Experimental measurement of reaction rates
Since reate of reaction is related via stoichiometry to the rate of consumption of a reactant and the rate of formation of a product, the rate of reaction can be determine by following the concentration of either a reactant or product as a function of time. A graph of the chemical specie's concentration versus time can be used to determine the rate reaction. This is done by drawing the tangent line to the concentration curve at the time one wishes to determine the rate of reaction. The slope of the tangent line at in any point in time is the change in specie's concentration with time at that time. The change in concentration with time is directly proportional to the rate of reaction, where the proportionality constant in the reciprocal of the stoiochiometric coefficient of the species in the balanced chemical reaction. Generally the slope of the graph changes so that the reaction rate changes with time. Usually reactions slow down as the reaction proceeds as the amount of reactants left decreases.
As an example consider the decomposition of HI at 508°C. 2HI(g) H2(g) + I2(g) HI and H2 are colourless but I2 has purple colour. So it is easiest to monitor I2. A spectrometer can be used to monitor the change in colour intensity of the reaction chamber and relate it back to the concentration of I2. Knowing the concentration of I2, the concentration of HI can be calculated. Plotting the concentration of HI versus time gives the following graph. To get the rate of HI consumption at 100 s, the tangent line to the concentration curve is drawn and the slope of the tangent line is calculated to be -2.5x10-4 M/s. Therefore the reaction rate is 1.3x10-4 M/s.
The initial rate can be determined by draw the tangent line at t = 0. From slope of the tangent line the rate of HI consumption is 7.7x10-4 M/s . Therefore the reaction rate is 3.9x10-4 M/s.
As can be seen the reaction rate is decreasing with time, i.e. the slope of the line is getting smaller as time increases.