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.
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.
Numerical example: If the tangent at minute 2 passes through \((0.2,20)\) and \((4,64)\), the rate at that time is \(\dfrac{64-20}{4-0.2}=11.58\text{ cm}^3\text{ min}^{-1}\).
For a decreasing reactant-mass graph, the tangent gradient is negative; its magnitude reports the rate of mass decrease as a positive value.
Try These Questions First
Answer mentally, then check the explanation.
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.
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.
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).
Common Mistakes
Avoid these errors when calculating rates of reaction.
Common Misconceptions
Compare each misconception with the correct fact.
The rate of a chemical reaction remains constant from beginning to end.
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.
Instantaneous rate at time \(t\) can be calculated simply by dividing the volume at time \(t\) by \(t\) directly from the table.
\(\dfrac{V_t}{t}\) 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.
When calculating mass loss of a reaction producing gas on an electronic balance, the flask must be sealed tightly with a rubber stopper.
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
Which of the following processes is classified as a slow reaction?
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
The rate of reaction at a particular point of time is called the ______ rate of reaction.
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
Keywords
Tap or hover over a term to read its meaning.
