AP Chemistry
Chemical Equilibrium
Chemical equilibrium is reached
when the concentrations of
reactants and products
cease changing with time.
reactants
products
these RATES are equal;
NOT
amt. of R = amt. of P
-- system must be closed
-- equilibrium is a dynamic process
(although it might look static)
For the reaction A
B:
-- Eq. can be reached starting with… either
A (i.e., A
B) or with B (i.e., A
B).
Eventually, A
B is reached.
law of mass action: expresses the relationship between
amounts of R and P in any reaction
-- this is the “muscle” behind an
equilibrium-constant expression
For the equilibrium system aA + bB
the law of mass action says that an
equilibrium constant K is given by:
p
q
(amt. P) (amt. Q)
K
(amt. A)a (amt. B)b
When amts. are given in terms of
concentration (i.e., molarity):
(
pP + qQ
P
i.e., K =
R
)
[P]p [Q] q
Kc 
a
b
[A] [B]
For a gaseous system:
(PP )p (PQ )q
Kp 
(PA )a (PB )b
where PX is the partial pressure of X at PX values must
eq., and Kp is the pressure eq. constant be in atm. (!)
The relationship between Kc and Kp is given by...
Kp = Kc(RT)Dn
Dn = SPcoeff – SRcoeff
R = universal gas const.
0.08206 L-atm/mol-K
T = absolute temp.
For the gaseous reaction
2 NO + O2
2 NO2
(1) write the Kc and Kp
expressions
(2) find Kp at T = 750oC, if
Kc = 2.19 x 10–3.
[NO2]2
(1) Kc =
[NO]2 [O2]
(2)
Kp =
When a flask filled with colorless
NO is opened to the air,
O2 enters, resulting
in brown NO2.
(PNO2)2
(PNO)2 (PO2)
T = 1023 K
Kp = Kc(RT)Dn = 2.19 x 10–3 [0.08206(1023)]–1
= 2.61 x 10–5
A few notes about K:
-- it depends only on the reaction stoichiometry,
not its mechanism
-- it is independent of initial concentrations
-- it is unaffected by other substances, as
long as they don’t react with R or P
-- it varies with temperature
-- it is written without units
-- it NEVER includes pure liquids
or pure solids
Write expressions for Kc and Kp.
CO2(g) + H2(g)
CO(g) + H2O(l)
[CO]
Kc 
[CO2 ][H 2 ]
PCO
Kp 
PCO2 PH2
SnO2(s) + 2 CO(g)
2
[CO2 ]
Kc 
[CO] 2
Kp 
(PCO2 )2
2
(PCO )
Sn(s) + 2 CO2(g)
In the refining of iron ore, a common way to reduce
Fe2+/Fe3+ from the iron ore is to react FeO/Fe2O3 w/CO
to form Fe and CO2.
Write the Kc expression for each reaction.
** substances w/only
1 or 2 nonmetals:
best guess = gases
N2 + 3 H2
2 NH3
[NH 3 ]2
Kc 
[N2 ][H 2 ]3
2 SO3
2 SO2 + O2
[SO 2 ]2 [O2 ]
Kc 
[SO 3 ]2
Fritz Haber (1868–1934) discovered
a way to generate ammonia from
hydrogen and nitrogen at high
pressure. The ammonia was needed
for Germany’s munitions industry,
which was cut off from the nitrate
sources of South America by the
British blockade during WWI.