The rate of a chemical reaction is the change in concentration of a reactant or product in unit time.
For the reaction (1), it is convenient to measure the time (t) required for formation of a fixed amount of iodine (I2).
2I– + S2O82–
I2 + 2SO42– (1)
The rate of the reaction is calculated using (2).
Δ[I2] is the change in iodine concentration during time t. If Δ[I2] is fixed, the time t for formation of that amount of iodine varies if the concentration of any reactants upon which rate depends is changed.
If a solution containing persulfate ion (S2O82–) is mixed with a solution containing iodide ion (I–) and a small quantity of starch, a blue colour due to the starch-iodine complex appears immediately.
For the reaction (1), it is convenient to measure the time (t) required for formation of a fixed amount of iodine (I2).
2I– + S2O82–
I2 + 2SO42– (1)The rate of the reaction is calculated using (2).
| rate = | Δ[I2] | (2) |
| Δt |
The appearance of the blue colour is delayed if a small amount of sodium thiosulfate (Na2S2O3) is present in the reaction mixture.
I2 + 2S2O32– 2I– + S4O62– (3)
2I– + S4O62– (3)Reaction (3) consumes the first few percent of I2 formed, and appearance of the blue colour is delayed until all S2O32– has reacted. The time delay before appearance of the blue colour can be measured, and this is t in (2). Δ[I2] during that time can be calculated from Δ[S2O32–] using the stoichiometry of reaction (3).