Bond Energy And Standard Enthalpy Page 2
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35.30 The first step in the industrial recovery of
zinc from the zinc sulfide ore is roasting, that is,
calculate the enthalpy of formation of methanol
the conversion of ZnS to ZnO by heating:
(CH
OH) from its elements:
3
1
(g) Æ 2ZnO(s) + 2SO
(g) Æ CH
2ZnS(s) + 3O
(g)
C(graphite) + 2H
(g) +
/
O
OH(l)
2
2
2
2
2
3
∆Hº
=-879 kJ
rxn
Calculate the heat evolved (in kJ) per gram of
5.36 Calculate the standard enthalpy change for
ZnS roasted.
the reaction
(s) Æ 2Fe(s) + Al
2Al(s) + Fe
O
O
(s)
2
3
2
3
5.31 Determine the amount of heat (in kJ) given
given that
4
off when 1.26 x 10
g of ammonia are produced
3
(g) Æ Al
according to the equation
2Al(s) +
/
O
O
(s) ∆Hº
=-1601kJ
2
2
2
3
rxn
3
(g) Æ Fe
2Fe(s) +
/
O
O
(s) ∆Hº
= -821kJ
2
2
2
3
rxn
(g) Æ 2NH
N
(g) + 3H
(g)
2
2
3
∆Hº
=-92.6 kJ
5.37 Consider the following two reactions:
rxn
A Æ 2B
Assume that the reaction takes place under
∆Hº
= ∆H
rxn
1
A Æ C
standardstate conditions at 25°C.
∆Hº
= ∆H
rxn
2
5.32 At 850°C, CaCO
undergoes substantial
Determine the enthalpy change for the process
3
2B Æ C
decomposition to yield CaO and CO
. Assuming
2
that the ∆Hº
values of the reactant and products
f
are the same at 850°C as they are at 25°C,
5.38 The standard enthalpy change ∆H˚ for the
calculate the enthalpy change (in kJ) if 66.8 g of
thermal decomposition of silver nitrate according
CO
are produced in one reaction.
to the following equation is +78.67 kJ:
2
1
(s) Æ AgNO
5.33 From these data,
AgNO
(s) +
/
O
(g)
3
2
2
2
(g) Æ SO
S(rhombic) + O
(g)
The standard enthalpy of formation of AgNO
(s)
2
2
3
∆Hº
=-296.06 kJ
is -123.02 kJ/mol. Calculate the standard
rxn
(g) Æ SO
S(monoclinic) + O
(g)
enthalpy of formation of AgNO
(s).
2
2
2
∆Hº
=-296.36 kJ
rxn
calculate the enthalpy change for the
5.39 Hydrazine, N
H
, decomposes according to
2
4
transformation
the following reaction:
S(rhombic) Æ S(monoclinic)
(l ) Æ 4NH
(Monoclinic and rhombic are different allotropic
3N
H
(g) + N
(g)
2
4
3
2
forms of elemental sulfur.)
(a) Given that the standard enthalpy of formation
5.34 From the following data,
of hydrazine is 50.42 kJ/mol, calculate ∆H° for
its decomposition. (b) Both hydrazine and
(g) Æ CO
C(graphite) + O
(g)
ammonia burn in oxygen to produce H
O(l) and
2
2
2
∆Hº
=-393.5 kJ
N
(g). Write balanced equations for each of these
rxn
2
1
(g) Æ H
H
(g) +
/
O
O(l) ∆Hº
= -258.8kJ
processes and calculate ∆H° for each of them.
2
2
2
2
rxn
(g) Æ 4CO
2C
H
(g) + 7O
(g) + 6H
O(l)
On a mass basis (per kg), would hydrazine or
2
6
2
2
2
∆Hº
=-3119.6 kJ
ammonia be the better fuel?
rxn
calculate the enthalpy change for the reaction
5.40 Photosynthesis produces glucose, C
H
O
,
6
12
6
(g) Æ C
2C(graphite) + 3H
H
(g)
and oxygen from carbon dioxide and water:
2
2
6
O Æ C
6CO
+ 6H
H
O
+ 6O
2
2
6
12
6
2
5.35 From the following heats of combustion,
(a) How would you determine experimentally the
∆Hº
value for this reaction? (b) Solar radiation
rxn
3
14
(g) Æ CO
CH
OH(l) +
/
O
(g) + 2H
O(l)
produces about 7.0 x 10
kg glucose a year on
3
2
2
2
2
∆Hº
=-726.4kJ
Earth. What is the corresponding ∆H° change?
rxn
(g) Æ CO
C(graphite) + O
(g) = ∆Hº
2
2
rxn
-393.5kJ
5.41 2.10-mole sample of crystalline acetic acid,
1
(g) Æ H
H
(g) +
/
O
O(l) = ∆Hº
-285.8kJ
initially at 17.0°C, is allowed to melt at 17.0°C
2
2
2
2
rxn
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