Experiment 3 Iodide Persulphate

Experiment 3 Iodide Persulphate - Experiment 3 The Iodide...

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Experiment # 3: The Iodide- Persulphate Reaction: The Effect of Concentration on Reaction Rate Introduction: The overall purpose of this experiment is to obtain the rate law reaction; this can be calculated through successive experiments based on the property that the rate of oxidation of the iodide ion by persulphate ion can be measured. The following chemical reaction (Figure 1) shall take place and along with the rate of oxidation we shall be able to look at concentrations of different species and see how this impacts the rate law equation directly. (NH 4 ) 2 S 2 O 8 + 2KI I 2 + (NH 4 ) 2 SO 4 + K 2 SO 4 Figure 1- Chemical Reaction involving KI and (NH 4 ) 2 S 2 O 8 The equation can be further simplified because when it enters and aqueous environment is dissociates to produce the following net ionic equation which is of most concern to us. S 2 O 8 -2 + 2I - I 2 + 2SO 4 -2 Figure 2- Net Ionic Equation In this experiment in order to measure the rate law there must be a species from the above reaction that can be measured at different intervals and reaction speeds. This process is rather time consuming, therefore luckily in this reaction by measuring the time it takes to produce a certain amount of iodine, the rate of production can be calculated. Iodine reacts with sodium thiosulphate, to giving the following net equation: I 2 + 2S 2 O 3 -2 2I - + S 4 O 6 -2 Figure 3- Net Ionic Equation
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From the net ionic equation 2 moles of thiosulphate for every mole of iodine is needed to remove the violet colour that forms when iodine forms in the chemical reaction. Thiosulphate is added so the solution remains clear. This is important because when all the thiosulphate is consumed we will know that all the iodine has reacted and the solution should turn back into a violet pigment, however in the experiment two drops of starch is added because it forms a blue complex revealing a more distinct colour so our timing will be more precise. The time attained from beginning to when the blue colour forms is needed for that rate calculation based off the following rate law. Rate = -d[S 2 O 8 -2 ] = -∆S 2 O 8 -2 = k [ S 2 O 8 -2 ] m [I - ] n dt ∆t Figure 4- Rate law Equation - Where the m and n exponents are the orders of the reaction of S 2 O 8 -2 and I - respectively - -∆S 2 O 8 -2 is the moles per litre of S 2 O 8 -2 consumed - K is the rate constant The two values of the exponents seen in the above rate law can be calculated by creating graphs of –log Δt vs. log [S 2 O 8 -2 ] and the –log ∆t vs. log [I
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Experiment 3 Iodide Persulphate - Experiment 3 The Iodide...

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