Honors General Chemistry 225H
Midterm 2
NAME: _____KEY_______
100 pts total, 50 minutes.
Exams are closed book and notes, calculators are required. Obviously, no external
communication devices are allowed. A periodic table will be provided.
1.
Which is the correct equilibrium constant expression for Kp for the following reaction?
Fe2O3(s) + 3H2(g)
2Fe(s) + 3H2O(g)
Kp = pH2O3 / pH23
2.
For A + B
C
Kc (20°C) = 4.2 x 103
What is Kc (20°C) for the equilibrium:
3C
3B + 3A
K = 1 / Kc3 = 1 / (4.2E3)3 = 1.3 x 10-11
3.
The equilibrium constant, Kc = 5.0 for the reaction:
SO2(g) + NO2(g)
SO3(g) + NO(g)
at 1000°C. If 1.0 mole of SO2 and 1.0 moles of NO2 are placed in a 1.0 L container, what
concentration of SO3 will be present at equilibrium at 1000 °C?
Compound
SO2
NO2
SO3
NO
Initial conc
1.0
1.0
0
0
change
-x
-x
+x
+x
equ conc
1.0 - x
1.0 – x
x
x
Kc = 5.0 = [SO3] [NO] / [SO2] [NO2] = x2 / (1.0 – x)2
Taking square roots of both sides:
2.236 = x / (1.0 - x)
2.236 – 2.236x = x
x = 2.236 / 3.236 = 0.69
[SO3]equ = 0.69
4.
Circle the two incorrect statements about equilibrium. Below, briefly explain why they are wrong.
1.
2.
3.
4.
At equilibrium the total concentration of products is not changing.
Equilibrium occurs when the rate of the forward reaction approaches zero.
There is a unique set of equilibrium concentrations that equals the Kc value.
At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
2.
Rate of forward and reverse reactions are equal, they are not zero (this is not a static
condition).
Infinite number of concentrations are possible, as long as the ratio is as expressed in
K c.
3.
5.
Give two examples of Lewis acid-base reactions that do NOT involve proton transfer. Indentify
the Lewis acid and base in each case.
Some examples are:
W (s) + 6 CO (g)
W(CO)6 (g)
BX3 + NR3
X3B:NR3
[Cu(OH2)6]2+ (aq) + 6NH3(aq)
[Cu(NH3)6]2+ (aq) + 6H2O (l)
In each case the Lewis acid is written first. (In the last example, Cu2+ is the Lewis acid site.)
6.
Calculate the pH of a 0.14 M HNO2 solution at 25°C. Ka (HNO2, 25 °C) = 4.5 x 10-4
HNO2
Initial []
Change in []
Equilibrium []
0.14
-x
0.14 - x
+
H2 O
H3O+
----
10-7
+x
x
+
NO20
+x
x
This assumes x >> 10-7
Ka = 4.5 x 10-4 = x2 / (0.14-x)
Assume x << 0.14
4.5 x 10-4 = x2 / 0.14
6.3 x 10-5 = x2
x = 7.9 x 10-3 = [H3O+]eq
pH = -log(7.9 x 10-3) = 2.1
7.
HA is an acid with pKa = 5.8 at 25°C. What is the pH of a 0.52 M solution of NaA (the sodium
salt of the conjugate base A-) at 25°C?
AInitial []
Change in []
Equilibrium []
+
0.52
-x
0.52 - x
H2 O
HA
----
0
+x
x
+
OH10-7
+x
x
This assumes x >> 10-7
pKb(A-) = 14.0 - pKa(HA) = 14.0 – 5.8 = 8.2
Kb = 10-8.2 = 6.3 x 10-9 = x2 / (0.52-x)
Assume x << 0.52
6.3 x 10-9 = x2 / 0.52
3.3 x 10-9 = x2
x = 5.7 x 10-5 = [OH-]eq
pOH = -log(5.7 x 10-5) = 4.2
pH = 14.0 – 4.2 = 9.8
8.
a) Write the equation for the autoprotolysis of ammonia.
2NH3(l)
NH4+(am) + NH2-(am)
b) If Kam for ammonia (analogous to Kw for water) is 1 x 10-33 at -35 °C, determine [NH4+],
pNH4, and pNH2 at -35 °C.
[NH4+] = (1 x 10-33)1/2 = 3 x 10-17
pNH4 = pNH2 = -log(3 x 10-17) = 17
9.
Use Pauling’s rules to estimate the pKa for the first and second dissociation of H2SO4. Show the
Lewis structure and your calculation using Pauling’s rules.
Lewis structure should have two oxo ligands and two hydroxo ligands on S.
p=2
pKa1 ≈ 8 – 5p = 8 – 5(2) = -2
pka2 ≈ pKa1 + 5 = -2 + 5 = +3
10.
What is the ratio of hypochlorite to hypochlorous acid concentrations in a bleach solution that has
pH adjusted to 7.0 using strong acids or bases? pKa(HOCl, 25°C) = 7.5
pH = pKa + log ([A-] / [HA])
log ([A-] / [HA]) = pH – pKa
[A-] / [HA] = 10(pH – pKa)
[OCl-] / [HOCl] = 10(7.0 – 7.5) = 10-0.5 = 0.32