Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.3 Experimental techniques-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.3 Experimental techniques- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 11.3 Experimental techniques- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
Key Concepts:
11.3 be able to select and justify a suitable experimental technique to obtain rate data for a given reaction, including:
i titration
ii colorimetry
iii mass change
iv volume of gas evolved
v other suitable technique(s) for a given reaction
Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
11.3 Experimental Techniques to Measure Rate of Reaction
Selection of Suitable Experimental Technique
The rate of a reaction can be determined by measuring how a physical property changes with time. The choice of technique depends on the nature of the reactants and products.
- Method must allow measurable change over time
- Chosen based on observable property (colour, gas, mass, concentration)
- Must be accurate and suitable for reaction conditions
(i) Titration
Titration is used to determine the concentration of a reactant at different times by reacting it with a standard solution.
- Samples removed at fixed time intervals
- Reaction must be quenched (stopped) immediately
- Sample is titrated with a standard solution
- Volume used → concentration calculated
- Suitable for: slow reactions in solution
- Justification: allows accurate measurement of concentration over time
- Limitation: not suitable for very fast reactions
(ii) Colorimetry
Colorimetry measures the change in absorbance of a solution as the concentration of a coloured species changes during a reaction.
- Absorbance measured using a colorimeter
- Absorbance ∝ concentration (Beer-Lambert law)
- Continuous readings can be taken
- Suitable for: reactions involving coloured species
- Justification: allows continuous, real-time monitoring of rate
- Limitation: requires coloured reactant or product
(iii) Mass Change
Mass change is used to measure the rate of a reaction when a gas is produced and escapes from the system.
- Reaction carried out on a balance
- Mass decreases as gas is lost
- Readings taken at regular time intervals
- Suitable for: reactions producing gases
- Justification: simple method allowing continuous measurement
- Limitation: gas must escape freely for accurate results
(iv) Volume of Gas Evolved
The volume of gas produced during a reaction is measured over time to determine the rate of reaction.
- Gas collected using gas syringe or inverted measuring cylinder
- Volume recorded at regular time intervals
- Volume increases as reaction proceeds
- Suitable for: reactions producing measurable gas
- Justification: direct measurement of product formed
- Advantage: more accurate than mass loss (no gas escape errors)
- Limitation: requires airtight setup
(v) Other Suitable Techniques
Other techniques may be used depending on the property that changes during the reaction. The method chosen must allow continuous or measurable monitoring of the reaction progress.
- pH measurement: for reactions involving acids/bases
- Conductivity: for reactions involving ions (change in ionic concentration)
- Turbidity (precipitation): time taken for solution to become cloudy
- Pressure measurement: for gas reactions in closed systems
- Justification: technique depends on measurable physical change
- Key idea: choose method that gives clear, continuous data
Example 1
The reaction between sodium thiosulfate and hydrochloric acid produces a yellow precipitate of sulfur, making the solution cloudy. Suggest a suitable technique to measure the rate and justify your answer.
▶️ Answer/Explanation
Use colorimetry (or turbidity method)
Solution changes appearance due to formation of sulfur
Light transmission decreases as precipitate forms
This allows monitoring of rate based on change in absorbance/cloudiness
Example 2
A reaction between magnesium and hydrochloric acid produces hydrogen gas rapidly. Suggest the most suitable technique and justify why it is better than titration.
▶️ Answer/Explanation
Use mass change method
Hydrogen gas escapes → mass decreases
Allows continuous measurement of rate
Titration is unsuitable because reaction is too fast and cannot be sampled accurately
Example 3
The reaction between zinc and hydrochloric acid produces hydrogen gas. Explain why measuring volume of gas is more suitable than measuring mass change.
▶️ Answer/Explanation
Hydrogen gas is produced → can be collected
Volume of gas gives direct measurement of product formed
More accurate as no loss of gas to surroundings
Mass change method may lose gas → less accurate
Example 4
A reaction involves an acid being neutralised by a base over time. Suggest a suitable technique to measure the rate and justify your choice.
▶️ Answer/Explanation
Use pH measurement
pH changes as acid is neutralised
Can be monitored continuously using pH probe
Provides real-time data of reaction progress