Enthalpy 2
Enthalpy 2
Enthalpy 2
∆H = H products - H reactants
Remember!
∆Hof
-104.0 0 -393.5 -241.8
(kJ/mol)
Solution
∆Horxn = Σ ∆Hof (products) - Σ ∆Hof (reactants)
Σ∆Horxn = [3∆Hof (CO2,g) + 4∆Hof (H2O,g)] -
[∆Hof (C3H8,g) + 5∆Hof (O2,g)]
= [3(-393.5 kJ) + 4(-241.8 kJ)] –
[(-104.0 kJ) + 5(0)]
= - 2 043.7 kJ
Another way to calculate ∆H
• Used series of steps of reactions
• Summarized by Hess’s law
- states that the enthalpy of a sum of a
series of reactions is equal to the sum of
the enthalpies of those two reactions
The following rules are considered:
• Enthalpy is an extensive property; it depends
on the amount of reactants and products
• The ∆H for a forward reaction is equal in
magnitude but opposite in sign to its reverse
reaction
• The ∆H for a reaction is the same whether it
occurs in one step or in a series of steps
Sample
The thermochemical equation for the
incomplete combustion of C into CO is
represented as
2C(s) + O2(g) → 2CO(g)
Compute for the ∆H of the reaction if the overall
process above can occur in 2 steps whose
thermochemical equations are given below:
(1) C(s) + O2(g) → CO2 (g) ∆H = -393.5 kJ
(2) 2CO(g) + O2(g) → 2CO2(g) ∆H = -566.0 kJ
Solution
For the first equation, multiply by 2
[C(s) + O2(g) → CO2(g) ∆H = -393.5 kJ] x 2
2C(s) + 2O2(g) → 2CO2 (g) ∆H = -787.0 kJ
For the second equation, reverse
2CO2(g) → 2CO(g) + O2(g) ∆H = 566.0 kJ
Using Hess’s law,
2C(s) + 2O2(g) → 2CO2 (g) ∆H = -787.0 kJ
2CO2(g) → 2CO(g) + O2(g) ∆H = 566.0 kJ
2C(s) + O2(g) → 2CO(g) ∆H = -221.0 kJ
Practice
1. Write the balanced equation for the
production of the brown NO2 gas from N and
O gases. Calculate the enthalpy of the
reaction if the process occurs in the
sequence of reactions given below:
N2(g) + N2(g) → 2NO(g) ∆H = 180 kJ
2NO2(g) → 2NO(g) + O2(g) ∆H = 112 kJ
2. Methylhydrazine, N2H3CH3 is a common liquid
propellant used in rocket fuels. Using the table
of standard molar enthalpies below, calculate
the ∆H0 for the reaction per mole of N2H3CH3.
4N2H3CH3(l) + 5N2O4(l) → 12H2O(g) + 9N2(g) + 4CO2(g)
∆Hof
54.14 9.66 -393.5 -241.8 0
(kJ/mol)