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South Pasadena  AP Chemistry
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6  Energy and Chemical Reactions
HESS’S LAW CALCULATIONS
The enthalpy of the reactants, Hreactants and the enthalpy of the products, Hproducts depend on the bonding of the
reactants and products… nothing else. So, the Hreaction only depends on the initial and final state of the
reaction, not how you got from one state to another state. It is called a “state function”.
Practically speaking, if we can find several equations that “add up” to the equation we want, the Hreactions
will add up to the overall H. This is called Hess’s Law.
Heats of Formation: Write the formation equations for the following.
Compound
Formation Equation
Hf (kJ·mol-1)
CH4(g)
C(s) + 2H2(g)  CH4(g)
-74.8
H2O(l)
-285.8
H2O(g)
-241.8
CO2(g)
-393.5
C2H6(g)
-84.7
C3H8(g)
???
C4H10(g)
-125.6
Example in class:
Calculate the Hcombustion for CH4:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
a)
Calculate the heat of combustion, Hcombustion, for ethane, C2H6(g)
Compound
Hf (kJ·mol-1)
H2O(g)
-241.8
CO2(g)
-393.5
C4H10(g)
-125.6
c)
Calculate the Hcombustion of butane, C4H10(g).
d)
The heat of combustion of propane, C3H8 , is -2220 kJ·mol-1.
Use this information to calculate the Hf of C3H8.
1. Calculate the ΔH for the reaction: C2H4(g) + H2(g) → C2H6(g), from the following Data.
C2H4(g) + 3 O2(g) → 2 CO2(g)+ 2 H2O(l)
ΔH = -1411.kJ
C2H6(g) + 3 ½ O2(g) → 2 CO2(g) + 3 H2O(l)
H2(g) + ½ O2 → H2O(l)
ΔH = -1560. kJ
ΔH = -285.8 kJ
2. Calculate the ΔH 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)
ΔH = -180.5 kJ
N2(g) + 3 H2(g) → 2 NH3(g)
ΔH = -91.8 kJ
2H2(g) + O2(g) → 2 H2O(g)
ΔH = -483.6 kJ
3. Calculate the ΔH for the reaction: 2 H2(g) + 2 C(s)+ O2(g) → C2H5OH(l), from the following Data.
C2H5OH(l) + 2 O2(g) → 2 CO2(g) + 2 H2O(l)
ΔH = -875. kJ
C(s)+ O2(g) → CO2(g)
ΔH = -394.51 kJ
H2(g) + ½ O2(g) → H2O(l)
ΔH = -285.8 kJ
4. Calculate the ΔH for the reaction: CH4 (g) + NH3 (g) --> HCN (g) + 3 H2 (g), from the following Data.
N2 (g) + 3 H2 (g) --> 2 NH3 (g) 
C (s) + 2 H2 (g) --> CH4 (g)

H = -91.8 kJ

H2 (g) + 2 C (s) + N2 (g) --> 2 HCN (g) 
H = -74.9 kJ
H = +270.3 kJ
5. Calculate the standard enthalpy change, ΔH, for the formation of 1 mol of strontium carbonate ( the
material that gives the red color in fireworks) from its elements.
Sr(s) + C(graphite) + 3/2 O2(g) → SrCO3(s)
ΔH = ?
The information available is:
(1) Sr(s) + ½ O2(g) → SrO(s)
ΔH = -592 kJ
(2) SrO(s) + CO2(g) → SrCO3(s)
ΔH = -234 kJ
(3) C(graphite) + O2(g) → CO2(g)
ΔH = -394 kJ
6. The combination of coke and steam produces a mixture called coal gas, which can be used as a
fuel or as a starting material for other reactions. If we assume coke can be represented by
graphite, the equation for the production of coal gas is:
2 C(s) + 2H2O(g) → CH4(g) + CO2(g) ΔH = ????
Determine the standard enthalpy for this reaction from the following standard enthalpies of reaction:
(1) C(s) + H2O(g) → CO(g) + H2(g)
ΔH = 131.3 kJ
(2) CO(g) + H2O(g) → CO2(g) + H2(g)
ΔH = -41.2 kJ
(3) CH4(g) + H2O(g) → 3 H2(g) + CO(g)
ΔH = 206.1 kJ
The next one is challenging!
7. One reaction involved in the conversion of iron ore to the metal is
FeO(s) + CO(g) → Fe(s) + CO2(g)
Calculate the standard enthalpy change for this reaction from these reactions of iron oxides with
CO:
ΔH = -47.0kJ
(1) 3 Fe2O3(s) + CO(g) → 2 Fe3O4(s) + CO2(g)
(2)
Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)
ΔH = -25.0 kJ
(3)
Fe3O4(s) + CO(g) → 3 FeO(s) + CO2(g)
ΔH = 19.0 kJ