2.2.4

# Experimental Methods for Rates (A2 Only)

Test yourself

## 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.