CIE AS/A Level Chemistry 8.1 Rate of reaction Study Notes- 2025-2027 Syllabus
CIE AS/A Level Chemistry 8.1 Rate of reaction Study Notes – New Syllabus
CIE AS/A Level Chemistry 8.1 Rate of reaction Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Chemistry latest syllabus with Candidates should be able to:
Rates of Reaction and Collision Theory
The rate of a chemical reaction can be explained using collision theory. This theory describes how often particles collide and whether those collisions lead to a reaction.
Rate of Reaction
The rate of reaction is defined as:
the change in concentration of a reactant or product per unit time.
It can be measured by:
- how fast a reactant is used up
- how fast a product is formed
Units: \( \mathrm{mol\ dm^{-3}\ s^{-1}} \)
Collision Theory
Collision theory states that:
For a reaction to occur, reacting particles must collide.
However, not all collisions lead to a reaction.
Frequency of Collisions
The frequency of collisions refers to:
the number of collisions between reacting particles per unit time.
- Higher collision frequency increases reaction rate.
- Increasing concentration, pressure (for gases), or surface area increases collision frequency.
Effective Collisions
An effective collision is a collision that:
- has energy equal to or greater than the activation energy
- has the correct orientation of particles
Only effective collisions result in a chemical reaction.
Non-Effective Collisions
A non-effective collision is a collision that:
- does not have enough energy
- or does not have the correct orientation
These collisions do not lead to a reaction.
Relationship Between These Terms
- Higher collision frequency increases the chance of effective collisions.
- Higher temperature increases the proportion of collisions that are effective.
- The rate of reaction depends on the number of effective collisions per unit time.
Example
Define the rate of reaction.
▶️ Answer / Explanation
The rate of reaction is the change in concentration of a reactant or product per unit time.
Example
Explain why increasing concentration increases the rate of reaction.
▶️ Answer / Explanation
Increasing concentration increases the frequency of collisions between reacting particles, leading to more effective collisions per second.
Example
Explain why not all collisions between reacting particles lead to a reaction.
▶️ Answer / Explanation
Some collisions do not have enough energy to overcome the activation energy or do not have the correct orientation, so they are non-effective collisions.
Effect of Concentration and Pressure on Rate of Reaction
Changes in concentration (for solutions) and pressure (for gases) affect the rate of reaction by altering the frequency of effective collisions between reacting particles.
Effect of Concentration
Increasing the concentration of reactants increases the number of reacting particles in a given volume.
- Particles are closer together.
- Collisions occur more frequently.
- The number of effective collisions per unit time increases.
Result: the rate of reaction increases.
Decreasing concentration has the opposite effect, reducing collision frequency and lowering the reaction rate.
Effect of Pressure (Gaseous Reactions Only)
Increasing the pressure of a gas reduces the volume, forcing gas particles closer together.
- Gas particles collide more often.
- The frequency of collisions increases.
- More collisions are effective collisions.
Result: the rate of reaction increases.
Lowering pressure increases volume, reducing collision frequency and decreasing the rate.
Key Comparison
- Concentration affects reactions in solution.
- Pressure affects reactions involving gases.
- Both work by changing the frequency of effective collisions.
- Neither changes the energy of individual collisions.
Example
What happens to the rate of reaction when the concentration of a reactant is increased?
▶️ Answer / Explanation
The rate increases because there are more frequent effective collisions between particles.
Example
Explain why increasing the pressure increases the rate of a reaction involving gases.
▶️ Answer / Explanation
Increasing pressure forces gas particles closer together, increasing collision frequency and the number of effective collisions.
Example
A reaction rate increases when concentration is doubled. Explain this in terms of collision theory.
▶️ Answer / Explanation
Doubling concentration increases the number of reacting particles per unit volume, leading to more frequent collisions and therefore more effective collisions per second.
Calculating the Rate of a Reaction Using Experimental Data
The rate of reaction can be calculated from experimental data by measuring how fast a reactant is used up or how fast a product is formed. Rates are calculated using changes in measured quantities over time.
The rate of reaction is the change in concentration of a reactant or product per unit time.
\( \text{Rate} = \dfrac{\text{change in amount}}{\text{time taken}} \)
Depending on the experiment, the “amount” measured may be mass, volume of gas, or concentration.
Common Experimental Measurements
- Decrease in mass (e.g. gas escaping)
- Volume of gas produced
- Change in concentration (from titration or colour change)
The same rate equation applies in all cases.
Average Rate of Reaction
The average rate over a time interval is calculated using:
\( \text{Average rate} = \dfrac{\Delta \text{quantity}}{\Delta t} \)
Where:
- \( \Delta \) quantity = change in mass, volume or concentration
- \( \Delta t \) = time taken
Units of Rate
- \( \mathrm{g\ s^{-1}} \) when using mass
- \( \mathrm{cm^3\ s^{-1}} \) when using gas volume
- \( \mathrm{mol\ dm^{-3}\ s^{-1}} \) when using concentration
Example
In an experiment, 24 cm³ of carbon dioxide gas is produced in 60 seconds.
Calculate the average rate of reaction.
▶️ Answer / Explanation
\( \text{Rate} = \dfrac{24}{60} \)
\( \text{Rate} = 0.40\ \mathrm{cm^3\ s^{-1}} \)
Example
The mass of a reaction mixture decreases from 50.0 g to 49.2 g in 100 seconds due to gas loss.
Calculate the average rate of reaction.
▶️ Answer / Explanation
Change in mass = \( 50.0 – 49.2 = 0.8\ \mathrm{g} \)
\( \text{Rate} = \dfrac{0.8}{100} \)
\( \text{Rate} = 8.0 \times 10^{-3}\ \mathrm{g\ s^{-1}} \)
Example
The concentration of a reactant decreases from \( 0.50\ \mathrm{mol\ dm^{-3}} \) to \( 0.20\ \mathrm{mol\ dm^{-3}} \) in 150 seconds.
Calculate the average rate of reaction.
▶️ Answer / Explanation
Change in concentration:
\( \Delta [\mathrm{A}] = 0.50 – 0.20 = 0.30\ \mathrm{mol\ dm^{-3}} \)
\( \text{Rate} = \dfrac{0.30}{150} \)
\( \text{Rate} = 2.0 \times 10^{-3}\ \mathrm{mol\ dm^{-3}\ s^{-1}} \)