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Determining activation energy (Ea) of a reaction potassium between peroxodisulphate and iodine

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Ho Ka Wing (9) Group: 3 Date: 09-11-09 Determining activation energy (Ea) of a reaction potassium between peroxodisulphate and iodine Objective To determine the activation energy for the reduction of peroxodisulphate(VI)ions. S2O82- (aq) by iodide ions I- , using 'clock reaction'. Theory Activation Energy (Ea)-The minimum amount of energy required to start a reaction. A certain amount of activation must be supplied to initiating a reaction (even if the reaction is exothermic) because this energy must be absorbed to weaken or even break the bond holding the atoms in the reactant particles before the bond formation. The activation energy is used to overcome the energy barrier in the reaction. If the reactants do not gain energy that is greater or equal to the activation energy, the reaction won't occur. The equation for the reduction of peroxodisulphate(VI)ions by iodide ions : S2O82- (aq)+2I-(aq) -> 2SO42-(aq) +I2(aq) The equation of the reaction between thiosulphate ions and iodine formed in the above equation: 2S2O32-(aq) +I2 (aq) ->S4O62-(aq) +2I-(aq) Controlled Variables: 1. Concentration of potassium peroxodisulphate(VI)solution, potassium iodide solution,sodium thiosulphate solution and starch solution 2. Volume of potassium peroxodisulphate(VI)solution, potassium iodide solution, sodium thiosulphate solution and starch solution Dependent variable: - Time taken for the appearance of the blue-black color of the iodine-starch complex Independent variables: -temperature of the mixture Small amount Na2S2O3of is used to restrict the main reaction (the first reaction) ...read more.


skin contact * May cause sensitization by inhalation Sodium Thiosulphate - Na2S2O3 General Hazards * Irritant * Irritating to eyes, skin and repository system Potassium Peroxodisulphate - K2S2O8 General Hazards * Harmful * May cause sensitization by skin contact * May cause sensitization by inhalation Special Handling Information * Eye protection (safety goggles) must be worn at all times. * Wear gloves * Avoid skin contact with the chemicals Procedures: 1. the beaker was half-filled with water and heated to between 49? and 51?.This would be used as a water-bath. 2. 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 3. 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´┐Ż1?), the contents of the second boiling-tube was poured into the first one, shake to mix, and the clock was started. ...read more.


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. Temperature usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more thermal energy. 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 activation energy: 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) ...read more.

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