1. AP Chemistry
Chemical Equilibrium

2. 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)

3. For the reaction A B: --
either
A (i.e., A B) or with B (i.e., A B).
Eq. can be reached starting with…
Eventually, A B is reached.

4. ) ( 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 pP + qQ
the law of mass action says that an
equilibrium constant K is given by:
i.e., K = P R ) (
When amts. are given in terms of
concentration (i.e., molarity):

5. For a gaseous system:
where PX is the partial pressure of X at
eq., and Kp is the pressure eq. constant
PX values must
be in atm. (!)
The relationship between Kc and Kp is given by...
Kp = Kc(RT)Dn
Dn = SPcoeff – SRcoeff
R = universal gas const.
L-atm/mol-K
T = absolute temp.

6. For the gaseous reaction 2 NO + O2 2 NO2 (1) write the Kc and Kp
When a flask filled with colorless
NO is opened to the air,
O2 enters, resulting
in brown NO2.
For the gaseous reaction
2 NO + O NO2
(1) write the Kc and Kp
expressions
(2) find Kp at T = 750oC, if
Kc = 2.19 x 10–3.
Kc =
[NO2]2
[NO]2 [O2]
Kp =
(PNO2)2
(PNO)2 (PO2)
(1)
(2) T = 1023 K
Kp = Kc(RT)Dn
= 2.19 x 10–3 [ (1023)]–1
= x 10–5

7. 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

8. Write expressions for Kc and Kp.
CO2(g) + H2(g) CO(g) + H2O(l)
SnO2(s) CO(g) Sn(s) 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.

9. Write the Kc expression for each reaction.**
substances w/only
1 or 2 nonmetals:
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.
best guess = gases
N H NH3
2 SO SO2 + O2

10. Based on CaCO3(s) CaO(s) + CO2(g), which
of the following can attain equilibrium? Start with...
(Kp = PCO2)
(a) ...pure CaCO3
“YEP.”
(it breaks down, forming products until the amt. of CO2 is “right”)
(b) ...CaO and some CO2 at P > Kp
“YEP.”
(too much CO2; it reacts w/available CaO until amt. of CO2 is “right”)
(c) ...CaCO3 and some CO2 at P > Kp
“NOPE.”
(too much CO2, but no CaO for it to react with in order to attain the
“right” amt. of CO2)
(d) ...CaCO3 and CaO
“YEP.”

11. The Magnitude of the Equilibrium Constant
If K >> 1...
products are favored.
Eq. “lies to the right.”
If K << 1...
reactants are favored.
Eq. “lies to the left.”
The K for the forward and reverse reactions
are NOT the same. --
they are reciprocals
-- You must write out the equation
and specify the temperature
when reporting a K.

12. Calculating Equilibrium Constants
(1) If the concentrations of all substances at
equilibrium are known, plug and chug.
Find 472oC for N2(g) H2(g) NH3(g)
At 472oC…[NH3] = M
[N2] = M
[H2] = M. =