Chemistry Concentration Investigation 

This investigation was conducted to see how changing the acid concentration affects the reaction rate between Potassium Iodide and Potassium Peroxydisulphate using a clock reaction.

2KI(aq) + K2S2O8(aq) � l2(aq) + 2K2SO4 (Reaction 1)

(CLOCK PROCESS= l2(aq)+2Na2S2O3(aq) � Na2S2O6(aq) + 2NaI(aq))


When Potassium Iodide and Potassium Peroxydisulphate, the mixture turns to a blue/black colour. There are two main methods for carrying out this experiment, one is the Clock Process, in which white card is placed under the reaction and the clock is stopped manually when the white card is no longer visible. This method is extremely straight forward, but may not be very accurate, as the clock is stopped manually when the card cannot be seen. There may be some inaccuracy in judging when the card is no longer visible, as we are simply using the naked eye. Also, human’s reactions are limited to around a tenth of a second; therefore, the stopping of the stopwatch may not be precise. The other method would be to use a Colorimeter which electronically detects and records the change of colour in the reaction. This method is extremely precise but is a lot more expensive and a lot harder work. A Colorimeter typically retails at around �800 and they also need to be calibrated and adjusted before the experiment, the colour also need to be measured every few seconds, and the results can only be interpreted in graphs.

Collision Theory-

What is Reaction Rate?

Reaction Rate is defined as how fast a reaction takes place. It is a measure of how fast a Chemical Reaction occurs.

What Factors Affect Reaction Rate?

Concentration and Pressure of Reactions

The higher the concentration or pressure of the reactants, the more collisions per unit time (e.g. milliseconds, seconds, minutes). More collisions lead to a higher reaction rate. This is due to the fact that, the more collisions happening, the more likely it is for the atoms to bond and cause a reaction, therefore, the more collisions, the higher/faster the reaction rate.


A higher temperature will give molecules more kinetic energy, therefore, they will move faster. The faster they move, the more collisions there will be. And the more collisions, the more chance of a reaction, which means that the higher the temperature, the higher the reaction rate.

Physical State

Reactions between liquids and gases are simple, as it is often easy for them to mix with one another. But with solids involved, the reaction is more complicated and there is a lower reaction rate. This is due to the fact that the reaction can only take place on the surface of the solid. Therefore, smaller sized particles are more likely to react than the large particles. This works in the same way as powder, a more divided powder has a higher reaction rate than a big lump.


A Catalyst has the ability to alter the rate of a chemical reaction without being used up or permanently changed. A Catalyst changes the energy in the reaction to provide an alternative route that lowers the Activation energy of each particle. This means that the energy will be more evenly dispersed, and more particles will have the energy required to collide successfully.

Techniques to Measure Reaction Rate

You are able to measure the rate at which reactants are used up in a reaction or the rate at which products are formed in a reaction.


To time how long the reaction takes at the specified concentration.

The stopwatch is being used to give an accurate measurement of time, the only limitation is how fast a human is able to stop the clock.

Table of Equipment




Test Tube Rack

To hold up and support the Test Tubes.

This will keep them positioned correctly, to ensure that there are no spills or leakage’s.

15 Boiling Tubes

The reaction with take place in the boiling tube.

I was originally going to use Test Tubes, but I found Boiling Tubes more suitable due to their larger width, which enabled me to see the reaction more clearly, and therefore determine when the white card was out of view.


To time how long the reaction takes at the specified concentration.

The stopwatch is being used to give an accurate measurement of time, the only limitation is how fast a human is able to stop the clock.

Potassium Iodide Solution

Potassium Peroxydisulphate Solution

Sodium Thiosulphate Solution

Starch Solution

2.5cm� Graduated Plastic Pipettes

To measure out each chemical.

The pipettes are extremely accurate in measuring out liquids, therefore, they should give a precise measurement.

White Card

To place underneath each Boiling Tube to determine when each reaction has fully completed.

Test Tube

To contain the Potassium Peroxydisulphate before adding it to the Boiling Tube

The Test Tube should be clean and new, it should be in perfect condition to prevent leakages. The Test Tube will effectively contain the chemical.



The same card – otherwise there may be a slight variation in colour which could lead to a misjudgement of when the reaction is complete

Stopclock – There may be a slight difference in time if another one is used, this is best for consistency

Style of pipettes – Using different make pipettes may cause inaccurate measuring, as they may slightly differ

Temperature – Temperature affects reaction rate, it is important to make the experiment completely fair

Volumes of each Chemical – This will ensure that the test is even and fair on each concentration

The same chemicals -It is important to use the same Starch Solution etc. Otherwise, the second starch solution may slightly differ, slightly affecting the results


Volume of KI – In order to make the different concentrations of the chemical that is being tested to see the change in reaction rate

Volume of Water- In order to make the different concentrations of the chemical that is being tested to see the change in reaction rate


Safety Goggles – Some chemicals are dangerous, and if an accident was to occur such as a splash, the Chemical may affect the eye.

Gloves – To avoid irritation of the skin, caused by the chemicals

Potassium Iodide Hazards

Potassium iodide may irritate the lungs or eyes.

Consumption of large amounts of iodide may harm the growing foetus, but normal use of this material in the laboratory will not expose you to dangerous levels.

This material is designated as “Not hazardous according to Directive 67/548/EC”, a categorization which is applied to chemicals which are not regarded as likely to cause harm.

Potassium Peroxydisulphate Hazards

It’s an oxidizing agent, which means it can be harmful if inhaled or if it comes into contact with eyes, nose, or other mucous membranes. Also, it has a relatively serious fire risk, where the gases could be poisonous if inhaled.

Sodium Thiosulphate Hazards

Sodium thiosulphate may be harmful if you swallow it. It may also irritate the eyes or lungs.

Starch Solution Hazards

May irritate the eyes or lungs, but this material presents a low hazard.


The following method refers to the test conducted on the highest possible concentration of KI

* The mixture is made up of the following chemicals-

Potassium Oidide ( KI ) 5cm�

Sodium Thiosulphate ( Na2S2O3 ) 2cm�

Starch Solution 1cm�

Potassium Peroxydisulphate ( K2S2O8 ) 2cm� – The Reactant

No Water ( H2O ) is needed as the highest concentration is being tested

* Measure out the measurements of each chemical given above precisely, using a new, clean Pipette each time. Squirt the chemical from the Pipette into a clean Boiling Tube. Do Not add the chemical Potassium Peroxydisulphate to the Boiling Tube, measure this chemical out in a separate clean Test Tube.

* When complete, there should be exactly 8cm� of liquid in the Boiling Tube. Mix the liquid in the Boiling Tube.

* Ready your stopwatch, check the temperature and ensure it is maintained throughout all the tests.

* Place the White Card with a black cross on it directly under the Boiling Tube

* Add the Potassium Peroxydisulphate from the Test Tube to the Boiling Tube, and immediately start the stopwatch.

* Mix the liquid with 4 even stirs, use 4 even stirs for each concentration to ensure each test is completely fair.

* Observe the colour change of the liquid, look at the White Card through the liquid, from above. Immediately stop the stopwatch when the White Card is out of visibility due to the colour change of the liquid.

* Record the result in a table as shown below-

* Conduct the exact same test twice more, and record the results in the allocated space in the Results Table. Make sure that all environmental conditions (temperature etc.), hygiene conditions and stirs are kept the same.

* The proceed to make the 2nd concentration by adding 1cm� of water to the Boiling Tube with the other chemicals.

* Repeat the experiment on the 2nd concentration and record all results on the Results Table

* Then create the 3rd concentration by adding 2cm� of water to the Boiling Tube and so on, until you have tested 5 concentrations, up to 4cm� of water. Each should have been tested 3 times, so in all, 15 tests have been conducted.