# HESS'S LAW PROBLEMS

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```					                              ENTHALPY PROBLEMS

1. Determine H for the reaction

Use Hess’s Law of Heat Summation and the following reactions to solve for the enthalpy
of the above reaction

2. Carbon occurs in two forms: graphite and diamond. The enthalpy of combustion of
graphite is -393.5 kJ, and that of diamond is -395.4 kJ

C(graphite) + O2(g) -> CO2(g) H = -393.5 kJ

C(diamond) + O2(g) -> CO2(g) H = -395.4 kJ

Calculate H for the conversion of graphite to diamond

3. Use the thermochemical equations shown below to determine
the enthalpy for the reaction:

trans-C4H8(g) cis-C4H8(g)

cis-C4H8(g) + 6O2(g) 4CO2(g) + 4H2O(g)         H=-4066.3KJ

trans-C4H8(g) + 6O2(g) 4CO2(g) + 4H2O(g)         H=-4060.3KJ
4. Use the thermochemical equations shown below to determine
the enthalpy for the reaction:

2C2H4O(l) + 2H2O(l) 2C2H6O(l) + O2(g)

C2H6O(l) + 3O2(g) 2CO2(g) + 3H2O(l)            H=-685.5KJ

C2H4O(l) + 5/2O2(g) 2CO2(g) + 2H2O(g)             H=-583.5KJ

5. Use the thermochemical equations shown below to determine
the enthalpy for the reaction:

6Fe(s) + 4O2(g) 2Fe3O4(s)

2Fe3O4(s) + 1/2O2(g) 3Fe2O3(s)          H=-232.2KJ

3Fe2O3(s) 6Fe(s) + 9/2O2(g)         H=2466.5KJ

6. Given the following reactions and their enthalpy changes, calculate the enthalpy
change for

H2(g) + C(s) + O2(g) -----> HCOOH(l)

Change in Heat
Equations
Energy
C(s) + O2(g) ---> CO2(g)                                  -394 kJ
H2(g) + 1/2 O2(g) ---> H2O(l)                             -286kJ
HCOOH(l) + 1/2 O2(g) ---> CO2(g) + H2O(l)                 -275kJ
7. Given the following reactions and their enthalpy changes, calculate the enthalpy
change for

2 NO2(g) ----> N2O4(g)

Equations                                    Change in Heat Energy
N2(g) + 2 O2(g) ---> 2 NO2(g)                       67.8 kJ
N2(g) + 2 O2(g) ---> N2O4(g)                        9.67 kJ

8. Given the following reactions and their enthalpy changes, calculate the enthalpy
change for
Ca(g) + C(s) + 1.5 O2(g) ----> CaCO3(s)

Change in Heat
Equations
Energy
Ca(g) + 2 C(s) ---> CaC2(S)                                   -62.8 kJ
CO2(g) ---> C(s)+ O2(g)                                       394 kJ
CaCO3(s) + CO2(g) ---> CaC2(s) + 5/2 O2(g)                    1538 kJ

9. Calculate the heat change for the formation of 1.00mol of strontium carbonate from its
elements. Given the following experimental information:

10. Calculate the   Hfo for 6 C(s) + 6 H2 (g) + 3 O2 (g)       C6H12O6 (s) given that:

C6H12O6 (s) + 6 O2 (g)      6 CO2 (g) + 6 H2O (g) Hc = -2537.3 kJ
H2 (g) + 1/2 O2 (g)    H2O (g)     Hfo = -241.8 kJ
C (s) + O2 (g)       CO2 (g) Hfo = -393.5 kJ
Hess's Law Worksheet

1. Calculate H for the reaction: C2H4 (g) + H2 (g) C2H6 (g), from the following
data.

C2H4 (g) + 3 O2 (g) 2 CO2 (g) + 2 H2O (l)                       H = -1411. kJ

C2H6 (g) + 3½ O2 (g) 2 CO2 (g) + 3 H2O (l)                      H = -1560. kJ

H2 (g) + ½ O2 (g) H2O (l)                                       H = -285.8 kJ

2.    Calculate H for the reaction 4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g), from
the following data.

N2 (g) + O2 (g) 2 NO (g)                                        H = -180.5 kJ

N2 (g) + 3 H2 (g) 2 NH3 (g)                                     H = -91.8 kJ

2 H2 (g) + O2 (g) 2 H2O (g)                                     H = -483.6 kJ

3.    Find H for the reaction 2H2(g) + 2C(s) + O2(g)  C2H5OH(l), using the
following thermochemical data.

C2H5OH (l) + 2 O2 (g) 2 CO2 (g) + 2 H2O (l)                     H = -875. kJ

C (s) + O2 (g) CO2 (g)                                          H = -394.51 kJ

H2 (g) + ½ O2 (g) H2O (l)                                       H = -285.8 kJ

4.   Calculate H for the reaction CH4 (g) + NH3 (g) HCN (g) + 3 H2 (g), given:

N2 (g) + 3 H2 (g) 2 NH3 (g)                                     H = -91.8 kJ

C (s) + 2 H2 (g) CH4 (g)                                        H = -74.9 kJ

H2 (g) + 2 C (s) + N2 (g) 2 HCN (g)                             H = +270.3 kJ

5.   Calculate for the reaction 2 Al (s) + 3 Cl2 (g) 2 AlCl3 (s) from the data.
2 Al (s) + 6 HCl (aq) 2 AlCl3 (aq) + 3 H2 (g)   H = -1049. kJ

HCl (g) HCl (aq)                                H = -74.8 kJ

H2 (g) + Cl2 (g) 2 HCl (g)                      H = -1845. kJ

AlCl3 (s) AlCl3 (aq)                            H = -323. kJ

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