The Effect of Concentration on Reaction Rate

The Effect of Concentration on Reaction Rate

Introduction:

In this experiment, we utilized the ability for the iodide ion to

become oxidized by the persulphate ion. Our general reaction can be

described as:

(NH4)2S2O8 + 2KI Ã I2 + (NH4)2SO4 + K2SO4 (1a)

However, we know that in an aqueous solution, all of these compounds

except iodine will dissociate into their ionic components. Thus we can

rewrite the equation in a more convenient manner:

S2O82- + 2I- Ã I2 + 2SO42- (1b)

It is important however to note that the NH4 and K ions are still in

the solution, they are just unreactive. In order to measure the rate

of the reaction, the conventional method would be to measure the

species in question at certain times. However, this would be

inconvenient, especially for a three hour laboratory period. Since the

iodide ion can be oxidized by the persulphate ion, we can use sodium

thiosulphate to be an indicator of the presence of iodine in the

solution. For this experiment, we can simply calculate the rate of the

reaction by timing the amount of iodine being produced in several

runs. The reaction between iodine and sodium persulphate can be

depicted as:

I2 + 2Na2S2O3 Ã 2NaI + Na2S4O6 (2a)

Similarly, this reaction above can also be simplified due to

dissociation of all the ions except for iodine and persulphate.

I2 + 2S2O3 Ã 2I- + S4O62- (2b)

An interesting property of reaction (1) is that it produces a

brilliant violet colour. However, this violet colour only results in

the presence of iodine, or in other words, when iodine is being

produced in the reaction. If sodium thiosulphate is added to reaction

(1), than as long as there are two moles of thiosulphate for every

mole of iodine, the solution will be colourless because the iodine is

being used up in reaction (2). However, as time passes, the

thiosulphate must run out at some point, and when it does, the violet

colour will appear.