Dougherty Valley HS AP Chemistry
Hess’s Law Problems
Class work
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Date:
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[1] Hydrogen peroxide, H2O2, is a colorless liquid whose solutions are used as a bleach and an antiseptic. H2O2 can be
prepared in a process whose overall change is: -187.8KJ
H2(g) + O2(g)  H2O2(l)
Calculate the enthalpy change using the following data:
1
H2O2(l)  H2O(l) + O2(g) ; H = -98KJ
2
2H2(g) + O2(g)  2H2O(l) ; H = -571.6KJ
[2] Hydrazine, N2H4, is a colorless liquid used as a rocket fuel. What is the enthalpy change for the process in which
hydrazine is formed from it elements?
N2(g) + 2H2(g)  N2H4(l)
Use the following reactions and enthalpy changes:
N2H4(l) + O2(g)  N2(g) + 2H2O(l) ; H = -622.2KJ
1
H2(g) + O2(g)  H2O(l) ; H = -285.8KJ
2
[3] Ammonia will burn in the presence of a platinum catalyst to produce nitric oxide, NO. -906.3KJ
4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)
What is the heat of reaction at constant pressure? Use the following thermochemical equations:
N2(g) + O2(g)  2NO(g) ; H = 180.6KJ
N2(g) + 3H2(g)  2NH3(g) ; H = -91.8KJ
2H2(g) + O2(g)  2H2O(g) ; H = -483.7KJ
[4] Hydrogen cyanide is a highly poisonous, volatile liquid. It can be prepared by the reaction
CH4(g) + NH3(g)  HCN(g) + 3H2(g)
What is the heat of reaction at constant pressure? Use the following thermochemical equations:
N2(g) + 3H2(g)  2NH3(g) ; H = -91.8KJ
C(graphite) + 2H2  CH4(g) ; H = -74.9KJ
H2(g) + 2C(graphite) + N2(g)  2HCN(g) ; H = 270.3KJ
[5] Compounds with carbon-carbon double bonds, such as ethylene, C2H4, add hydrogen in a reaction called
hydrogenation. -137KJ
C2H4(g) + H2(g)  C2H6(g)
Calculate the enthalpy change for this reaction, use the following combustion data:
C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(l) ; H = -1401KJ
7
C2H6(g) + O2(g)  2CO2(g) +3H2O(l); H = -1550KJ
1
2
H2(g) + O2(g)  H2O(l) ; H = -286KJ
2
[6] Acetic acid, CH3COOH, is contained in vinegar. Suppose acetic acid was formed from its elements, according to the
following equation:
2C(graphite) + 2H2(g) + O2(g)  CH3COOH(l)
Find the enthalpy change, H, for this reaction, using the following data:
CH3COOH(l) + 2O2(g)  2CO2(g) + 2H2O(l) ; H = -871KJ
C(graphite) + O2(g)  CO2(g) ; H = -394KJ
1
H2(g) + O2(g)  H2O(l) ; H = -286
2