Predicting rate expressions
- A rate expression indicates of how the concentration of a particular reactant affects the rate of a reaction.
|
You can use the computer model below to explore how the starting concentration of dinitrogen pentoxide, N2O5, affects the inital rate of the reaction:
2N2O5(g) --> 4NO2(g) + O2(g)
Make a measurement of initial rate of reaction (i.e. at time = 0 seconds) for various starting concentrations of nitrogen pentoxide, [N2O5]. The model is set to work at a temperature of 338K.
Carry out the following steps: (Model)
- set the starting concentration of N2O5 to 0.055 M.
- set the rate to be measured at a time at 0 seconds.
- run the model. The gradient and inital rate of reaction will be displayed.
- enter the concentration value, [N2O5], and the rate into the text fields for the graph plotting program below the computer model
- clear the reaction model but not the graph plotting program.
- repeat this procedure for 4 other starting concnetrations of N2O5
- press plot to see the relationship between starting concentration and initial rate of reaction.
Reaction Model: Decomposition of dinitrogen pentoxide at 338K.
Graph Plotting: Rate of reaction against concentration.
Back to instructions.
You should have found that there is a straight line relationship between the initial rate of reaction and the starting concentration of reactant. You should have found that the straight line has a gradient of about 5.2 x 10-3 s-1.
This can be more generally expressed as saying there is a linear relationship between the rate of reaction and the reactant concentration. Mathematically this is:
rate of reaction is proportional to the reactant concentration
In the case you have been studying:
rate
[N2O5]
or
rate = k[N2O5]
where k is the constant of proportionality, commonly called the rate constant, with units in this case of s-1 and the equation relating rate to concentration is known as a rate expression.
The value of the rate constant varies with the temperature, so the value of k = 5.2 x 10-3s-1 for the decomposition of dinitrogen pentoxide is specific to the temperatue at which the measurements were made, in this case 338K.
The equation of rate = k[N2O5] is one of the simpler types of rate expressions.
To explore more complex examples, move on to the next page.