CHEM_100                                                                            2012

 

 

 

ASSIGNMENT # 2    SOLUTIONS   (out of 100 scores)            Due to October31st

 

 

REDOX REACTIONS, THERMOCHEMISTRY

 

 

 

 

 

  1. {10} Rules for Assigning an Oxidation Number (O.N.). Complete the table

 

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

 

 

  1. {10} Identify oxidation number for N and S in the following:

(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

 

  1. {10} Identify the oxidizing and reducing agent in each of the following:

 

(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, CO- reducing

(e) 2Na (s) + 2H2O (l) à 2NaOH (aq)  + H2 (g)

Na(s) –reducing; H2O- oxydizing

 

  1. {20} Balance the following equations using the method of half-reactions (9 scores). Identify the oxidizing and reducing agents (1 score).

(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

 

  1. {5} For which system ΔE is always negative?

(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

 

  1. {5} Of the following, which one is not a state function?

(a)   H

(b)   E

(c)    q

(d)   T

(e)    P

Hess’s Law

  1.  {10} Calculate the enthalpy of the rxn:

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