Chapter 5 (3rd edition)
5.29 We can calculate U by rearranging Equation 5.10 of the text.
H−P V
U
Substituting into the above equation:
U
483.6 kJ/mol − (1.00 atm )(32.7 L) 101.3 J
L atm
U
5.42 Specific heat
C
m
85.7 J/ C
362 g
4.80
1 kJ
1000 J
102 kJ
0.237 J/g C
5.49 Making the necessary changes to the reactions given in the problem, and making the
corresponding changes to the H values:
Reaction
CO2(g)
H (kJ/mol)
2H2O(l)
C(graphite)
2H2(g)
C(graphite)
CH3OH(l)
O2(g)
O2(g)
2H2(g)
3
2
O2(g)
CO2(g)
2H2O(l)
1
2
O2(g)
726.4
393.5
2(–285.8)
CH3OH(l)
H rxn
238.7 kJ/mol
We have just calculated an enthalpy at standard conditions, which we abbreviate H rxn . In
this case, the reaction in question was for the formation of one mole of CH3OH from its elements
in their standard state.
Therefore, the H rxn that we calculated is also, by definition, the standard heat of formation H f
of CH3OH ( 238.7 kJ/mol).
5.62 a.
H
H
[(2)( 393.5 kJ/mol)
b.
H
H
5.67
w
= [2 Hf (CO2 )
2 H f (SO2 )]
U
[2 Hf (H2S)
98 L atm
P V
or
101.3 J
1 L atm
U
H
(3)(0)]
3 Hf (O2 )]
(2)( 296.4 kJ/mol)] [(2)( 20.15 kJ/mol)
H
1124 kJ/mol
(1 atm)( 98 L)
H
3 Hf (O2 )] ]
[ Hf (C2 H4 )
(2)( 285.8 kJ/mol)] [(1)(52.3 kJ/mol)
H
1411 kJ/mol
= [2 H f (H2O)
[(2)( 285.8 kJ/mol)
P V
2 Hf (H2O)]
9.9
103 J
(3)(0)]
9.9 kJ
P V
Using H as 185.2 kJ (2
92.6 kJ), (because the question involves the formation of 4
moles of ammonia, not 2 moles of ammonia for which the standard enthalpy is given in the
question), and P V as 9.9 kJ (for which we just solved):
U
185.2 kJ
9.9 kJ
175.3 kJ
5.73 The balanced equation showing the formation of Ag2O(s) from its elements is:
2Ag(s)
1
2
O2(g)
Ag2O(s)
Knowing that the standard enthalpy of formation of any element in its most stable form is zero,
and using Equation 5.18 of the text, we write:
H rxn
H rxn
n Hf (products)
m Hf (reactants)
H f (Ag 2 O) - 2 H f (Ag)
H rxn
1
H f (O 2 )
2
H f (Ag 2 O) - 0 0
Hf (Ag2O)
Hrxn
In a similar manner, you should be able to show that Hf (CaCl 2 )
Ca(s)
Cl2(g)
Hrxn
CaCl2(s)
In both cases, calorimetry could be used to measure the enthalpy changes.
for the reaction