditions and repeat trials when possible.
Report observed results separately from predicted or explained trends.

4. Comparing and reporting results

When the same volume of gas is measured in two trials, divide that volume by the time taken in each trial to compare average gas-production rates. A larger volume-per-time value means gas was produced more quickly over the measured interval. Alternatively, when the same amount is used to define an endpoint, the shorter endpoint time indicates the faster trial.

For example, a measured gas volume of

30.0\,\mathrm{mL}

produced in

15.0\,\mathrm{s}

corresponds to an average rate of

2.00\,\mathrm{mL\,s^{-1}}

. The units show that the measured volume is divided by time. Keep units in each calculation and in the final answer.

Report a calculated value with an appropriate number of significant digits. Significant digits are the meaningful digits supported by the measurements. If the measured values are recorded to three significant digits, a calculated rate reported to three significant digits is usually suitable. Do not add digits that the measuring tools do not support.

A graph can help display results. Put the changed condition on the horizontal axis and the measured rate or time on the vertical axis. State which quantity is plotted. A higher rate means a faster reaction, while a shorter endpoint time means a faster reaction. Rate and time do not increase in the same direction when the amount measured is held constant.

Conclusions apply to the reaction and conditions tested. State what was changed, what was measured, and the pattern in the recorded results. Use the particle model to explain a pattern, but do not claim that one result proves the same pattern for every reaction.

Compare rates using the same kind of measured change and consistent units.
A larger amount-per-time rate means a faster change; a shorter time to the same endpoint also means faster.
Report only the precision supported by measurements.
Limit conclusions to the reaction and conditions tested.

Worked example

Comparing gas-production rates

Two trials produce the same measured gas volume. Trial A produces

24.0\,\mathrm{mL}

12.0\,\mathrm{s}

, and Trial B produces

24.0\,\mathrm{mL}

18.0\,\mathrm{s}

. Calculate each average rate and identify which trial is faster.

Choose a comparison
Both trials produce the same gas volume. Dividing that volume by the time taken gives an average rate that can be compared directly.
$\text{average rate}=\frac{\text{gas volume}}{\text{time}}$
Calculate Trial A
Divide the measured volume by the measured time. Millilitres divided by seconds gives millilitres per second.
$\frac{24.0\,\mathrm{mL}}{12.0\,\mathrm{s}}=2.00\,\mathrm{mL\,s^{-1}}$
Calculate Trial B
Use the same calculation for Trial B. Keeping the method and units the same makes the comparison fair.
$\frac{24.0\,\mathrm{mL}}{18.0\,\mathrm{s}}=1.33\,\mathrm{mL\,s^{-1}}$
Compare the rates
Trial A has the larger average gas-production rate. It also took less time to produce the same volume, so both comparisons identify Trial A as faster.
$2.00\,\mathrm{mL\,s^{-1}}>1.33\,\mathrm{mL\,s^{-1}}$

Answer: Trial A is faster over the measured interval. Its average rate is

2.00\,\mathrm{mL\,s^{-1}}

, compared with

1.33\,\mathrm{mL\,s^{-1}}

for Trial B.

Check: Each rate has three significant digits, matching the three-significant-digit measurements. Both rates use gas volume per time.

Common mistakes and how to avoid them

Saying a reaction is faster because it produced more product, without considering time or starting amounts.

Correction: Compare the same measured change over time, or compare times to the same endpoint under matched starting conditions.

Changing temperature and concentration in the same comparison, then attributing the result to only one condition.

Correction: Change one independent variable at a time and control other relevant conditions.

Treating a catalyst as a reactant that is permanently consumed.

Correction: A catalyst increases reaction rate and is not used up overall.

Assuming that a shorter endpoint time means a smaller rate.

Correction: For the same endpoint and matched starting conditions, less time means a faster average rate.

Lesson summary

Reaction rate describes how quickly a measurable amount of reactant is used or product is formed.
Temperature, concentration, gas pressure, exposed solid surface area, and catalysts can affect reaction rate.
Particle encounters and particle motion provide a course-level model for explaining these effects.
A fair investigation changes one condition, measures a clear outcome, and controls other relevant conditions.
Use consistent units and appropriate significant digits, and distinguish recorded observations from explanations.

Check your understanding

Question 1

Two equal samples reach the same endpoint in

20\,\mathrm{s}

and

35\,\mathrm{s}

. Which sample has the faster average rate?

The sample that took $20\,\mathrm{s}$
The sample that took $35\,\mathrm{s}$
Both must have the same rate because the endpoint is the same
There is not enough information to compare their times to the same endpoint

Show answer and explanation

The sample that took

20\,\mathrm{s}

With equal samples and the same endpoint, reaching that endpoint in less time indicates a faster average rate.

Question 2

A student investigates how crushing a solid reactant affects rate. Which condition should be kept the same?

The amount of solid used
The particle size
The exposed surface area
The condition being deliberately changed

Show answer and explanation

The amount of solid used

Keep the amount of solid the same while changing its particle size. Changing particle size changes the exposed surface area.

Question 3

Why can warming a reaction mixture increase its rate?

Particles move faster on average, and a greater share of encounters can lead to reaction
Warming makes atoms in the reactants disappear
Warming always increases the amount of reactant present
Warming turns every particle encounter into a reaction

Show answer and explanation

Particles move faster on average, and a greater share of encounters can lead to reaction

Higher temperature increases average particle motion and the share of encounters able to react. It does not make every encounter successful.

Key terms

Reaction rate: How quickly reactants are used or products are formed.
Effective encounter: An encounter between particles that can lead to products.
Concentration: The amount of a substance present in a given volume of solution.
Surface area: The part of a solid that is exposed to and in contact with its surroundings.
Catalyst: A substance that increases reaction rate and is not used up overall.
Activation energy: The energy needed to get a reaction started.
Independent variable: The condition deliberately changed in an investigation.
Dependent variable: The result measured or observed in an investigation.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation D2.8. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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