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7.2 Factors Affecting the Rate of Reaction

7.2 Factors Affecting Rate of Reactions
CHEMISTRY • Form 4 • Chapter 7: Rate of Reaction

7.2Factors Affecting Rate of Reactions

Different chemical substances have distinct chemical properties, causing reactions to proceed at different speeds. The speed at which reactants convert into products can be manipulated by changing specific physical and environmental conditions.

Learning Objectives

  • Investigate factors affecting the rate of reactions through experiments, based on size of reactants, concentration of solutions, temperature of reaction mixtures and presence of a catalyst.
  • Understand the effect of pressure on reactions involving gases.
  • Interpret experimental data, calculate rates of reaction, and deduce relationships using graphical methods.

Factors Affecting the Rate of Reaction & Curve Comparisons

See how each factor changes particle exposure and how rate curves compare on a volume–time graph.

Surface Area

Smaller pieces expose more total surface area.

Concentration

More particles per unit volume increases collision frequency.

Pressure (Gases)

Compression raises gas concentration.

Rate Curves

Rate curve comparison Curve II rises faster than Curve I but both reach the same final volume. Time Volume Curve II Curve I

Faster reactions are steeper but end at the same plateau.

Brief Explanation

The four main experimental factors and the special case of pressure.

Chemical reactions occur when reactant particles interact. The speed of this interaction depends directly on four main conditions: total surface area, concentration, temperature and the presence of a catalyst.

1. Size of Reactants

When a fixed mass of solid is divided into smaller pieces, its total surface area increases while its total mass remains constant. The smaller the solid particles, the larger the exposed surface area and the higher the rate of reaction.

Example: A \(2\text{ cm} \times 2\text{ cm} \times 2\text{ cm}\) cube has a total surface area of \(24\text{ cm}^2\). Cutting it into two halves increases the total surface area to \(32\text{ cm}^2\), and into four pieces to \(40\text{ cm}^2\).

2. Concentration of Reactants

Concentration is the quantity of solute dissolved in a given volume of solution. The higher the concentration of a reacting solution, the higher the rate of reaction.

Dilution follows \(M_1V_1 = M_2V_2\), where \(M_1\) and \(V_1\) are the initial concentration and volume, and \(M_2\) and \(V_2\) are the diluted concentration and total volume.

3. Temperature

Most reactions proceed faster at higher temperatures. For reactions occurring around room temperature, each increase of \(10\ ^\circ\text{C}\) roughly doubles the rate of reaction.

The graph of \(\dfrac{1}{\text{time}}\) against temperature curves upwards, showing that the rate increases non-linearly with rising temperature.

4. Presence of Catalyst

A catalyst alters the rate of a chemical reaction without undergoing any chemical changes at the end of the reaction. Its chemical properties and total mass remain unchanged, although its physical properties may change.

Catalysts do not alter the final quantity or yield of products; they only shorten the time needed to reach the end of the reaction.

Factor Effect on rate Key experimental method
Smaller solid size Increases total surface area, so rate increases. Marble chips + HCl; measure CO₂ volume over time.
Higher concentration More particles per unit volume, so rate increases. Na₂S₂O₃ + H₂SO₄; disappearing 'X' mark.
Higher temperature Particles gain thermal energy, so rate increases. Same thiosulphate system at different temperatures.
Catalyst Provides an alternative pathway, so rate increases. H₂O₂ decomposition with MnO₂; glowing splinter relights.
Higher pressure Increases gas concentration, so rate increases. Only for reactions involving gaseous reactants.

Try to Answer First

Answer mentally, then check the explanation.

1

Why does powdered calcium carbonate react much faster with hydrochloric acid than large marble chips of the same mass?

Answer: Powdered calcium carbonate has a much larger total surface area exposed for reaction than large marble chips of the same mass, which increases the rate of reaction.
2

In the reaction between sodium thiosulphate solution and sulphuric acid, what causes the solution to turn cloudy, and how is the reaction rate determined?

Answer: The solution turns cloudy due to the formation of an insoluble yellow precipitate of solid sulphur, S. The rate of reaction is determined by measuring the time taken for a black 'X' mark placed beneath the flask to disappear from view, calculated as Rate = 1/time.
3

A student adds manganese(IV) oxide powder to hydrogen peroxide solution. What happens to the mass and chemical identity of the manganese(IV) oxide at the end of the reaction?

Answer: The mass and chemical identity of manganese(IV) oxide remain completely unchanged because it acts as a catalyst; only its physical appearance may change.

Common Mistakes

Avoid these errors when explaining reaction rates.

!
Confusing particle size with surface area: writing that larger marble chips have a larger surface area. In fact, smaller pieces or powders have a significantly larger total surface area for the same mass.
!
Assuming catalysts increase product yield. A catalyst only increases the speed of the reaction; it never changes the total quantity of products formed.
!
Applying pressure to liquids and solids. Pressure changes only affect systems containing gaseous reactants.
!
Misinterpreting graph plateaus: assuming that a faster reaction produces a higher volume plateau. If the limiting reactant mass and volume remain constant, the curve is merely steeper and reaches the exact same final volume earlier.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

A catalyst participates in the reaction, so some of it must be permanently used up or chemically converted.

Correct Fact

Although a catalyst interacts during the reaction, it is chemically regenerated by the end. Its chemical properties and overall mass are strictly conserved.

Misconception

Increasing the volume of acid used always increases the rate of reaction.

Correct Fact

Increasing the volume of acid without changing its concentration does not alter the rate of reaction. Rate depends on concentration, not total volume.

Misconception

Diluting an acid with water makes the reaction faster because there is more total liquid.

Correct Fact

Adding water lowers the concentration of acid particles per unit volume, which actually decreases the reaction rate.

Summary

Key facts to remember.

  • Factors increasing reaction rate: smaller solid reactant size, higher solution concentration, higher temperature, presence of a suitable catalyst, and higher pressure for gaseous reactants only.
  • Acronym: CSTP — Concentration, Size, Temperature, Pressure.
  • Catalyst properties: speeds up reaction, remains chemically unchanged, mass is conserved, does not change total product yield.
  • Experimental measurements: gas syringe or inverted burette for gas volume over time, and disappearing cross method with Rate = 1/t for precipitate formation.
  • On a volume–time graph, a faster reaction is steeper at the start but reaches the same final volume if reactant quantities are identical.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

What happens to the mass and chemical properties of a catalyst once the chemical reaction is complete?

2

For a chemical reaction occurring at room temperature, what is the approximate effect on the reaction rate if the temperature is increased by \(10\ ^\circ\text{C}\)?

B. Fill in the Blanks

3

In the experiment between sodium thiosulphate solution and sulphuric acid, the insoluble yellow solid that causes the solution to turn cloudy is ______.

4

Increasing pressure increases the rate of reaction only for reactions involving ______ reactants.

C. Match the Factor with Its Characteristic

Drag each factor on desktop. On mobile, tap a factor and then tap the matching box.

Answer Choices

Smaller solid reactant size Higher solution concentration Addition of a catalyst Higher pressure

Characteristic / Effect

1Provides a larger total surface area exposed for reaction
2Contains more solute particles per unit volume of solution
3Speeds up reaction while retaining unchanged mass and chemical identity
4Compresses gas particles into a smaller volume, increasing their concentration
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Total surface area Concentration Catalyst Precipitate Initial rate of reaction CSTP
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

7.2 Factors Affecting the Rate of Reaction

7.2 Factors Affecting Rate of Reactions
CHEMISTRY • Form 4 • Chapter 7: Rate of Reaction

7.2Factors Affecting Rate of Reactions

Different chemical substances have distinct chemical properties, causing reactions to proceed at different speeds. The speed at which reactants convert into products can be manipulated by changing specific physical and environmental conditions.

Learning Objectives

  • Investigate factors affecting the rate of reactions through experiments, based on size of reactants, concentration of solutions, temperature of reaction mixtures and presence of a catalyst.
  • Understand the effect of pressure on reactions involving gases.
  • Interpret experimental data, calculate rates of reaction, and deduce relationships using graphical methods.

Factors Affecting the Rate of Reaction & Curve Comparisons

See how each factor changes particle exposure and how rate curves compare on a volume–time graph.

Surface Area

Smaller pieces expose more total surface area.

Concentration

More particles per unit volume increases collision frequency.

Pressure (Gases)

Compression raises gas concentration.

Rate Curves

Rate curve comparison Curve II rises faster than Curve I but both reach the same final volume. Time Volume Curve II Curve I

Faster reactions are steeper but end at the same plateau.

Brief Explanation

The four main experimental factors and the special case of pressure.

Chemical reactions occur when reactant particles interact. The speed of this interaction depends directly on four main conditions: total surface area, concentration, temperature and the presence of a catalyst.

1. Size of Reactants

When a fixed mass of solid is divided into smaller pieces, its total surface area increases while its total mass remains constant. The smaller the solid particles, the larger the exposed surface area and the higher the rate of reaction.

Example: A \(2\text{ cm} \times 2\text{ cm} \times 2\text{ cm}\) cube has a total surface area of \(24\text{ cm}^2\). Cutting it into two halves increases the total surface area to \(32\text{ cm}^2\), and into four pieces to \(40\text{ cm}^2\).

2. Concentration of Reactants

Concentration is the quantity of solute dissolved in a given volume of solution. The higher the concentration of a reacting solution, the higher the rate of reaction.

Dilution follows \(M_1V_1 = M_2V_2\), where \(M_1\) and \(V_1\) are the initial concentration and volume, and \(M_2\) and \(V_2\) are the diluted concentration and total volume.

3. Temperature

Most reactions proceed faster at higher temperatures. For reactions occurring around room temperature, each increase of \(10\ ^\circ\text{C}\) roughly doubles the rate of reaction.

The graph of \(\dfrac{1}{\text{time}}\) against temperature curves upwards, showing that the rate increases non-linearly with rising temperature.

4. Presence of Catalyst

A catalyst alters the rate of a chemical reaction without undergoing any chemical changes at the end of the reaction. Its chemical properties and total mass remain unchanged, although its physical properties may change.

Catalysts do not alter the final quantity or yield of products; they only shorten the time needed to reach the end of the reaction.

Factor Effect on rate Key experimental method
Smaller solid size Increases total surface area, so rate increases. Marble chips + HCl; measure CO₂ volume over time.
Higher concentration More particles per unit volume, so rate increases. Na₂S₂O₃ + H₂SO₄; disappearing 'X' mark.
Higher temperature Particles gain thermal energy, so rate increases. Same thiosulphate system at different temperatures.
Catalyst Provides an alternative pathway, so rate increases. H₂O₂ decomposition with MnO₂; glowing splinter relights.
Higher pressure Increases gas concentration, so rate increases. Only for reactions involving gaseous reactants.

Try to Answer First

Answer mentally, then check the explanation.

1

Why does powdered calcium carbonate react much faster with hydrochloric acid than large marble chips of the same mass?

Answer: Powdered calcium carbonate has a much larger total surface area exposed for reaction than large marble chips of the same mass, which increases the rate of reaction.
2

In the reaction between sodium thiosulphate solution and sulphuric acid, what causes the solution to turn cloudy, and how is the reaction rate determined?

Answer: The solution turns cloudy due to the formation of an insoluble yellow precipitate of solid sulphur, S. The rate of reaction is determined by measuring the time taken for a black 'X' mark placed beneath the flask to disappear from view, calculated as Rate = 1/time.
3

A student adds manganese(IV) oxide powder to hydrogen peroxide solution. What happens to the mass and chemical identity of the manganese(IV) oxide at the end of the reaction?

Answer: The mass and chemical identity of manganese(IV) oxide remain completely unchanged because it acts as a catalyst; only its physical appearance may change.

Common Mistakes

Avoid these errors when explaining reaction rates.

!
Confusing particle size with surface area: writing that larger marble chips have a larger surface area. In fact, smaller pieces or powders have a significantly larger total surface area for the same mass.
!
Assuming catalysts increase product yield. A catalyst only increases the speed of the reaction; it never changes the total quantity of products formed.
!
Applying pressure to liquids and solids. Pressure changes only affect systems containing gaseous reactants.
!
Misinterpreting graph plateaus: assuming that a faster reaction produces a higher volume plateau. If the limiting reactant mass and volume remain constant, the curve is merely steeper and reaches the exact same final volume earlier.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

A catalyst participates in the reaction, so some of it must be permanently used up or chemically converted.

Correct Fact

Although a catalyst interacts during the reaction, it is chemically regenerated by the end. Its chemical properties and overall mass are strictly conserved.

Misconception

Increasing the volume of acid used always increases the rate of reaction.

Correct Fact

Increasing the volume of acid without changing its concentration does not alter the rate of reaction. Rate depends on concentration, not total volume.

Misconception

Diluting an acid with water makes the reaction faster because there is more total liquid.

Correct Fact

Adding water lowers the concentration of acid particles per unit volume, which actually decreases the reaction rate.

Summary

Key facts to remember.

  • Factors increasing reaction rate: smaller solid reactant size, higher solution concentration, higher temperature, presence of a suitable catalyst, and higher pressure for gaseous reactants only.
  • Acronym: CSTP — Concentration, Size, Temperature, Pressure.
  • Catalyst properties: speeds up reaction, remains chemically unchanged, mass is conserved, does not change total product yield.
  • Experimental measurements: gas syringe or inverted burette for gas volume over time, and disappearing cross method with Rate = 1/t for precipitate formation.
  • On a volume–time graph, a faster reaction is steeper at the start but reaches the same final volume if reactant quantities are identical.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

What happens to the mass and chemical properties of a catalyst once the chemical reaction is complete?

2

For a chemical reaction occurring at room temperature, what is the approximate effect on the reaction rate if the temperature is increased by \(10\ ^\circ\text{C}\)?

B. Fill in the Blanks

3

In the experiment between sodium thiosulphate solution and sulphuric acid, the insoluble yellow solid that causes the solution to turn cloudy is ______.

4

Increasing pressure increases the rate of reaction only for reactions involving ______ reactants.

C. Match the Factor with Its Characteristic

Drag each factor on desktop. On mobile, tap a factor and then tap the matching box.

Answer Choices

Smaller solid reactant size Higher solution concentration Addition of a catalyst Higher pressure

Characteristic / Effect

1Provides a larger total surface area exposed for reaction
2Contains more solute particles per unit volume of solution
3Speeds up reaction while retaining unchanged mass and chemical identity
4Compresses gas particles into a smaller volume, increasing their concentration
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Total surface area Concentration Catalyst Precipitate Initial rate of reaction CSTP
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Rate of Reaction
Topic : Factors Affecting Rate of Reactions
Form 4 Chemistry
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