How do we determine a rate of reaction for a reaction and what kinds of rates exist? Rate is just a slope of a line when plotting concentration versus time for an experiment. So a rate can be determined by plotting the concentration of a reactant (or product) versus time. There are two types of rates; average rate and instantaneous rate. The average rate is determine by drawing the slope of a line through two data points. An instantaneous rate is obtained by determining the slope of the line tangent to the data curve. The initial rate is defined as the instantaneous rate at t = 0, the rate of the reaction at the very beginning of the reaction. At this point the rate is at its maximum. Also measuring the rate at t = 0 minimizes any side reactions which might effect the rate of the reaction of interest.
Earlier I demonstrated how the rate of a reaction was effected when the concentration of a reacting species was changed. Let's look at a reaction in more detail to discover a more quantitative relationship between rate and concentration of reactants. We'll study the following reaction;
| [NO2] | Time (mins) | [NO2] | Time (mins) | |
| 0.350 M | 0 | 0.700 M | 0 | |
| 0.245 M | 4 | 0.379 M | 4 | |
| 0.190 M | 8 | 0.258 M | 8 | |
| 0.130 M | 16 | 0.158 M | 16 | |
| 0.090 M | 24 | 0.115 M | 24 | |
| 0.062 M | 40 | 0.074 M | 40 |
Let's plot the data and see what happens.
