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7.1 Determination of Rate of Reaction

7.1 Determining Rate of Reaction
CHEMISTRY • Form 4 • Chapter 7: Rate of Reaction

7.1Determining Rate of Reaction

Chemical reactions occur all around us and inside our bodies at various speeds. Some reactions happen within a split second, such as the explosion of fireworks or the combustion of gas in a stove, while others take days, weeks, or even millions of years, such as the rusting of iron or rock erosion. Understanding the rate of reaction allows chemists and chemical engineers to monitor, predict, and control how quickly reactants turn into products in laboratory and industrial processes.

Learning Objectives

  • Classify fast and slow reactions that occur in daily life.
  • Explain the meaning of the rate of reaction.
  • Identify changes which can be observed and measured during chemical reactions through activity.
  • Determine the average rate of reaction and instantaneous rate of reaction.
  • Solve numerical problems based on the average and instantaneous rate of reaction.

Comparison of Average Rate and Instantaneous Rate Determination

Average rate is calculated from an interval, while instantaneous rate is found from the gradient of a tangent at a specific time.

Brief Explanation

Understand the key ideas before solving numerical problems.

Rate of reaction is the change in the quantity of reactant per unit time or the change in the quantity of product per unit time. During a chemical reaction, the quantity of reactant decreases with time while the quantity of product increases with time.

1. Fast Reactions

Reactions that occur rapidly within a short period of time. Examples include combustion of gases, ignition of matches, fireworks, reaction inside electric cells, and explosion at an oil factory.

2. Slow Reactions

Reactions that occur slowly and take a long duration to complete. Examples include rusting or corrosion of metals, rock erosion, fermentation, food decay, and photosynthesis.

Quantity measured Common unit
Mass of solids grams (g)
Volume of gases cm³ or dm³
Concentration of solutions mol dm⁻³
Time seconds (s) for fast reactions, minutes (min) for slow reactions

3. Observable and Measurable Changes

Scientists measure property changes that can be observed and measured over time:

  • Formation of precipitate
  • Decrease in mass of reactants
  • Increase in volume of gas released
  • Change in pressure
  • Change in electrical conductivity
  • Change in pH values

4. General Formula

\[\text{Rate of reaction} = \dfrac{\text{Change in the quantity of reactant or product}}{\text{Time taken for the change to occur}}\]

Common units of rate of reaction:

  • g s⁻¹ or g min⁻¹
  • cm³ s⁻¹ or cm³ min⁻¹
  • mol dm⁻³ s⁻¹ or mol dm⁻³ min⁻¹

5. Average Rate of Reaction

The average rate is the mean value of the reaction rate over a specific time interval.

\[\text{Overall average rate} = \dfrac{\text{Total volume of gas collected}}{\text{Total time taken for the reaction to complete}}\]

For the first \(t_1\) seconds:

\[\text{Average rate} = \dfrac{V_1 - 0}{t_1 - 0} = \dfrac{V_1}{t_1}\]

From time \(t_1\) to \(t_2\):

\[\text{Average rate} = \dfrac{V_2 - V_1}{t_2 - t_1}\]

6. Instantaneous Rate of Reaction

The instantaneous rate is the rate of reaction at a particular point in time. It is determined by drawing a tangent line to the curve on a plotted graph at that exact time \(t\) and finding its gradient.

\[\text{Rate of reaction at time } t = \text{Gradient of tangent} = \dfrac{\Delta V}{\Delta t} = \dfrac{V_2 - V_1}{t_2 - t_1}\]

At the completion of the reaction, the curve becomes horizontal; the tangent is horizontal (gradient = 0), so the rate of reaction is 0 cm³ s⁻¹.

Try These Questions First

Answer mentally, then check the explanation.

1

Classify each of the following chemical reactions as either a fast reaction or a slow reaction: (a) Explosion of fireworks during a celebration. (b) Rusting of an iron gate exposed to air and rain. (c) Neutralisation between hydrochloric acid and sodium hydroxide solution.

Answer: (a) Fast reaction. (b) Slow reaction. (c) Fast reaction.
2

Excess calcium carbonate chips are added to dilute hydrochloric acid. The volume of carbon dioxide released is recorded every minute. At 0 min: 0.0 cm³, 1 min: 22.0 cm³, 2 min: 36.0 cm³, 3 min: 45.0 cm³, 4 min: 50.0 cm³, 5 min: 50.0 cm³. Calculate: (a) The average rate of reaction in the second minute. (b) The overall average rate of reaction.

Answer: (a) Average rate in the 2nd minute: \(\dfrac{36.0 - 22.0}{2 - 1} = 14.0\text{ cm}^3\text{ min}^{-1}\). (b) The maximum volume of 50.0 cm³ is reached at 4 minutes, so the reaction stops at 4 minutes: \(\dfrac{50.0\text{ cm}^3}{4\text{ min}} = 12.5\text{ cm}^3\text{ min}^{-1}\).
3

State the observable and measurable change that can be used to determine the rate of reaction for each of the following: (a) Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). (b) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).

Answer: (a) Increase in the volume of hydrogen gas (H₂) collected per unit time (or decrease in mass of reactants over time). (b) Formation of white precipitate of barium sulphate (BaSO₄) per unit time / time taken for a mark 'X' to become obscured.

Common Mistakes

Avoid these errors when calculating rates of reaction.

!
Using total recorded experimental time instead of the time when reaction stopped. When calculating overall average rate, students frequently divide total gas volume by the last time printed on the table (e.g., 10 minutes), even though the gas volume stopped changing at 7 minutes. The divisor must be the exact time the reaction completed (7 minutes).
!
Confusing 'rate for the first \(n\) minutes' with 'rate in the \(n\)-th minute'. Rate for the first 3 minutes \(= \dfrac{V_3 - 0}{3 - 0}\). Rate in the 3rd minute \(= \dfrac{V_3 - V_2}{3 - 2}\).
!
Omitting or writing incorrect units. Omitting the time unit or writing volume units without time (e.g., writing cm³ instead of cm³ s⁻¹ or cm³ min⁻¹).
!
Drawing inaccurate tangents. Drawing a chord across the curve instead of a tangent that touches the curve at exactly one point, or drawing too small of a gradient triangle, which causes significant rounding errors.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

The rate of a chemical reaction remains constant from beginning to end.

Correct Fact

The rate of reaction is highest at the beginning because the concentrations/quantities of reactants are at their maximum. The rate gradually decreases as reactants are depleted, eventually becoming zero when one or more reactants are completely exhausted.

Misconception

Instantaneous rate at time \(t\) can be calculated simply by dividing the volume at time \(t\) by \(t\) directly from the table.

Correct Fact

\(\dfrac{V_t}{t}\) gives the average rate from 0 to \(t\). The instantaneous rate at time \(t\) must be obtained by measuring the gradient of the tangent to the curve at that specific point.

Misconception

When calculating mass loss of a reaction producing gas on an electronic balance, the flask must be sealed tightly with a rubber stopper.

Correct Fact

If sealed tightly with a rubber stopper, gas cannot escape, and the balance reading will not decrease. A loose cotton wool plug is used so gas can escape while preventing liquid droplets from splashing out.

Summary

Key facts to remember.

  • Rate of reaction measures the change in quantity of reactant or product per unit time.
  • Observable and measurable changes include the volume of gas released, mass loss due to escaping gas, precipitate formation, electrical conductivity, pressure, and pH.
  • Average rate of reaction is calculated over an interval of time: \(\dfrac{\Delta \text{quantity}}{\Delta \text{time}}\).
  • Overall average rate \(= \dfrac{\text{Total product formed}}{\text{Time when reaction stops}}\).
  • Instantaneous rate of reaction is the gradient of the tangent line drawn to the curve at that specific instant: \(\dfrac{\Delta V}{\Delta t}\).
  • Reactions start at their fastest rate, slow down gradually as reactants are consumed, and reach a rate of zero when the reaction is complete.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following processes is classified as a slow reaction?

2

The reaction between excess zinc powder and dilute hydrochloric acid is completed at the 4th minute, producing a total of 48 cm³ of hydrogen gas. What is the overall average rate of reaction?

B. Fill in the Blanks

3

The rate of reaction at a particular point of time is called the ______ rate of reaction.

4

When excess marble chips react with nitric acid, the mass of the flask decreases over time due to the release of ______ gas.

C. Match the Reaction or Concept with the Measurable Change

Drag each item on desktop. On mobile, tap an item and then tap the matching box.

Reaction / Concept

AgNO₃(aq) + NaCl(aq) Mg(s) + 2HCl(aq) Instantaneous rate of reaction 2SO₂(g) + O₂(g) → 2SO₃(g)

Measurable Change or Property

1Formation of precipitate that causes an 'X' mark to disappear
2Increase in the volume of hydrogen gas measured using a gas syringe
3Gradient of the tangent line drawn to the curve at time \(t\)
4Change in pressure measured using a pressure meter
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Rate of reaction Average rate of reaction Instantaneous rate of reaction Tangent Precipitate Overall average rate
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

7.1 Determination of Rate of Reaction

7.1 Determining Rate of Reaction
CHEMISTRY • Form 4 • Chapter 7: Rate of Reaction

7.1Determining Rate of Reaction

Chemical reactions occur all around us and inside our bodies at various speeds. Some reactions happen within a split second, such as the explosion of fireworks or the combustion of gas in a stove, while others take days, weeks, or even millions of years, such as the rusting of iron or rock erosion. Understanding the rate of reaction allows chemists and chemical engineers to monitor, predict, and control how quickly reactants turn into products in laboratory and industrial processes.

Learning Objectives

  • Classify fast and slow reactions that occur in daily life.
  • Explain the meaning of the rate of reaction.
  • Identify changes which can be observed and measured during chemical reactions through activity.
  • Determine the average rate of reaction and instantaneous rate of reaction.
  • Solve numerical problems based on the average and instantaneous rate of reaction.

Comparison of Average Rate and Instantaneous Rate Determination

Average rate is calculated from an interval, while instantaneous rate is found from the gradient of a tangent at a specific time.

Brief Explanation

Understand the key ideas before solving numerical problems.

Rate of reaction is the change in the quantity of reactant per unit time or the change in the quantity of product per unit time. During a chemical reaction, the quantity of reactant decreases with time while the quantity of product increases with time.

1. Fast Reactions

Reactions that occur rapidly within a short period of time. Examples include combustion of gases, ignition of matches, fireworks, reaction inside electric cells, and explosion at an oil factory.

2. Slow Reactions

Reactions that occur slowly and take a long duration to complete. Examples include rusting or corrosion of metals, rock erosion, fermentation, food decay, and photosynthesis.

Quantity measured Common unit
Mass of solids grams (g)
Volume of gases cm³ or dm³
Concentration of solutions mol dm⁻³
Time seconds (s) for fast reactions, minutes (min) for slow reactions

3. Observable and Measurable Changes

Scientists measure property changes that can be observed and measured over time:

  • Formation of precipitate
  • Decrease in mass of reactants
  • Increase in volume of gas released
  • Change in pressure
  • Change in electrical conductivity
  • Change in pH values

4. General Formula

\[\text{Rate of reaction} = \dfrac{\text{Change in the quantity of reactant or product}}{\text{Time taken for the change to occur}}\]

Common units of rate of reaction:

  • g s⁻¹ or g min⁻¹
  • cm³ s⁻¹ or cm³ min⁻¹
  • mol dm⁻³ s⁻¹ or mol dm⁻³ min⁻¹

5. Average Rate of Reaction

The average rate is the mean value of the reaction rate over a specific time interval.

\[\text{Overall average rate} = \dfrac{\text{Total volume of gas collected}}{\text{Total time taken for the reaction to complete}}\]

For the first \(t_1\) seconds:

\[\text{Average rate} = \dfrac{V_1 - 0}{t_1 - 0} = \dfrac{V_1}{t_1}\]

From time \(t_1\) to \(t_2\):

\[\text{Average rate} = \dfrac{V_2 - V_1}{t_2 - t_1}\]

6. Instantaneous Rate of Reaction

The instantaneous rate is the rate of reaction at a particular point in time. It is determined by drawing a tangent line to the curve on a plotted graph at that exact time \(t\) and finding its gradient.

\[\text{Rate of reaction at time } t = \text{Gradient of tangent} = \dfrac{\Delta V}{\Delta t} = \dfrac{V_2 - V_1}{t_2 - t_1}\]

At the completion of the reaction, the curve becomes horizontal; the tangent is horizontal (gradient = 0), so the rate of reaction is 0 cm³ s⁻¹.

Try These Questions First

Answer mentally, then check the explanation.

1

Classify each of the following chemical reactions as either a fast reaction or a slow reaction: (a) Explosion of fireworks during a celebration. (b) Rusting of an iron gate exposed to air and rain. (c) Neutralisation between hydrochloric acid and sodium hydroxide solution.

Answer: (a) Fast reaction. (b) Slow reaction. (c) Fast reaction.
2

Excess calcium carbonate chips are added to dilute hydrochloric acid. The volume of carbon dioxide released is recorded every minute. At 0 min: 0.0 cm³, 1 min: 22.0 cm³, 2 min: 36.0 cm³, 3 min: 45.0 cm³, 4 min: 50.0 cm³, 5 min: 50.0 cm³. Calculate: (a) The average rate of reaction in the second minute. (b) The overall average rate of reaction.

Answer: (a) Average rate in the 2nd minute: \(\dfrac{36.0 - 22.0}{2 - 1} = 14.0\text{ cm}^3\text{ min}^{-1}\). (b) The maximum volume of 50.0 cm³ is reached at 4 minutes, so the reaction stops at 4 minutes: \(\dfrac{50.0\text{ cm}^3}{4\text{ min}} = 12.5\text{ cm}^3\text{ min}^{-1}\).
3

State the observable and measurable change that can be used to determine the rate of reaction for each of the following: (a) Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). (b) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).

Answer: (a) Increase in the volume of hydrogen gas (H₂) collected per unit time (or decrease in mass of reactants over time). (b) Formation of white precipitate of barium sulphate (BaSO₄) per unit time / time taken for a mark 'X' to become obscured.

Common Mistakes

Avoid these errors when calculating rates of reaction.

!
Using total recorded experimental time instead of the time when reaction stopped. When calculating overall average rate, students frequently divide total gas volume by the last time printed on the table (e.g., 10 minutes), even though the gas volume stopped changing at 7 minutes. The divisor must be the exact time the reaction completed (7 minutes).
!
Confusing 'rate for the first \(n\) minutes' with 'rate in the \(n\)-th minute'. Rate for the first 3 minutes \(= \dfrac{V_3 - 0}{3 - 0}\). Rate in the 3rd minute \(= \dfrac{V_3 - V_2}{3 - 2}\).
!
Omitting or writing incorrect units. Omitting the time unit or writing volume units without time (e.g., writing cm³ instead of cm³ s⁻¹ or cm³ min⁻¹).
!
Drawing inaccurate tangents. Drawing a chord across the curve instead of a tangent that touches the curve at exactly one point, or drawing too small of a gradient triangle, which causes significant rounding errors.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

The rate of a chemical reaction remains constant from beginning to end.

Correct Fact

The rate of reaction is highest at the beginning because the concentrations/quantities of reactants are at their maximum. The rate gradually decreases as reactants are depleted, eventually becoming zero when one or more reactants are completely exhausted.

Misconception

Instantaneous rate at time \(t\) can be calculated simply by dividing the volume at time \(t\) by \(t\) directly from the table.

Correct Fact

\(\dfrac{V_t}{t}\) gives the average rate from 0 to \(t\). The instantaneous rate at time \(t\) must be obtained by measuring the gradient of the tangent to the curve at that specific point.

Misconception

When calculating mass loss of a reaction producing gas on an electronic balance, the flask must be sealed tightly with a rubber stopper.

Correct Fact

If sealed tightly with a rubber stopper, gas cannot escape, and the balance reading will not decrease. A loose cotton wool plug is used so gas can escape while preventing liquid droplets from splashing out.

Summary

Key facts to remember.

  • Rate of reaction measures the change in quantity of reactant or product per unit time.
  • Observable and measurable changes include the volume of gas released, mass loss due to escaping gas, precipitate formation, electrical conductivity, pressure, and pH.
  • Average rate of reaction is calculated over an interval of time: \(\dfrac{\Delta \text{quantity}}{\Delta \text{time}}\).
  • Overall average rate \(= \dfrac{\text{Total product formed}}{\text{Time when reaction stops}}\).
  • Instantaneous rate of reaction is the gradient of the tangent line drawn to the curve at that specific instant: \(\dfrac{\Delta V}{\Delta t}\).
  • Reactions start at their fastest rate, slow down gradually as reactants are consumed, and reach a rate of zero when the reaction is complete.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following processes is classified as a slow reaction?

2

The reaction between excess zinc powder and dilute hydrochloric acid is completed at the 4th minute, producing a total of 48 cm³ of hydrogen gas. What is the overall average rate of reaction?

B. Fill in the Blanks

3

The rate of reaction at a particular point of time is called the ______ rate of reaction.

4

When excess marble chips react with nitric acid, the mass of the flask decreases over time due to the release of ______ gas.

C. Match the Reaction or Concept with the Measurable Change

Drag each item on desktop. On mobile, tap an item and then tap the matching box.

Reaction / Concept

AgNO₃(aq) + NaCl(aq) Mg(s) + 2HCl(aq) Instantaneous rate of reaction 2SO₂(g) + O₂(g) → 2SO₃(g)

Measurable Change or Property

1Formation of precipitate that causes an 'X' mark to disappear
2Increase in the volume of hydrogen gas measured using a gas syringe
3Gradient of the tangent line drawn to the curve at time \(t\)
4Change in pressure measured using a pressure meter
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Rate of reaction Average rate of reaction Instantaneous rate of reaction Tangent Precipitate Overall average rate
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 : Determining Rate of Reaction
Form 4 Chemistry
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