Reaction Conditions and rate
Several factors-reactant concentrations, temperature, and presence of catalysts-affect the rate of a reaction. If the reactant is a solid, the surface area available for reaction is also a factor.
The “iodine clock reaction” illustrates the effect of concentration and temperature. The reaction mixture contains hydrogen peroxide (H2O2), iodide ion (I-), vitamin C (ascorbic acid), and starch (which is an indicator of the presence of iodine, I2). A sequence of reaction begins with the slow oxidation of iodide ion to I2 by H2O2
H2O2 (aq) + 2I- (aq) + 2 H3O+ (aq) -> 4 H2O (l) + I2 (aq)
As soon as I2 is formed in the solution, vitamin C rapidly reduces it to I-.
2H2O (l) + I2 (aq) + C6H8O6 (aq) -> C6H6O6 (aq) + 2 H3O+ (aq) + 2I- (aq)
When all of the vitamin C has been consumed, I2 remains in solution and forms a blue-black complex with starch. The time measured represents how long it has taken for the given amount of vitamin C to react. For the first experiment the time required is 51 seconds. When the concentration of iodide ion is smaller