4.1 Introduction to Rate of Reaction
A chemical reaction changes reactants into products. The rate of reaction measures how fast or slow this change takes place per unit time.
Learning Objectives
- State the definition of a chemical reaction and rate of reaction.
- Compare and contrast fast reactions and slow reactions in daily life.
- Identify observable and measurable changes in reactants or products used to determine the rate of reaction.
- Calculate the average rate of reaction over a given time interval and for the whole reaction.
- Determine the instantaneous rate of reaction at a particular point in time using the gradient of a tangent to a curve.
Comparison of Fast and Slow Reactions
Graphs and a concept map show how reactant and product quantities change over time, and how a tangent gives the instantaneous rate.
Short Explanation
Definition
A chemical reaction is a process in which one or more reactants are converted to one or more products.
\[ \text{Reactant} \xrightarrow{\text{Chemical reaction}} \text{Product} \]
During a reaction, the quantity of reactant decreases with time, while the quantity of product increases with time.
Example
Colourless potassium iodide solution reacts with colourless lead(II) nitrate solution to produce a yellow precipitate of lead(II) iodide and colourless potassium nitrate solution.
\[ \text{Lead(II) nitrate} + \text{Potassium iodide} \rightarrow \text{Lead(II) iodide} + \text{Potassium nitrate} \]
Fast Reactions vs Slow Reactions
Similarities
In both fast and slow reactions, the quantity of reactant decreases and the quantity of product increases.
Differences
A fast reaction occurs quickly, has a high rate of reaction and takes a shorter time to complete. A slow reaction occurs slowly, has a low rate of reaction and takes a longer time to complete.
Rate of Reaction
Definition
The rate of reaction is the change in the quantity of reactant or product per unit time.
\[ \text{Rate of reaction} = \dfrac{\text{Change in the quantity of reactant or product}}{\text{Time taken for the change to occur}} \]
Example Calculation
\(0.3\text{ g}\) of magnesium tape reacts completely with excess dilute hydrochloric acid in \(30\text{ s}\).
\[ \text{Rate of reaction} = \dfrac{\text{Decrease in mass of magnesium}}{\text{Time taken}} = \dfrac{0.3\text{ g}}{30\text{ s}} = 0.01\text{ g s}^{-1} \]
Observable and Measurable Changes
- Decrease in mass, volume or concentration of reactant.
- Increase in mass, volume or concentration of product.
- Decrease or increase in pressure, temperature, pH value, electrical conductivity, heat conductivity or colour intensity of the reacting mixture.
- Increase in the volume or pressure of gas released.
- Increase in the height of the precipitate formed.
Average Rate of Reaction
The average rate of reaction is the average value for the rate of reaction that occurs in a specific time interval.
In the first \(t\) seconds:
\[ \text{Average rate} = \dfrac{\text{Total volume of gas collected in } t \text{ seconds}}{t} \]
In a specific time interval from \(t_1\) to \(t_2\):
\[ \text{Average rate} = \dfrac{V_2 - V_1}{t_2 - t_1} \]
For the whole reaction:
\[ \text{Average rate} = \dfrac{\text{Total volume of gas collected}}{\text{Time taken for the reaction to complete}} \]
The time taken for the whole reaction to complete is the exact point when the product curve becomes horizontal, not necessarily the end time recorded in the table or graph.
Instantaneous Rate of Reaction
The instantaneous rate of reaction is the rate of reaction at any particular point of time or specific instance.
It is determined from the gradient of the tangent to the curve at time \(t\):
\[ \text{Rate of reaction at time } t = \text{Gradient of the tangent to the curve at time } t = \dfrac{\Delta y}{\Delta x} \]
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is meant by the rate of reaction?
In an experiment, \(20.00\text{ cm}^3\) of gas is collected in the first \(60\text{ seconds}\). Calculate the average rate of reaction for the first minute.
How do you determine the instantaneous rate of reaction from a curve graph of volume of gas against time?
Common Mistakes
- Using the entire duration shown on the x-axis, such as \(360\text{ s}\), to calculate the average rate for the whole reaction, instead of using the time when the reaction actually stopped, where the curve becomes completely horizontal, such as \(180\text{ s}\).
- Confusing “for the first 2 minutes” (\(0\text{ s}\) to \(120\text{ s}\)) with “in the second minute” (\(60\text{ s}\) to \(120\text{ s}\)).
- Forgetting to write the correct units, such as writing \(\text{cm}^3\) instead of \(\text{cm}^3\text{ s}^{-1}\), or \(\text{g}\) instead of \(\text{g s}^{-1}\).
- Drawing an inaccurate tangent that crosses through the curve like a secant line rather than touching the curve at exactly one point.
Concept Misunderstandings
The rate of reaction remains constant throughout the entire chemical reaction.
The rate of reaction is usually fastest at the beginning when reactant concentration is highest, decreases gradually as reactants are used up, and eventually reaches zero when the reaction stops.
A slow reaction produces less total product than a fast reaction.
Speed does not determine the total quantity produced. A slow reaction simply takes a longer time to produce its maximum yield.
Summary
- A chemical reaction changes reactants into products.
- As time increases, the quantity of reactants decreases and the quantity of products increases.
- Fast reactions have a high rate of reaction and short reaction time; slow reactions have a low rate of reaction and long reaction time.
- Observable changes include change in mass, volume of gas, height of precipitate, pH, temperature or colour intensity.
- Average rate of reaction \(= \dfrac{\text{Change in quantity}}{\text{Time taken}}\).
- Instantaneous rate of reaction \(=\) gradient of the tangent to the curve at time \(t\).
Short Activity
A. Objective Quiz
1 Which of the following is an example of a fast reaction in daily life?
2 How is the instantaneous rate of reaction determined from a graph of volume of gas against time?
B. Fill in the Blanks
3 Rate of reaction is defined as the change in the quantity of reactant or product per unit .
4 If \(0.3\text{ g}\) of magnesium tape reacts completely with excess acid in \(30\text{ s}\), the rate of reaction is \(\text{g s}^{-1}\).
C. Drag and Drop: Match the Description to Its Term
Drag each term to the correct description. 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 Description to Its Term
Write the correct term next to each description.
| Characteristic / Definition | Concept / Reaction Type |
|---|---|
| 1. Takes a shorter time to complete and has a high rate of reaction | |
| 2. Takes a longer time to complete and has a low rate of reaction | |
| 3. Average value of the rate of reaction over a specific time interval | |
| 4. Rate of reaction at a specific point of time given by the gradient of the tangent |
Answer choices: Fast reaction • Slow reaction • Average rate of reaction • Instantaneous rate of reaction
