# iv thermodynamics pg 1 hess s law

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```							IV. Thermodynamics (pg 1)                                                               AP Chemistry

Hess’s Law
1) Given the following reactions:
S (s) + O2 (g)  SO2 (g)                              ∆H = - 71.0 Kcal
SO2 (g) + ½ O2 (g)  SO3 (g)                          ∆H = - 23.5 Kcal

Calculate ∆H for the reaction:
S (s) + 1 ½ O2 (g)  SO3 (g)

2) Given the enthalpies of reaction
P4 (s) + 3 O2 (g)  P4O6 (s)              ∆H = - 1640.1 kJ
P4 (s) + 5 O2 (g)  P4O10 (s)             ∆H = - 2940.1 kJ
Calculate the enthalpy change for the reaction
P4O6 (s) + 2 O2 (g)  P4O10 (s)

3) From the heats of reaction
2 H2 (g) + O2 (g)  2 H2O (g)               ∆H = - 483.6 kJ
3 O2 (g)  2 O3 (g)                         ∆H = + 284.6 kJ
Calculate the heat of reaction
3 H2 (g) + O3 (g)  3 H2O (g)

4) Use the reactions below to determine ∆H for this single displacement reaction.

Cl2 (g) + 2 HBr (g)  2 HCl (g) + Br2 (g)

a) H2 (g) + Cl2 (g)  2 HCl (g)                       ∆H = - 185 kJ
b) H2 (g) + Br2 (g)  2 HBr (g)                       ∆H = - 73 kJ

5) Use the reactions below to determine ∆H for the reaction of carbon monoxide and hydrogen to form methanol (CH3OH).

CO (g) + 2 H2 (g)  CH3OH (l)

a) 2 CO (g) + O2 (g)  2 CO2 (g)                               ΔH = - 566 kJ
b) 2 H2 (g) + O2 (g) -> 2 H2O (l)                              ΔH = -572 kJ
c) 2 CH3OH (l) + 3 O2 (g)  2 CO2 (g) + 4 H2O (l)              ΔH = -1452 kJ

6) Use the following reactions to determine ∆H for the oxidation of ethanol (C 2H5OH) to form acetaldehyde (C2H4O) and water.
2 C2H5OH (l) + O2 (g)  2 C2H4) (g) + 2 H2O (l)

a) 2 C2H4O (g) + 5 O2 (g)  4 CO2 (g) + 4 H2O (l)                       ∆H = - 2385 kJ
b) C2H5OH (l) + 3 O2 (g)  2 CO2 (g) + 3 H2O (l)                        ∆H = -1367 kJ

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