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.

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Graph Plotting: Rate of reaction against concentration.

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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.