4.2 Factors Affecting Rate of Reaction
There are five main factors that affect the rate of a chemical reaction: temperature of reactants, concentration of reactants, size of solid reactants, presence of a catalyst, and pressure for reactions involving gaseous reactants.
Learning Objectives
- State the five factors affecting the rate of reaction.
- Describe how temperature, concentration, size of solid reactants, presence of a catalyst, and pressure affect the rate of reaction.
- Explain the procedures, variables, and observations of laboratory experiments investigating the effect of temperature, concentration, size of solid reactants, and catalyst on the rate of reaction.
Summary of the 5 Factors Affecting Rate of Reaction
Each factor causes the rate of reaction to increase in a different way.
Short Explanation
What is rate of reaction?
Rate of reaction measures how quickly reactants turn into products.
Five main factors control this rate:
1. Temperature
Heating reactants speeds up the reaction, so the time taken is shorter.
2. Concentration
More concentrated solutions result in a faster reaction.
3. Size of Solid Reactants
Smaller pieces, such as crushed marble chips, react faster than large pieces because they offer more surface contact.
4. Presence of Catalyst
Adding a suitable catalyst, such as copper(II) sulphate in the reaction between zinc and hydrochloric acid, increases the speed of reaction without being consumed.
5. Pressure
Higher pressure increases reaction rates only when reactants are gases.
In laboratory investigations, rate can be quantified either as \(\frac{1}{\text{time taken for a visual mark to disappear}}\), such as the mark 'X' obscured by sulphur, or as \(\frac{\text{volume of gas collected}}{\text{time taken}}\).
Laboratory Experiments on Factors Affecting Rate of Reaction
Four experiments investigate temperature, concentration, size of solid reactants, and catalyst.
Experiment 4.1: Effect of Temperature of Reactants
Aim: To study the effect of temperature of reactants on the rate of reaction.
Problem Statement: How does temperature of reactants affect the rate of reaction?
Hypothesis: The higher the temperature of reactants, the higher the rate of reaction.
Variables:
- Manipulated: Temperature of sodium thiosulphate solution.
- Responding: Time taken until 'X' is no longer visible.
- Constant: Concentration and volume of sodium thiosulphate solution, concentration and volume of sulphuric acid, and size of conical flask.
Procedure Outline:
- Measure \(50\text{ cm}^3\) of \(0.2\text{ mol dm}^{-3}\) sodium thiosulphate solution into a conical flask; leave for 5 minutes; record initial room temperature.
- Place conical flask over 'X' on white paper.
- Add \(5\text{ cm}^3\) of \(1\text{ mol dm}^{-3}\) sulphuric acid and immediately start stopwatch.
- Observe 'X' vertically from the mouth of the conical flask.
- Stop stopwatch when 'X' is completely obscured by the yellow sulphur precipitate.
- Repeat by heating sodium thiosulphate solution to \(35^\circ\text{C}\), \(40^\circ\text{C}\), \(45^\circ\text{C}\), and \(50^\circ\text{C}\).
Data Analysis & Relationship: As temperature increases, the time taken for 'X' to disappear decreases. The value of \(\frac{1}{\text{time}}\) increases, showing the rate of reaction increases. Operational definition: Rate of reaction is inversely proportional to the time taken for the 'X' mark to be no longer visible.
Experiment 4.2: Effect of Concentration of Reactants
Aim: To study the effect of concentration of reactants on the rate of reaction.
Problem Statement: How does concentration of reactants affect the rate of reaction?
Hypothesis: The higher the concentration of reactants, the higher the rate of reaction.
Variables:
- Manipulated: Concentration of sodium thiosulphate solution (\(0.20\), \(0.16\), \(0.12\), \(0.08\), \(0.04\text{ mol dm}^{-3}\)).
- Responding: Time taken until 'X' is no longer visible.
- Constant: Volume of sodium thiosulphate solution, concentration and volume of sulphuric acid, size of conical flask.
Procedure Outline:
- Add \(50\text{ cm}^3\) of \(0.20\text{ mol dm}^{-3}\) sodium thiosulphate solution to conical flask placed over 'X'.
- Add \(5\text{ cm}^3\) of \(1\text{ mol dm}^{-3}\) sulphuric acid and start stopwatch simultaneously.
- Record time taken until 'X' is no longer visible.
- Repeat with \(0.16\), \(0.12\), \(0.08\), and \(0.04\text{ mol dm}^{-3}\) sodium thiosulphate solutions.
Data Analysis & Relationship: The higher the concentration, the shorter the time taken for 'X' to become invisible. The value of \(\frac{1}{\text{time}}\) increases as concentration increases, indicating an increased rate of reaction.
Experiment 4.3: Effect of Size of Solid Reactants
Aim: To study the effect of size of solid reactants on the rate of reaction.
Problem Statement: How does the size of reactants affect the rate of reaction?
Hypothesis: The smaller the size of solid reactants, the higher the rate of reaction.
Variables:
- Manipulated: Size of marble chips (large vs. small pieces).
- Responding: Time taken to collect \(30.00\text{ cm}^3\) of gas.
- Constant: Temperature, mass of marble (\(2\text{ g}\)), concentration (\(0.1\text{ mol dm}^{-3}\)) and volume (\(40\text{ cm}^3\)) of hydrochloric acid.
Procedure Outline:
- Invert a water-filled burette into a basin of water and clamp vertically; record initial reading \(V_0\).
- Place \(40\text{ cm}^3\) of \(0.1\text{ mol dm}^{-3}\) dilute hydrochloric acid into a conical flask.
- Add \(2\text{ g}\) of large marble chips, immediately close with stopper connected to delivery tube leading under the burette, and start stopwatch.
- Record time taken to collect \(30.00\text{ cm}^3\) of carbon dioxide gas.
- Repeat using \(2\text{ g}\) of small marble chips.
Data Analysis & Relationship: Time taken to collect \(30.00\text{ cm}^3\) of gas is shorter when using small marble chips compared to large pieces. Rate of reaction is higher with smaller particle size.
Experiment 4.4: Effect of Catalyst
Aim: To study the effect of presence of catalyst on rate of reaction.
Problem Statement: How does the presence of a catalyst affect the rate of reaction?
Hypothesis: Presence of catalyst increases the rate of reaction.
Variables:
- Manipulated: Presence of catalyst (copper(II) sulphate solution).
- Responding: Time taken to collect \(30.00\text{ cm}^3\) of gas.
- Constant: Temperature, volume and concentration of hydrochloric acid.
Materials & Setup: Zinc pieces (\(2\text{ g}\)), \(0.1\text{ mol dm}^{-3}\) dilute hydrochloric acid (\(40\text{ cm}^3\)), \(0.5\text{ mol dm}^{-3}\) copper(II) sulphate solution (\(5\text{ cm}^3\)). Gas is collected over water in an inverted burette.
Caution: The mixture of hydrogen and air in the burette can explode when ignited. Do not ignite the gas in the burette.
Procedure Outline:
- React \(2\text{ g}\) zinc pieces with \(40\text{ cm}^3\) of \(0.1\text{ mol dm}^{-3}\) dilute hydrochloric acid; measure time taken to collect \(30.00\text{ cm}^3\) of hydrogen gas.
- Repeat by adding \(5\text{ cm}^3\) of \(0.5\text{ mol dm}^{-3}\) copper(II) sulphate solution to the acid mixture as a catalyst.
Data Analysis & Relationship: Time taken to collect \(30.00\text{ cm}^3\) of hydrogen gas is shorter in the presence of copper(II) sulphate solution. The presence of a catalyst increases the rate of reaction.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Name the five factors that affect the rate of reaction.
In Experiment 4.1, what solid precipitate causes the 'X' mark to disappear, and what happens to the time taken when the temperature of the solution is increased?
Why does pressure only affect reactions involving gaseous reactants and not solids or liquids?
Common Mistakes
- Writing “the larger the size of solid reactants, the higher the rate” instead of “the smaller the size of solid reactants, the higher the rate”.
- Stating that pressure affects reactions involving solids and liquids; pressure only affects reactions involving gaseous reactants.
- Confusing the responding variable with rate of reaction directly: time taken is inversely proportional to the rate of reaction (a shorter time means a higher rate).
Concept Misunderstandings
A higher rate of reaction means it takes more time to finish.
A higher rate means the reaction is faster, so the time taken is shorter.
Adding a catalyst increases the total amount of product formed.
A catalyst only increases the rate (speed) of reaction, not the final quantity or yield of products.
Summary
- Rate of reaction is affected by five factors: temperature, concentration, size of solid reactants, catalyst, and pressure.
- Rate of reaction increases when temperature of reactants increases.
- Rate of reaction increases when concentration of reactants increases.
- Rate of reaction increases when size of solid reactants decreases.
- Rate of reaction increases when a catalyst is present.
- Rate of reaction increases when pressure increases (for gaseous reactants).
- In experiments, a faster reaction is shown by a shorter time taken (\(t\)), giving a larger value of \(\frac{1}{t}\).
Short Activity
A. Objective Quiz
1 What happens to the rate of reaction when the size of solid marble chips is decreased?
2 Which of the following factors only affects the rate of reaction involving gaseous reactants?
B. Fill in the Blanks
3 In Experiment 4.4, adding copper(II) sulphate solution acts as a to increase the rate of reaction between zinc and dilute hydrochloric acid.
4 The higher the temperature of reactants, the the time taken for the mark 'X' to no longer be visible.
C. Matching: Condition Change to Effect on Rate of Reaction
Match each reaction condition change to its effect on the rate of reaction. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Condition Change to Its Effect
Write the correct effect next to each condition change.
| Condition Change | Effect on Rate of Reaction |
|---|---|
| 1. Increasing the temperature of sodium thiosulphate solution | |
| 2. Decreasing the concentration of sodium thiosulphate solution | |
| 3. Using large pieces instead of small pieces of marble chips | |
| 4. Adding a suitable catalyst to the reaction mixture |
Answer choices: Increases the rate of reaction • Decreases the rate of reaction • Lowers the rate of reaction • Speeds up the rate of reaction
