Chemistry 114
Third Hour Exam
Name:____________
1. (10 points) Using the collision model for chemical kinetics we developed the equation:
k=zpe-Ea/RT
What do the various terms in this equation represent?
k=the rate of a reaction
z= collision frequency ( how often molecules collide)
p= steric factor ( How often molecules are correctly oriented)
e-Ea/RT = fraction of collisions with sufficient energy to produce a reaction
Ea = activation energy - minimum energy required for a reaction
R = gas constant in J/K mol
T = temp in K
2. (10 points) I have a reaction with k=1.5 mol/sec at 0 o C and 4.6 mole/sec at 30oC. What
is the activation energy for this reaction
ln(k2/k1) = Ea/R (1/T1 - 1/T2)
ln(4.6/1.5) = Ea/8.3145 (1/273 - 1/303)
1.12=Ea/8.3145 (3.663x10-3 -3.300x10-3 )
1.12(8.3145) = Ea(.363x10-3 )
Ea=1.12(8.3145) /.363x10-3
=2.57x104 J/mol = 25.7 kJ/mol
3. (10 points) Write an equilibrium expression for the reaction:
Fe2O3(s) + 6 H2C2O4(aq) 6 2Fe(C 2O4)33-(aq) + 3H2O(l) + 6H+ (aq)
[ Fe(C2 O4 ) 2− 3 ]2 [ H + ]6
K=
[ H2 C2 O6 ]6
2
4. (10 points) The gas phase reaction 2NO(g) + Cl2(g) W 2NOCl(g) has a K p of 1.9x103.
What is the K c for this reaction? (Assume T=25oC)
KP = K C(RT))n
)n = 2-3 = -1
1.9x103 = K c(RT)-1 = K C/RT
Kc = 1.9x103 (RT) = 1.9x103 (.08206)298K = 46,000
5. The gas phase reaction H2 + F2 W 2HF has a K c of 1.13x102.
A. (2 points) If I have 1 mole each of H2, F2, and HF in a 1 liter container, is the system
at equilibrium?
Q=1, Q…K, system not at equilibrium
B. (2 points) How will the system shift to get to equilibrium, toward reactants or toward
products?
Q<K, not enough products, system will shift toward products
C. What will the concentrations of these gases be when the system reaches
equilibrium? (3 points for set up, 3 points for answer)
X = decrease in concentration of H2 or F 2
10.63 − 10.63 X = 1 + 2 X
[ HF ]2
K=
[ H2 ][ F2 ]
113
. x10 2 = 113 =
1+ 2X
1− X
1+ 2 X
10.63 =
1− X
113 =
(1 + 2 X ) 2
(1 − X )(1 − X )
9.63 = 12.63 X
X = 9.63 / 12.63
X = .76
[H 2 ]= 1-.76 = .24
[F2 ]= 1-.76 = .24
[HF] = 1+2(.76) = 2.52
3
6. A. (5 points) What is the pH of a .045 M HCl solution
HCl is a strong acid, so it will dissociate 100%, so
[H+]=[Cl-] = .045
pH= -log[H+] = -log(.045)
= -log(-1.35)
=1.35
B. (5 points) What is the pH of a .7x10-7 M NaOH solution
NaOH is a strong base so it will dissociate 100% so:
[Na+] = [OH-] = .7x10-7
Notice, this is less than the [OH-] coming from the dissociation of water itself (1x10-7 )
Thus something is fishy here. For a quick and dirty answer, I will add the [H+]
from the two different sources together to get
[OH-] = 1x10-7 + .7x10-7 = 1.7x10-7
pOH = -log [OH-] = -log(1.7x10-7 ) = -log(-6.77) = 6.77
pH = 14-pOH = 14-6.77 = 7.23
7. (10 points) What is the pH of a 1M solution of hypochlorous acid (HOCl), K a = 3.5x10-8
[ H + ][ A − ]
Ka =
[ HA]
35
. x10
−8
X2
=
1− X
Ka is small, so assume that 1-X = 1
3.5 x10 − 8 =
X =
X2
; 3.5 x10 − 8 = X 2
1
3.5 x10 − 8 = 1.87 x10 − 4
[H+] = 1.87x10-4 ; pH =-log[H+] = 3.73
8. (10 points) Rank the following compounds from most acidic to least acidic
NaNO3, HNO2, Cr3+, FeO2, Be(OH)2, NH4NO3
Acid
HNO3 (Cr3+ or NH4NO3 )
neutral
NaNO3
base
(FeO2 or Be(OH)2 )
4
9. (10 points) Nitrous acid is a weak acid with a Ka of 4.0x10-4 . What is the pH of a .01M
solution of the sodium salt of this acid (NaNO2)
Weak acid rxn HA W H+ + A The anion of the weak acid will undergo the base reaction
A- + H2O W HA + OHKB = [HA][OH-]/[A -]
KB? K BKA = K w; KB = K W/KA = 1x10-14/4.0x10-4 = 2.5x10-11
X=[OH-] = [HA] , [A -] = .01-X
2.5x10-11 = X 2/.01-X
KB is small so assume .01-X •.01
2.5x10-11 = X 2/.01
2.5x10-11(.01) = X 2
X = 5.0x10-7 ; pOH = -log[OH-] = 6.30, pH = 14-pOH = 7.70
But this is another one where the OH- of water itself is significant so, more correctly
[OH-] = 5.0x10-7 + 1x10-7 = 6.0x10-7 , pOH = 6.22, pH = 7.78
10. (10 points) The pH of normal human blood is 7.450. What hydrogen ion concentration
does this correspond to? What is the correct number of significant figures in your answer?
pH = -log [H+]
7.450 = -log[H+]
-7.450 = log[H+]
10-7.450 = [H+]
[H+] = 3.54813x10-8
But the correct number fo significant figures is 3
So [H+] = 3.55x10-8