CHEM_100 2012
ASSIGNMENT # 2 SOLUTIONS (out
of 100 scores) Due to October31st
REDOX REACTIONS, THERMOCHEMISTRY
1. For
an atom in its elemental form (Na, O2 etc.): O.N. = 0
2. For a monoatomic ion : O.N.
= charge of the ion
3. For Group 1A (1)
in all compounds O.N.
= +1
4. For Group 2A (2)
in all compounds O.N.= +2
5. For hydrogen in
combination with nonmetals O.N.
= +1
in combination with
metals O.N.= -1
6. For
fluorine in all compounds O.N.
= -1
7. For oxygen in
peroxides O.N.= -1
in
all other compounds (except with F) O.N.= -2
8. For group 7A (17)
in combination with metals,
nonmetals (except
with O), and other halogens
lower in the group O.N.=
-1
(a)
NO3- +5
(b)
NO2- +3
(c)
NH3 -3
(d)
N2O5 +5
(e)
NO +2
(f)
NO2 +4
(g)
N2O3 +3
h) S8 0
I SO42- +6
(a) 2Al (s) + 3 H2SO4 (aq) à Al2(SO4)3(aq) + 3 H2 (g)
Al(s)- reducing, H+ - oxydizing (H2SO4
is acceptable answer)
(b) PbO (s) + CO(g) à
Pb (s) + CO2 (g)
PbO(s) – oxidizing,
CO- reducing
(c) 2H2 (g) + O2 (g) à 2H2O (g)
O2 (g) –
oxidizing, H2 (g)- reducing
(d) 5CO(g) + I2O5 (s) à I2 (s) + 5 CO2 (g)
I2O5
(s) – oxidizing,
(e) 2Na (s) + 2H2O (l) à 2NaOH (aq) + H2 (g)
Na(s) –reducing; H2O- oxydizing
(a) SO32-(aq) + MnO4-
(aq) à SO42-
(aq) +
Mn2+ (aq) (acidic
solution)
SO32- + H2O à SO42- + 2H+ + 2e x5
MnO4- + 8H+ +
5e à
Mn2+ + 4H2O x 2
6H+
(aq) + 5 SO32-(aq) + 2MnO4- (aq) à 2Mn2+ (aq) + 3 H2O (l) + 5SO42-(aq)
SO32-(aq)
–reducing agent
MnO4- (aq) -
oxidizing agent
(b) MnO4-
(aq) + Br- (aq) à MnO2 (s) + BrO3- (aq) (basic solution)
MnO4- + 4H+ + 3e
--> MnO2 + 2 H2O x2
Br- + 3 H2O --> BrO3-
+ 6H+ + 6e
2MnO4- + 8H+
+ Br- + 3H2O -->
2 MnO2 + 4H2O +
BrO3- +6H+
2MnO4- + 2H+
+ Br- --> 2 MnO2
+ H2O + BrO3-
2OH-
+ 2MnO4- + 2H+
+ Br- --> 2 MnO2 + H2O + BrO3- +
2OH-
2MnO4-
+ 2H2O + Br- -->
2 MnO2 + H2O +
BrO3- +
2OH-
2MnO4-
+H2O + Br- -->
2 MnO2 + BrO3-
+ 2OH-
Oxidizing agent: MnO4- (aq)
Reducing agent: Br-
(aq)
THERMOCHEMISTRY
5. {10} When 1 mole of nitroglycerine decomposes, it causes a violent
explosion and releases 5.72 x103 kJ of heat.
C3H5(NO3)3 (l) à 3 CO2(g) + 5/2
H2O (g) +1/4O2(g) + 3/2 N2 (g)
(a) {2} Is the
reaction exothermic or endothermic? Write a thermochemical equation.
(b) {3} Draw an
enthalpy diagram for it.
(c) {5} How much heat
will be released when 1kg of nitroglycerine decomposes?
(a)
C3H5(NO)3 (l) à 3 CO2(g) + 5/2 H2O (g) +1/4O2(g) + 3/2 N2 (g) ΔH = -5.72x103 kJ
Reaction is
exothermic
ΔH, kJ
C3H5(NO)3
(l)
-5.72x103 kJ
3
CO2(g) + 5/2 H2O (g) +1/4O2(g) + 3/2 N2 (g)
(c) ΔH = -5.72 x103
kJ/mol x 1000g/227g/ mol = -2.52 x 104 kJ
(a)
absorbs heat
and does work
(b)
gives off heat and does work
(c)
absorbs heat
and has work done on it
(d)
gives off
heat and has work done on it
(a) H
(b) E
(c)
q
(d) T
(e) P
Hess’s Law
N2 + 2O2
à 2NO2
Using the following data:
2NO à N2 + O2 ΔH = -180 kJ
2NO + O2 à 2NO2 ΔH = -112
kJ
N2 + O2 à 2NO
ΔH = +180 kJ
+
2NO + O2 à 2NO2 ΔH
= -112 kJ
N2 + 2O2 à 2NO2 ΔH = 68 kJ
9. {20}. Determine the enthalpy
change for the oxidation of ammonia:
4NH3
(g) + 5 O2 (g) à 4 NO(g)
+ 6 H2O(g) ΔH =
from the following data:
N2
(g) + 3 H2 (g) à 2 NH3 (g) ΔH = -99.22 kJ
N2 (g) + O2 (g) à 2 NO (g) ΔH = +180.5 kJ
2H2 (g) + O2 (g) à 2 H2O (l) ΔH = -571.6 kJ
H2O (g) à H2O (l) ΔH = -44.01 kJ
2H2
(g) + O2 (g) à
2 H2O (l) ΔH
= -571.6 kJ
2H2O(l) à 2H2O(g) ΔH = +88 kJ
2H2
(g) + O2 (g) à
2 H2O (g) ΔH
= -483.6 kJ
4 NH3 (g) à 2N2 (g) + 6H2 (g) ΔH = 2x99.22 kJ
2N2
(g) + 2O2 (g) à
4 NO (g) ΔH = 2x180.5
kJ
6H2
(g) + 3O2 (g) à 6 H2O (l) ΔH = 3x(-483.6) kJ
4NH3
(g) +
5 O2 (g) à 4 NO(g) + 6 H2O(g)
ΔH = 2x99.22 + 2x180.5 +
3x(-483.6) kJ
= -891.4 kJ