Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.4 Initial-rate and continuous monitoring methods-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.4 Initial-rate and continuous monitoring methods- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.4 Initial-rate and continuous monitoring methods- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
Key Concepts:
11.4 understand experiments that can be used to investigate reaction rates by:
i an initial-rate method, carrying out separate experiments where different initial concentrations of one reagent are used
A ‘clock reaction’ is an acceptable approximation of this method.
ii a continuous monitoring method to generate data to enable concentration-time or volume-time graphs to be plotted
Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
11.4 Investigating Reaction Rates
(i) Initial-Rate Method
The initial-rate method involves measuring the rate of a reaction at the very start by carrying out separate experiments with different initial concentrations of a reactant.
Method
- Perform multiple experiments
- Change concentration of one reactant only
- Keep all other conditions constant (temperature, volume, etc.)
- Measure initial rate (rate at \( t = 0 \))
- Initial rate determined from:
- Gradient of tangent at start of graph
- Or time taken for a fixed small change
Purpose
- To determine how rate depends on concentration
- Used to find order of reaction
- Used to derive rate equation
Clock Reaction (Approximation)
A clock reaction measures the time taken for a visible change (e.g. colour change) to occur, which corresponds to a fixed amount of product formed.
- Time measured instead of rate directly
- Rate ∝ \( \frac{1}{\text{time}} \)
- Common example: iodine clock reaction
Key Considerations
- Only ONE variable changed at a time
- Accurate timing required
- Initial rate avoids complications from changing concentrations
Example 1
In an experiment, the concentration of reactant A is doubled while all other conditions are kept constant. The initial rate increases by a factor of 4. Explain how the initial-rate method is used here and deduce the order with respect to A.
▶️ Answer/Explanation
Separate experiments carried out with different initial concentrations of A
Initial rate measured at start of each reaction
Doubling [A] causes rate to increase by 4
\( 2^n = 4 \Rightarrow n = 2 \)
Reaction is second order with respect to A
Example 2
In a clock reaction, the time taken for a colour change is measured. When the concentration of reactant B is tripled, the time taken decreases from 60 s to 20 s. Deduce the order with respect to B.
▶️ Answer/Explanation
Rate ∝ \( \frac{1}{\text{time}} \)
Initial rate increases from \( \frac{1}{60} \) to \( \frac{1}{20} \)
Rate increases by factor of 3
Concentration increases by factor of 3
\( 3^n = 3 \Rightarrow n = 1 \)
Reaction is first order with respect to B
(ii) Continuous Monitoring Method
The continuous monitoring method involves measuring a changing physical property of a reaction mixture continuously over time to obtain rate data.
Method
- Reaction is followed in real time
- A measurable property is recorded continuously
- Data is used to plot graphs
- Common measurements include:
- Concentration vs time
- Volume of gas vs time
- Mass vs time
- Absorbance vs time
Use of Graphs
- Gradient of curve at any point = rate at that time
- Initial rate = gradient at \( t = 0 \)
- Curve becomes less steep as reaction proceeds
- Allows analysis of how rate changes over time
- Can determine half-life and reaction order
Purpose
- To obtain detailed rate data throughout the reaction
- To construct concentration–time or volume–time graphs
- To analyse rate changes as reaction progresses
Key Considerations
- Requires suitable measurable property
- Must ensure accurate and continuous data recording
- No need to remove samples (unlike titration)
Example 1
A reaction produces a gas which is collected in a gas syringe. The volume of gas is recorded every 10 seconds and a graph of volume vs time is plotted. Explain how the rate at 30 seconds can be determined.
▶️ Answer/Explanation
Draw a tangent to the curve at 30 s
Calculate gradient of tangent
Gradient = rate at that time
This gives instantaneous rate at 30 s
Example 2
A concentration–time graph shows that the slope becomes less steep as time increases. Explain what this indicates about the rate of reaction and why this occurs.
▶️ Answer/Explanation
Rate decreases over time
Gradient represents rate, so less steep → slower rate
Concentration of reactants decreases
Fewer effective collisions occur
Therefore, rate decreases as reaction proceeds