Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

2.1 Definitions and conventions 9
• Convenient units for gas phase reactions:
— molar units:
[] = mol cm 3 s (2.8)
— molecular units: 14 [] =molecules cm 3 s ⇒ cm −3 s −1 (2.9)
— We prefer to use molar units for rate constants, because rate constants are
inherently quantities that are “averages” of some other quantity, e.g., a molecular
cross section weighted by the relevant statistical distribution function
(usually the Boltzmann distribution). Rate constants are thus properties of
an ensemble of molecules, they are not original molecular quantities.
• Convenient units for liquid phase reactions:
[] = mol ls=M s (2.10)
2.1.2 Rate laws and rate coefficients
I Therateequation(ratelaw): Experiments show that the reaction rate generally
depends on the concentrations of the reactants. The functional dependence is expressed
by the rate equation (also called rate law), 15 e.g.
for reaction 2.3.
= − 1 [B 1 ]
=
| 1 |
(2.11)
= ([B 1 ] [B 2 ] [B ] [B +1 ] ) (2.12)
Theratelawsofcomplexreactions(i.e.,the ([B 1 ] , [B 2 ] , , [B ] , [B +1 ] , ))
are often rather complicated; only rarely (as in the case of elementary reactions) are
they simple and intuitive.
Often (but not always), ([B 1 ] , [B 2 ] , , [B ] , [B +1 ] , ) can be written as a
product of a rate coefficient and some power series of the concentrations, for
example
= − 1 [B 1 ]
= [B 1 ] [B 2 ] ... (2.13)
| 1 |
Note that the exponents . . . are not usually equal to 1 2 ...
I Therateconstant(ratecoefficient): The rate constant (also called rate coefficient)
16 usually depends on temperature ( = ( )) and sometimes also on pressure
( = ()), but not on the concentrations ( 6= ()).
14 IUPAC has decided that “molecules” is not a unit; see (Mills 1989) or (Homann 1995).
15 Rate law = Geschwindigkeits- oder Zeitgesetz.
16 Rate constant = Geschwindigkeitskonstante. Es wird manchmal auch die Übersetzung “Ratenkonstante”
verwendet (aber bitte nicht “Ratekonstante”).