Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.5 Order determination-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.5 Order determination- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.5 Order determination- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
Key Concepts:
11.5 be able to deduce the order (0, 1 or 2) with respect to a substance in a rate equation, using data from:
i a concentration-time graph
ii a rate-concentration graph
iii an initial-rate method
Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
11.5 Order determination
(i) Using Concentration–Time Graphs
The order of a reaction with respect to a reactant can be deduced by analysing how its concentration changes with time using a concentration–time graph.
Method
- Examine how concentration decreases over time
- Determine half-life values from the graph
- Compare half-life at different points
Identification of Order
Zero order:
- Concentration decreases linearly (straight line)
- Rate is constant
- Half-life decreases over time
First order:
- Curve decreases exponentially
- Constant half-life
- Rate ∝ concentration
Second order:
- Curve decreases more steeply at start
- Half-life increases over time
- Rate depends on concentration squared
Key Indicators Summary
- Constant half-life → first order
- Decreasing half-life → zero order
- Increasing half-life → second order
Example 1
A concentration–time graph shows that the concentration decreases from \( \mathrm{1.00 \rightarrow 0.50} \) in 20 s, \( \mathrm{0.50 \rightarrow 0.25} \) in 20 s, \( \mathrm{0.25 \rightarrow 0.125} \) in 20 s.
Deduce the order of the reaction and justify your answer.
▶️ Answer/Explanation
Half-life = 20 s for each halving
Half-life is constant
Therefore, reaction is first order
Example 2
A concentration–time graph shows that the concentration halves as follows:
10 s → 30 s → 70 s
Deduce the order of the reaction and explain your reasoning.
▶️ Answer/Explanation
Half-life increases (10 → 20 → 40 s)
Time taken for each halving increases
This indicates second-order reaction
Rate decreases more rapidly as concentration decreases
(ii) Using Rate–Concentration Graphs
The order of a reaction with respect to a reactant can be determined by analysing how the rate of reaction changes with the concentration of that reactant.
Method
- Plot rate (y-axis) against concentration (x-axis)
- Observe the shape of the graph
- Determine relationship between rate and concentration
Identification of Order
Zero order:
- Horizontal straight line
- Rate independent of concentration
- Rate = constant
First order:
- Straight line through origin
- Rate ∝ concentration
- Gradient = \( \mathrm{k} \)
Second order:
- Curved line (upward curve)
- Rate ∝ concentration²
- Gradient increases with concentration
Key Relationships
- Zero order: \( \mathrm{rate = k} \)
- First order: \( \mathrm{rate = k[A]} \)
- Second order: \( \mathrm{rate = k[A]^2} \)
Example 1
A graph of rate against concentration gives a straight line passing through the origin. Explain what this indicates about the reaction and determine the order.
▶️ Answer/Explanation
Rate is directly proportional to concentration
Straight line through origin → linear relationship
Therefore, reaction is first order
Rate equation: \( \mathrm{rate = k[A]} \)
Example 2
A graph of rate against concentration is curved, with the gradient increasing as concentration increases. Deduce the order of the reaction and explain your reasoning.
▶️ Answer/Explanation
Rate increases more rapidly than concentration
Graph is not linear → not first order
Upward curve indicates rate ∝ concentration²
Therefore, reaction is second order
(iii) Using Initial-Rate Method
The order of a reaction with respect to a reactant can be determined by comparing initial rates from experiments where only the initial concentration of that reactant is changed.
Method
- Carry out multiple experiments
- Change concentration of one reactant only
- Keep all other conditions constant
- Measure initial rate for each experiment
Determining Order
- Compare how rate changes when concentration changes
- Use relationship:
\( \mathrm{rate \propto [A]^n} \)
- If concentration doubles:
- Rate unchanged → zero order
- Rate doubles → first order
- Rate quadruples → second order
Key Approach
- Choose two experiments where only one concentration changes
- Calculate ratio of rates
- Calculate ratio of concentrations
- Use powers to determine order
Example 1
Experimental data for a reaction is shown:
Exp 1: \( \mathrm{[A] = 0.10,\ Rate = 2.0 \times 10^{-3}} \) Exp 2: \( \mathrm{[A] = 0.20,\ Rate = 8.0 \times 10^{-3}} \)
Determine the order with respect to A.
▶️ Answer/Explanation
[A] doubles (0.10 → 0.20)
Rate increases by factor of 4 \( 2.0 \rightarrow 8.0 \)
\( 2^n = 4 \Rightarrow n = 2 \)
Reaction is second order with respect to A
Example 2
Data for a reaction:
Exp 1: \( \mathrm{[B] = 0.50,\ Rate = 1.5 \times 10^{-3}} \) Exp 2: \( \mathrm{[B] = 1.00,\ Rate = 1.5 \times 10^{-3}} \)
Deduce the order with respect to B.
▶️ Answer/Explanation
[B] doubles (0.50 → 1.00)
Rate remains unchanged
\( 2^n = 1 \Rightarrow n = 0 \)
Reaction is zero order with respect to B