Relative Rates of Reaction

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For a reaction:

the relative rates of reaction are:

which basically says that things appear or disappear at rates proportional to their coefficients in the balanced chemical equation.

Example 1

For the reaction:

the relative rates of appearance and disappearance are:

What this means is that Ca(2+) is disappearing 3 times faster than Ca3(PO4)2 is appearing.  PO4(3-) is disappearing twice as fast as Ca3(PO4)2 is appearing.  Suddenly, if we can measure how fast one thing is appearing or disappearing, we know how the other substances' concentrations are changing as well.  These relative rates may also be readily apparent from the balanced chemical equation.

Example 2

Since pressure is also a measure of concentration, we can use pressure if everything is in the gas phase.  For:

We can write the relative rates as:

This means that H2 is disappearing at the same rate that H2O is appearing.  O2 is disappearing at half of this rate. These relative rates may also be readily apparent from the balanced chemical equation.