The Chemistry of Gases, Part 2 - Rate Law Determination For an Ideal Gas

Pressure as a Measurement to Calculate Order of a Reaction

A slight manipulation of the ideal gas law equation, PV=nRT, will show that pressure is directly proportional to molar concentration at a constant temperature.

PV=nRT ==>  PV/V = nRT/V ==> P = (RT)(n/V)

where:

"RT" is now a proportionality constant and

"n/V" is molar concentration.

This result is useful, in determining the Rate Law Equation of a gas-phase reaction, because a relative change in pressure is equal to a relative change in molar concentration of an ideal gas.  Since the relative change in molar concentration is used to determine reactant orders, the result above allows us to use changes in reactant partial pressures; a more convenient measurement for the gas phase.

The diagram below shows the rate-law calculations for a hypothetical gas-phase reaction.

As indicated in the diagram, once the rate law equation is determined for a specific reaction, the value of "k" can be calculated from any single set of reactant partial pressures and the reaction rate.  Then, a new predicted reaction rate can be calculated for another set of reactant partial pressures using the known-value of "k".

Thank you for reading.