top-rated free essay

To Determine the Activation Energy for the Reduction of Peroxodisulphate(Vi) Ions

By kellykaying Feb 26, 2011 882 Words
Determining activation energy (Ea) of a reaction
© KCl      http://hk.geocities.com/fatherofchemistry
Objective
           To determine the activation energy for the reduction of peroxodisulphate(VI) ions. S2O82-, by iodide ions I-, using a 'clock' reaction. Principle
           The equation for reduction of S2O82- by I- is:                       S2O82-   +   2I-   →   2SO42-   +   I2            The formation of iodine is 'monitored' by small & known amount of thiosulphate ions, S2O32-:                      2S2O32-   +   I2   →   S4O62-   +   2I-            Once the reactants are mixed, the stop-watch is started. At the time when all of the thiosulphate is reacted, any free iodine produced will turn starch solution (added before) into dark-blue, the time is then recorded. The amount of thiosulphate added monitors the time in which starch turns blue and the reaction rate is directly proportional to 1 ÷ time taken for starch changes to dark blue.            By plotting a graph of log10(1/t) against 1/T (T=absolute temperature), activation energy (Ea) can be found. Chemicals

           0.020M K2S2O8, 0.50M KI, 0.010M Na2S2O3, 0.2% starch solution Apparatus
           400 cm3 beaker, boiling tubes x2, pipettes (10ml), thermometer x2, water bath, stop-watch Procedure
         1.> Half-fill the beaker with hot water at temperature between 49 - 51℃ (as water bath).          2.> Pipette 10 cm3 of  0.020M K2S2O8 solution into the first boiling tube,  place a thermometer in this solution and keep this in the water bath.          3.> Pipette 5 cm3 of both 0.50M KI and 0.010M Na2S2O3 and 2.5 cm3 of starch solution into the second boiling tube. Place another thermometer in                this solution and stand it in the water bath.          4.> When the temperatures of the two solutions are equal and constant (to within ± 1℃), pour the contents of the second boiling tube into the first,                 shake to mix, and start the stop-watch.          5.> When the blue colour of the starch-iodine complex appears, stop the stop-watch and record the time.          6.> Repeat the experiment at temperatures at 50℃, 45℃, 40℃, 35℃, 30℃.          7.> Plot a graph of log10(1/t) against 1/T to calculate the Ea. Data Analysis

Temperature /℃ |50 |45 |40 |35 |30 | |Temperature (T) /K |323 |318 |313 |308 |303 | |Time (t) /sec |52 |62 |95 |135 |182 | |log10 (1/t) |- 1.72 |- 1.79 |- 1.98 |- 2.13 |- 2.26 | |1/T /K-1 |3.10x10-3 |3.14x10-3 |3.19x10-3 |3.25x10-3 |3.30x10-3 | |                                  ● Plot log10(1/t) against 1/T:                                   [pic]

                    Let the rate equation be:   Rate = k[S2O82-]a [I-]b                     From Arrhenius equation, we have k = Ae-Ea/RT, where A is Arrhenius constant.                     By taking logarithm of above equation, we have log10k = log10A - Ea÷(2.303RT)   [1]                  ∵ All concentration terms have been kept constant                 ∴ Rate only varies as k (rate constant)                     ∵ Rate is directly proportional to 1/t & varies as k in the rate equation                     ∴ We can substitute k by 1/t in [1]:                                log10(1/t) = log10A - Ea÷(2.303RT), which is the equation of the previous graph                     From the graph, the slope is,                     -Ea ÷ (2.303R) = [-1.75 - (-1.98)] ÷ [(3.12 - 3.20)x10-3] = - 2875                  ∴ Ea (activation energy of reduction of S2O82- by I-)                      = (- 2875)(- 2.303)(R)                      = (2875)(2.303)(8.314) Jmol-1                      = 55.05 kJmol-1

© KCl      http://hk.geocities.com/fatherofchemistry
Precaution
        1.> The amount of S2O82-, I- and S2O32- must keep constant at the start of each experiment, so it's better to use a pipette to transfer these chemicals                instead of measuring cylinder.

        2.> The temperature measured should be the temperature of the mixtures inside two boiling tubes, not the water-bath. It's because boiling tubes have                their own heat capacities, they will absorb part of heat from water-bath. Hence, the temperature inside boiling tubes are slightly lower than that of                water-bath.

        3.> It is necessary to cover the reagent bottle containing Na2S2O3 immediately after use because it can be easily oxidized by oxygen in air. Discussion
        In this experiment, known amount of thiosulphate is added. Some thiosulphate can reduce I2 produced. When all thiosulphate is used up, any excess I2 produced will turn starch to dark blue. Hence, the reaction is monitored by thiosulphate ion. Without adding thiosulphate, the starch will turn dark blue immediately.         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.         For peroxodisulphate(VI) ion, S2O82-, we may expect the oxidation number of S should be +7. However, oxidation number of S only exists from -2 to +6. This interesting phenomenon can be explained by its structure:                  [pic]

        There are actually two oxidation numbers of oxygen, one with oxidation number -1 and another one -2. Oxygen has oxidation number -2 if attached only to less electronegative than itself (e.g. sulphur) whereas when attached to itself, is -1. Hence, oxidation number of S must be +6, not +7. Conclusion

         By using a 'clock' reaction, the experimental value of activation energy of reduction of S2O82- by I- can be easily obtained by measuring the slope of Arrhenius plot of log10(1/t) against 1/T.

Cite This Document

Related Documents

  • Investigation Into the Kinetics of the Reaction Between Peroxodisulphate(Vi) Ions and Iodide Ions

    ...find the rate equation for this particular reaction. • Investigate the effect of temperature on the rate and use the results to find the activation enthalpy for this particular reaction. Background detail The Reaction:2 The reaction I am studying is often referred to as an ‘iodine clock reaction.' A clock reaction is where the time tak...

    Read More
  • Rate Law and Activation Energy

    ...Absorption to Activation Energy Calculating the rate constants, half-lives, and activation of a reaction by monitoring the change in absorption Department of Chemistry Dusten Dussex Lab partner: John Drury Lab date: February 7th, 2013 Due date: February 21st, 2013 Introduction In this experiment we are analyzing the relations...

    Read More
  • Activation Energy of a Reaction

    ...Activation energy of a reaction Mengyuan Wu IB Chem HL Richard Forster March 20, 2013 Introduction: In this experiment, a reduction of peroxodisulphate (VI) ions by iodine ions is investigated. Solution of 10cm3 of K2S2O8, potassium peroxodisulphate, represented as ‘Solution A’. Mixture of 5cm3 of KI (Potassium iodide solution), and ...

    Read More
  • Investigating the Kinetics of the Reaction Between Iodide Ions and Peroxodisulphate (Vi) Ions (Entire Plan)

    ...PLANNING Investigating the Kinetics of the reaction between Iodide ions and Peroxodisulphate (VI) ions By the use of an Iodine clock reaction I hope to obtain the length of time taken for Iodine ions (in potassium iodide) to react fully with Peroxodisulphate ions (in potassium Peroxodisulphate). I will do three sets of experiments changing...

    Read More
  • Investigating the Rate of Reaction Between Peroxydisulphate (Vi) Ions and Iodide Ions

    ...the rate equation of the reaction of hydrogen peroxide and iodide ions. This will be achieved by using an iodine clock method and colorimetric analysis. 2. Draw a graph of rate against concentration for each reactant (Hydrogen peroxide, potassium iodide and H+ ions). 3. Finding the order for each reactant 4. Finding the rate-determining ste...

    Read More
  • Determination of the Activation Energy of an Enzyme Catalysed Reaction

    ...Determination of the activation energy of an enzyme catalysed reaction Introduction In this practical the aim for this experiment was to find out the catalytic power of alkaline phosphate, as well as the rate of reaction and the activation energy of p-nitrophenol phosphate. Enzymes are biological molecules that catalyse a chemical reactio...

    Read More
  • ENERGY

    ...Operation, Sizing and Installation Manual AIR HANDLER MANUFACTURED WITH AIRCOM ELECTRONICS Models AH-U-S4A-24-P16 (E16) AH-U-L4A-36-P16 (E16) AH-U-S2A-30-P16 (E16) AH-U-L2A-36-P16 (E16) AH-U-L2B-48-P16 (E16) * LEAVE FOR HOMEOWNER NOTE: Due to ongoing research and product development, specifications, ratings and dimensions are...

    Read More
  • Activation Energy of the Reaction between Br— and BrO3— in Acid Solution

    ...Kwok Tak Seng Catholic Secondary School S.6 Chemistry Experiment [9] – To Determine the Activation Energy of the Reaction between Br— and BrO3— in Acid Solution S.6 ( ) Name: [ ] ( ) Time allowed: 55 minutes Introduction: The reaction can be represented by 5Br—(aq) + Br...

    Read More

Discover the Best Free Essays on StudyMode

Conquer writer's block once and for all.

High Quality Essays

Our library contains thousands of carefully selected free research papers and essays.

Popular Topics

No matter the topic you're researching, chances are we have it covered.