1
Equilibrium: The Extent of
Reactions
– Chemical equilibrium – studies the extent of reactions
and the ways it can be altered
– Kinetics and equilibrium are two different aspects of
chemical reactions (fast reactions may proceed to a
great, lesser or a limited extent; same is true for slow
reactions)
16.1 The Dynamic Nature of the
Equilibrium State
•Chemical equilibrium – a state in which the
concentrations of reactants and products no
longer change
¾Equilibrium is not a stationary state or a
unidirectional process
Example: (A →B →C)
If the rates of step 1 and step 2 are equal, [B]
remains constant →not an equilibrium state
¾Equilibrium is a dynamic state achieved by the
equalization of the forward and reverse rates of a
reversible (bidirectional) process
Example: (A ↔B)
If the rates of the forward and reverse reactions are
equal, [A] and [B] remain constant →an
equilibrium state
⇒At equilibrium →Ratefwd = Raterev
[A] = constant
[B] = constant
C
oncentrat
i
on
Time
[A]
[B]
Equilibrium
Rate
forward
reverse
Equilibrium
Time
– At a given T, the same equilibrium state is
reached even if the process is started from
different starting points
Ratefwd = Raterev
Example: N2O4(g; colorless)
↔
2NO2(g; brown)
¾The reaction can be started from pure N2O4(g;
colorless) or from pure NO2(g; brown).
¾In both cases at equilibrium, the same light-brown
color is reached (the same proportion of N2O4 and
NO2is produced)
¾The reaction has a single step mechanism (the
forward and reverse reactions are elementary), so at
equilibrium:
Rate1= Rate-1 →k1[N2O4] = k-1[NO2]2
]O[N
][NO
42
2
2
1
1==⇒
−
K
k
k
→Kis a constant which depends
on T(K= 0.211 at 100°C)
→Kdetermines the proportion of
N2O4and NO2at equilibrium
