Experimental Methods for Rates (A2 Only)
Chemistry: AQA A Level, topic 2.2.4
The Initial Rates Method
The initial rates method can be used to find the rate equation for a reaction.
Initial rates method
- The initial rate is the rate of reaction at the beginning of the reaction.
- If you measure the initial rate over a variety of concentrations of a reactant, you can work out how rate depends on the concentration of that reactant.
- To find the initial rate, construct a tangent to the graph at time = 0.
- We showed you how to do this at the beginning of the rates section.
The iodine clock reaction
- The iodine clock reaction has a sudden colour change.
- The reaction happens between hydrogen peroxide and iodide ions according to the equation:
- H2O2 + 2I- + 2H+ → 2H2O + I2
- By varying the concentrations of hydrogen peroxide or iodide, you can get different reaction times.
How does it work?
- You add sodium thiosulfate and starch to the solution.
- Initially, the thiosulfate will react with any iodine formed and remove it from the solution.
- Once the thiosulfate is all gone, any iodine formed will turn the solution black by reacting with the starch.
Continuous Monitoring
Continuous monitoring is another method for determining rate equations.
Continuous monitoring
- Continuous monitoring is the name given to taking measurements of rates at specific intervals.
- You tend to measure the change in a quantity of a substance.
- These can include:
- Loss of mass.
- Volume of gas evolved.
- Change in colour intensity.
- pH change.
Colorimetry
Colorimetry can be best explained by looking at redox titrations.
Redox titrations
- Titration is an experimental technique for finding the concentrations of solutions.
- It uses balanced equations to calculate the concentration of an unknown solution from a volume of a known solution (measured by a burette).
- The experimental technique often involves an indicator to measure the endpoint of a reaction.
- A small volume of solution is dropped from the burette into the reaction flask, and this is repeated until the indicator shows no further reaction happening.
Test yourself on Experimental Methods for Rates (A2 Only)
What must you construct on a concentration vs time graph to determine the initial rate?
The Iodine Clock Reaction
What is NOT a suitable factor to measure for continuous monitoring?
Which of these statements are most true of a titration?