If Na2S2O3 was absent. After mixing the solution, the reaction will take place immediately. The iodine produced will immediate react with starch to form blue complex .The time taken for the appearance of blue colour of each 5 temp reading will be would be too quick and immeasurable
The reaction between peroxodisulphate (VI) ions and iodide ions in solution forms sulphate (VI) ions and iodine. The equation for this reaction is as follows:
S2O82- (aq) + 2I- (aq) → 2SO42- (aq) + 2I2 (aq)
The iodine formed immediately reacts with sodium thiosulphate.
2S2O32-(aq)+I2(aq)->S4O62-(aq)+2I-(aq)
When all the sodium thiosulphate solution is used, free iodine is presented. Starch solution is used as an indicator to indicate the production of free bromine by the appearance of the blue black colour of the iodine-starch complex. However a small amount of starch is needed since we just only focus on the sudden colour change of blue. This can reduce the chemical wastes.
This Ea value can be found by relating the rate constant, k, with the temperature, T, of the reaction. The two are related by the Arrhenius equation.
To find the (Ea) firstly we need to take the natural logs of all the items in the Arrhenius equation:
Since the rate constant:
(Since concentration is constant)
Put into the equation in place of k:
So the graph which would be plotted would be
The Ea can then be found from the gradient of the graph
Apparatus and Reagents Used
Safety spectacles
Beaker, 400 cm3
2 thermometers,0-100℃
Bunsen burner, tripod, gauze and mat
4 burettes and stands, with beakers and funnels for filling
6 boiling tubes
4 stopper
2 clamp and stand
Potassium peroxodisulphate(VI)solution, 0.020M K2S2O8
Potassium iodide solution, 0.50M KI
Sodium thiosulphate solution, 0.010M Na2S2O3
Starch solution, 0.2%
Stopwatch
Safety Precautions
Potassium Iodide – KI General Hazards
Sodium Thiosulphate – Na2S2O3 General Hazards
Potassium Peroxodisulphate – K2S2O8 General Hazards
Special Handling Information
- Eye protection (safety goggles) must be worn at all times.
- Wear gloves
- Avoid skin contact with the chemicals
Procedures:
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the beaker was half-filled with water and heated to between 49℃ and 51℃.This would be used as a water-bath.
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Burette was used to measure the volume of 10.00 cm3 potassium peroxodisulphate(VI)solution into the first boiling-tube. The first tube was clamped in the water-bath and a thermometer was placed in the solution in the first boiling tube
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Burette was used to measure the volume of 5.00 cm3 for both potassium iodide and sodium thiosulphate solutions and the volume of 2.5 cm3 starch solution into the second boiling tube. The second tube was clamped in the water-bath and a thermometer was placed in the solution in the second boiling tube
4.When the temperatures of the two solution were equal and constant(to within±1℃), the contents of the second boiling-tube was poured into the first one, shake to mix, and the clock was started.
5.When the blue colour of the starch-iodine complex appeared, the clock was stopped and the time was written down in a table.
6.The experiment was repeated at temperatures close to 45℃, 40℃, 35℃, 30℃.
Results Table
Calculation:
The Ea can then be found from the slope of the graph:
Slope=
Ea=-6250x-8.314
=51963J mol-1
=52kJ mol-1
Discussion on Errors
1. Error: the temperature measured may be the temperature of the boiling tube but not the mixture.
Improvement: thermometer should not touch the boiling tube. Boiling tubes have their own heat capacities; they will absorb part of heat from water-bath. Hence, the temperature inside boiling tubes is slightly lower than that of water-bath.
2. Error: the temperature of the mixture may vary since heat loss to the surrounding when the time for the appearance of blue colour is very long.
Improvement: Adjust the air hole of the Bunsen burner to maintain the temp of the water bath remain constant.
3. Random error: reaction time for members to press stop watch is not the same, so the time recorded may not be so accurate.
ImprovementL taking average value of the experiment by repeating the experiment
4. It is necessary to cover the reagent bottle containing NaS2O3 immediately after use because it can be easily oxidized by oxygen in air.
To solve the difficulty encountered in monitering the sudden colour change, we can use the calorimeter.We can set a standard meter reading for stopping the time. For each experimental measurement, once the meter reading reach the standard meter reading, stop the stopwatch immediately.
Conclusion
By using a 'clock' reaction, the experimental value of activation
energy of reduction of NaS2O3 by KI can be easily obtained by
measuring the slope of Arrhenius plot of log10(1/t) against 1/T.
the activation energy of a reaction between potassium peroxodisulphate and iodine is 52kJ mol-1
Comments
Reaction rate is directly proportional to temperature and inversely proportional to activation energy. Higher the temperature, higher the rate of reaction whereas higher the activation energy, lower the reaction rate. However, a rise in temperature has a more significant effect on the reactions with high activation energy.
usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more . Although collision frequency is greater at higher temperatures, this alone contributes only a very small proportion to the increase in rate of reaction. If we increase the temperature from 300 K to 310 K then the average speed increases by a factor of (310/300)1/2 is about 1.016 which is an increase of 1.6%. However the rate usually increases by 200%-300%. The effect of increasing collision frequency on the rate of the reaction is very minor.
Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than : E > Ea) is significantly higher.The distribution curves below in diagram 1 for 300 K and 310 K with energy greater than 52kJ mol-1 shows what happens to the rate.
(Diagram 1: Enthalpy Profiles)
As can be seen in the graph a higher proportion of molecules have enough energy to react.
Reference
Textbook ( New Way Chemistry for Hong Kong A-level)
Study guide
Website ( www.wikipedia.org)
