Bond Energy And Standard Enthalpy
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3CHAPTER FIVE
Bond Energy and Standard Enthalpy
(Look up the standard enthalpy of formation of
5.11 What is meant by the standard-state
the reactant and products in Table 6.3.)
condition?
5.24 The standard enthalpies of formation of ions
5.12 How are the standard enthalpies of an
in aqueous solutions are obtained by arbitrarily
+
element and of a compound determined?
assigning a value of zero to H
ions; that is, ∆Hº
f
+
[H
(aq)] = 0. (a) For the following reaction
5.13 What is meant by the standard enthalpy of a
+
-
HCl(g) Æ H
reaction?
(aq) + Cl
(aq) ∆H° = 74.9 kJ
-
5.14 Write the equation for calculating the
calculate ∆Hº
for the Cl
ions. (b) Given that
f
enthalpy of a reaction. Define all the terms.
∆Hº
for
f
-
OH
ions is -229.6 kJ/mol, calculate the enthalpy
5.15 State Hess’s law. Explain, with one
of neutralization when 1 mole of a strong
example, the usefulness of Hess’s law in
monoprotic acid (such as HCl) is titrated by 1
thermochemistry.
mole of a strong base (such as KOH) at 25°C.
5.16 Describe how chemists use Hess’s law to
5.25 Calculate the heats of combustion for the
determine the ∆Hº
of a compound by measuring
following reactions from the standard enthalpies
f
its heat (enthalpy) of combustion.
of formation listed in Appendix 3:
(g) Æ 2H
(a) 2H
(g) + O
O(l)
2
2
2
(g) Æ 4CO
5.17 Which of the following standard enthalpy of
(b) 2C
H
(g) + 5O
(g) + 2H
O(l )
2
2
2
2
2
formation values is not zero at 25°C? Na(s),
Ne(g), CH
(g), S
(s), Hg(l ), H(g).
5.26 Calculate the heats of combustion for the
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following reactions from the standard enthalpies
5.18 The ∆Hº
values of the two allotropes of
of formation listed in Appendix 3:
f
(g) Æ 2CO
oxygen, O
and O
, are 0 and 142.2 kJ/mol,
(a) C
H
(g) + 3O
(g) + 2H
O(l )
2
3
2
4
2
2
2
(g) Æ 2H
respectively, at 25°C. Which is the more stable
(b) 2H
S(g) + 3O
O(l ) + 2SO
(g)
2
2
2
2
form at this temperature?
5.27 Methanol, ethanol, and n-propanol are three
5.19 Which is the more negative quantity at
common alcohols. When 1.00 g of each of these
25°C: ∆Hº
for H
O(l ) or for ∆Hº
H
O(g)?
alcohols is burned in air, heat is liberated as
f
2
f
2
follows: (a) methanol (CH
OH), -22.6 kJ; (b)
3
5.20 Predict the value of ∆Hº
(greater than, less
ethanol (C
H
OH), -29.7 kJ; (c) n-propanol
f
2
5
than, or equal to zero) for these elements at
(C
H
OH), -33.4 kJ. Calculate the heats of
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25°C: (a) Br
(g); Br
(l ), (b) I
(g); I
(s).
combustion of these alcohols in kJ/mol.
2
2
2
2
5.21 In general, compounds with negative ∆Hº
5.28 The standard enthalpy change for the
f
values are more stable than those with positive
following reactionis 436.4 kJ:
∆Hº
values. H
O
(l ) has a negative ∆Hº
(see
f
2
2
f
(g) Æ H(g) + H(g)
Table 6.3). Why, then, does H
O
(l) have a
H
2
2
2
tendency to decompose to H
O(l) and O
(g)?
2
2
Calculate the standard enthalpy of formation of
5.22 Suggest ways (with appropriate equations)
atomic hydrogen (H).
that would allow you to measure the ∆Hº
f
values of Ag
O(s) and CaCl
(s) from their
5.29 From the standard enthalpies of formation,
2
2
elements. No calculations are necessary.
calculate ∆Hº
for the reaction
rxn
(g) Æ 6CO
5.23 Calculate the heat of decomposition for this
C
H
(l) + 9O
(g) + 6H
O(l)
6
12
2
2
2
process at constant pressure and 25°C:
For C
H
(l), ∆Hº
=-151.9 kJ/mol.
6
12
f
(s) Æ CaO(s) + CO
CaCO
(g)
3
2
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