(b) The balanced equation shows that 2 mol N 2 O 5 is consumedfor every 4 mol NO 2 that is
formed.Because [N 2 O 5 ] is decreasing as [NO 2 ] increases, we should write the reaction ratio as.rate of change of [N 2 O 5 ]0.0036The rate of changeof [N 2 O 5 ] with time,
[N
2
tO 5 ]
, is 0.0036 mol
LN
s^2 O^5 , a negativenumber,
because N 2 O 5 , a reactant, is being used up.
(c) The rate of reaction can be calculated from the rate of decrease of any reactant concentra-
tion or the rate of increase of any product concentration.rate of reaction1
21
20.0036 0.0018
rate of reaction 0.0072 0.0018
rate of reaction 0.0018
We see that the rate of reaction is the same, no matter which reactant or product we use to
determine it. Remember that the mol in these units is interpreted as “moles of reaction.”You should now work Exercise 10.FACTORS THAT AFFECT REACTION RATES
Often we want a reaction to take place rapidly enough to be practical but not so rapidly
as to be dangerous. The controlled burning of fuel in an internal combustion engine is
an example of such a process. On the other hand, we want some undesirable reactions,
such as the spoiling of food, to take place more slowly.
Four factors have marked effects on the rates of chemical reactions. They are (1) nature
of the reactants, (2) concentrations of the reactants, (3) temperature, and (4) the presence
of a catalyst. Understanding their effects can help us control the rates of reactions in desir-
able ways. The study of these factors gives important insight into the details of the
processes by which a reaction occurs. This kind of study is the basis for developing theo-
ries of chemical kinetics. Now we study these factors and the related theories—collision
theory and transition state theory.mol
Ls[O 2 ]
t1
1mol
Lsmol
Ls1
4[NO 2 ]
t1
4mol
Lsmol
Ls[N 2 O 5 ]
tmol N 2 O 5
Ls2 mol N 2 O 5
4 mol NO 20.0072 mol NO 2
Ls[N 2 O 5 ]
t2 mol N 2 O 5
4 mol NO 2654 CHAPTER 16: Chemical Kinetics