The effect of concentration and temperature on reaction rate

In this experiment, two colourless solutions are mixed to make a solution which becomes dark blue. Changing the concentration or temperature of the solutions changes the time required for the blue colour to develop.

This experiment should take 30 minutes.

Equipment

Apparatus

• Eye protection
• Beakers, 250 cm3, x2
• Water bath (or some means of warming solution A)

Chemicals

• Solution A – 4.3 g of KIO₃ per dm³
• Solution B – starch solution 

Health, safety and technical notes

• Read our standard health and safety guidance.
• Always wear eye protection.
• Both solutions are of low hazard.

Procedure

1. Place 50 cm³ of solution A in a 250 cm³ beaker.
2. Place the same volume of solution B in a second beaker.
3. Mix the two solutions by pouring from one beaker into the other several times.
4. Note the time required for a reaction to occur (formation of blue colour).
5. Repeat, but use solution A that has been diluted to one half the concentration. Note the time for the reaction to occur.
6. Repeat using solution A warmed to 35 °C. Note the time for a reaction to occur. 

Notes

The colour change takes about 5–6 minutes.

A colorimeter sensor or a light sensor set up as a colorimeter can be used to monitor colour change on the computer.

The result, in the form of graphs on the computer, provides very useful material for analysis using data logging software.

While a colorimeter sensor is ideal, it is easy to substitute a light sensor clamped against a plastic cuvette filled with the reactants.

The data logging software should clearly show the change occurring on a graph.

Measure the rate of change by using its slope or the time taken for a change to occur. 

Background theory

The mechanism is not clearly understood, but the following simplified sequence has been proposed.

IO₃− reacts with HSO₃− to form I⁻:

IO₃− + 3HSO₃− → I⁻ + 3H⁺ + 3SO₄²⁻ 

I⁻ reacts with IO₃− to form I₂. 

I₂ is immediately consumed by reacting with HSO₃−:

I₂ + HSO³⁻ + H₂O → 2 I⁻ + SO₄²⁻ + 3H⁺

When all of the HSO³⁻ has been used up, I₂ accumulates.

Iodine reacts with starch to form a coloured complex.

Questions

1. Why does increasing the concentration usually result in an increased rate of reaction?
2. Why does increasing the temperature usually result in an increased rate of reaction? 
3. How could this experiment be set up, so it took exactly 10 min to turn blue?

Answers

1. There are more molecules of reactant in the solution, therefore more chance of reacting.
2. Increasing the temperature has two effects. Since the particles are moving faster, they will travel a greater distance in a given time and so will be involved in more collisions. In addition, because the particles are moving faster, a larger proportion of the collisions will exceed the activation energy, the energy needed to react. The rate of the reaction therefore increases.
3. Depending on the results of the experiment, increase/decrease concentration to a specific amount AND/OR increase/decrease the temperature by a specific amount.