hess The following is a list of some extra Hess's Law problems.  They will not be collected, nor will these particular questions be asked on an exam.  Doing these problems, however, will certainly help you understand Hess’s Law better.  Good luck!






(1)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
PCl5(g)  →  PCl3(g)  +  Cl2(g)

P4(s)  +  6Cl2(g)  →  4PCl3(g)            ΔH = -2439 kJ
4PCl5(g)  →  P4(s)  +  10Cl 2(g)         ΔH = 3438 kJ
answer = 249.8 kJ
 

(2)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
2CO2(g)  +  H2O(g)  →  C 2H2(g) +  5/2O2(g)

C2H2(g) + 2H2(g)  →  C2H6(g)                              ΔH  =-94.5 kJ
H2O(g)  →  H2(g) + 1/2O2 (g)                               ΔH  =71.2 kJ
C2H6(g) +  7/2O2(g)  →  2CO2(g)  +  3H2O(g)     ΔH  =-283 kJ
answer = 235 kJ
 

(3)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
N2H4(l)  +  H2(g)  →  2NH3(g)

N2H4(l)  +  CH4O(l)  →  CH2O(g)  +  N2(g)  +  3H2 (g)         ΔH = -37 kJ
N2(g)  +  3H2(g)  →  2NH 3(g)                                                ΔH = -46 kJ
CH4O(l)  →  CH2O(g) +  H 2(g)                                              ΔH = -65 kJ
answer = -18 kJ
 

(4)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
H2SO4(l)  →  SO3(g)  +  H2O(g)

H2S(g)  +  2O2(g)  →  H2SO4(l)                                  ΔH = -235.5 kJ
H2S(g)  +  2O2(g)  →  SO 3(g)  +  H2O(l)                    ΔH = -207 kJ
H2O(l)  →  H2O(g)                                                      ΔH = 44 kJ
answer = 72 kJ
 


(5)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
2C2H4O(l) + 2H2O(l)  →  2C2H6O(l) +  O2(g)

C2H6O(l)  +  3O2(g)  →  2CO2(g)  +  3H2O(l)            ΔH = -685.5 kJ
C2H4O(l)  +  5/2O2(g)  →  2CO2(g)  +  2H2O(l)         ΔH = -583.5 kJ
answer = 204.0 kJ

(6)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
N2(g) +  2O2(g)  →  2NO 2(g)

N2(g)  +  3H2(g)  →  2NH3(g)                                  ΔH = -115 kJ
2NH3(g)  +  4H2O(l)  →  2NO2(g)  +  7H2(g)          ΔH = -142.5 kJ
H2O(l)  →  H2(g)  +  1/2O 2(g)                                 ΔH = -43.7 kJ
answer = -83 kJ

(7)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
CO2(g)  →  C(s) +  O2(g)

H2O(l)  →  H2(g)  +  1/2O2(g)                                    ΔH = 643 kJ
C2H6(g)  →  2C(s) +  3H 2(g)                                     ΔH = 190.6 kJ
2CO2(g) +  3H2O(l)  →  C 2H6(g) + 7/2O2(g)            ΔH = 3511.1 kJ
answer = 886 kJ

(8)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
N2H4(l)  +  CH4O(l)  →  CH2O(g)  +  N2(g)  +  3H2 (g)

2NH3(g)  →  N2H4(l)  +  H2(g)                 ΔH = 22.5 kJ
2NH3(g)  →  N2(g)  +  3H 2(g)                 ΔH = 57.5 kJ
CH2O(g) +  H2(g)  →  CH 4O(l)               ΔH = 81.2 kJ
answer = -46.2 kJ

(9)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
1/2H2(g)  +  1/2Cl2(g)    HCl(g)

COCl2(g)  +  H2O(l)    CH2Cl2(l)  +  O2(g)                               ΔH = 47.5 kJ
2HCl(g)  +  1/2O2(g)    H 2 O(l)  +  Cl2(g)                                 ΔH = 105 kJ
CH2Cl2(l) +  H2(g)  +  3/2O 2(g)    COCl2(g)  +  2H 2O(l)         ΔH = -402.5 kJ
answer = -230 kJ


 

 

(10)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
C2H2(g) +  5/2O2(g)    2CO2(g)  +  H2O(g)

C2H6(g)    C2H 2(g) + 2H2(g)                              ΔH = 283.5 kJ
H2(g) + 1/2O2(g)    H2O(g)                                ΔH = -213.7 kJ
2CO2(g)  +  3H2O(g)    C2H6(g) +  7/2O2(g)      ΔH = 849 kJ
answer = -705 kJ

(11)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
HCl(g) + NaNO2(s) HNO2(l) + NaCl(s)

2NaCl(s) + H2O(l) →  2HCl(g) + Na2O(s)                 ΔH = 507 kJ
NO(g) + NO2(g) + Na2O(s) 2NaNO2(s)             ΔH = -427 kJ
NO(g) + NO2(g) N2O(g) + O2(g)                         ΔH = -43 kJ
2HNO2(l) N2O(g) + O2(g) + H 2O(l)                    ΔH = 34 kJ
Answer = -78 kJ

(12)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
Zn(s) +  1/8S8(s)  +  2O2(g)    ZnSO4(s)

Zn(s) + 1/8S8(s)      ZnS(s)                                   ΔH = -183.92 kJ
2ZnS(s) + 3O2(g)      2ZnO(s) + 2SO2(g)            ΔH = -927.54 kJ
2SO2(g) + O2(g)      2SO3(g)                              ΔH = -196.04 kJ
ZnO(s) + SO3(g)      ZnSO4 (s)                           ΔH = -230.32 kJ
Answer = -976.03 kJ



These problems involve using heat of formation values that are found in the appendix of your textbook.

(13)  What is the enthalpy of the following reactions?

(a)   SiF4(g)    →    Si(s)    +    2F2(g)    (answer:  +1615.0 kJ)

(b)   SiF4(g)    →    Si(g)    +    2F2(g)    (answer:  +2065.0 kJ)

(c)   SO3(g)    +    H2O(g)    →    H2SO4(aq)    (answer: -271.8 kJ)

(d)   3K2O2(s)    +    3H2O(l)    →    6KOH(aq)    +    O3(g)    (answer: -412 kJ) (not -411.7 kJ)

(e)   Fe3O4(s)   +   8HCl(g)  →   2FeCl3(s)   +   FeCl2(s)  +  4H2O(g)  (answer: -252 kJ) (not -251.6 kJ)