Kinetics of the Iodine Clock Reaction

Objective• To fully determine the rate law for the

iodine clock reaction using the method of initial rates

The Reaction

The Rate Lawx,y,z are the orders of reaction

k is the rate constant

A is the collision factor

Ea is the Activation energy

The Reaction

The Rate

These are colorless

How will we measure the Rate?

• We could monitor [I2]t using UV/VIS spectroscopy

• But we will instead use the I2 clock reaction and the method of initial rates

What is the I2 Clock Reaction?

• To the reaction we add thiosulfate S2O32-(aq)

• S2O32-(aq) reacts very fast with any I2

• Only when all the S2O32-(aq) is used up do we

start to see I2

Why do we add Starch?

• In addition to thiosulfate we will be adding a small amount of starch

• Starch is an indicator for I2

• In the presence of I2 starch turns blue/black

• This will help us to see the I2 when all the thiosulfate is used up

Measuring the Rate• We have 3 reactions going on simultaneously

• As soon as all the S2O32- is used up I2 stays in solution

and reacts with starch and the solution turns green/blue/black

Measuring the Rate• We measure the time it takes for the solution to change

color

• This is the time it takes all of the [S2O32-]o to get used up

Measuring the Rate

• Measure the time Δt it takes to turn blue then the rate is calculated as

Method of Initial Rates

• Make up flask 1 and flask 2 for a given mixture

• Pour flask 2 into flask 1 and start timing• When solution starts to appear colored

stop timer• The time on the stop watch is Δt• Write time on board• Measure T(oC)• Dispose down sink • Repeat second trial

Day 2: Getting Ea and A

ignition tubes

hotplate

reaction vessel 2reaction vessel 1

thermometer

• Allow the contents of vessel 1 and vessel 2 to reach thermal equiibrium with the water• Add the contents of vessel 2 into vessel 1 – and leave it in the beaker• Stir with the thermometer

• 2 measurements above RT• 2 measurements below RT• 2 measurements at RT• Write T(oC) and Δt on board• Use class data