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The average reaction
rate is defined as the amount of change of the concentration of a species
for an amount of time. For example, in the reaction A + B -> C +
D, we can define the rate as the change in the concentration of C over a
given time,
- rate = -D[A] /DT
or
- rate = D[C] /DT
Since the rate of a reaction changes with the concentration of the
various species in the reaction, it is often useful to know the instantaneous
rate of the reaction, found by letting the time interval go to zero as usual
in calculus. The tangent of the line at the desired point is related to the
instantaneous rate.
For example, conside the reaction
- 2N2O5 -> 4NO2 + O2
If we graph the concentration of N2O5 vs. time,
we will get a curve like the one below. We can look at the change in the
concentration over a large time, such as 2 minutes, to get an average rate (Red
line), or we can look at the tangent to the slope at a given point. (Green line)
Note that the two rates
are quite different. |