Determining amount of reactant remaining at time t
When determining reaction order it is necessary somehow to detect the number of particles (that is, amount in moles, n) of reactant remaining at time t. The method used depends on the system.
For reactions in solution, a quantity directly proportional to concentration is measured.
The amount of a coloured substance in solution can be determined by measuring the absorbance (A) of the solution as this is directly proportional to the concentration through Beer's Law (A = εbc).
If the amount of substance remaining is being detected by titration, the volume of titrant is measured.
For gas phase reactions, either pressure or volume measurements can be used.
The properties of a gas are related by the ideal gas law
(PV = nRT - R is the ideal gas constant).
Pressure is caused by particles colliding with the walls of the vessel
If V and T are constant (PV = nRT) measured pressures are directly proportional to n.
If P and T are constant (PV = nRT) measured volumes are directly proportional to n.
P and concentration (c) are directly proportional at constant T because P = (n/V) x RT = c x RT.
!!! For the purpose of determining reaction order graphically, ANY quantity that is directly proportional to the number of particles of reactant (A, P.....) may be plotted as ln x or 1/x against t. For second order reactions care must be taken with the unit of the rate constant.
The amount of a coloured substance in solution can be determined by measuring the absorbance (A) of the solution as this is directly proportional to the concentration through Beer's Law (A = εbc).
If the amount of substance remaining is being detected by titration, the volume of titrant is measured.
The properties of a gas are related by the ideal gas law
(PV = nRT - R is the ideal gas constant).
Pressure is caused by particles colliding with the walls of the vessel
If V and T are constant (PV = nRT) measured pressures are directly proportional to n.
If P and T are constant (PV = nRT) measured volumes are directly proportional to n.
P and concentration (c) are directly proportional at constant T because P = (n/V) x RT = c x RT.