Page 189 - Physical Chemistry

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PROBLEMS

Section 5.1 are 4.180 and 0.450 J/(g °C), respectively. The density of water
3
5.1 True or false? (a) The term standard state implies that at 24°C is 0.9973 g/cm . Assuming the heat capacity of the
the temperature is 0°C. (b) The term standard state implies chemicals in the bomb is negligible compared with the heat ca-
that the temperature is 25°C. (c) The standard state of a pure pacity of the bomb and surrounding water, find the final tem-
gas is the pure gas at a pressure of 1 bar and temperature T. perature of the system. Neglect the temperature dependence of
c . Neglect the changes in thermodynamic functions that occur
P
Section 5.2 when the reactants and products are brought from their standard
5.2 True or false? (a) The SI units of H° for a reaction are J. states to those that occur in the calorimeter.
(b) Doubling the coefficients of a reaction doubles its H°. 5.12 Repeat Prob. 5.11b, taking account of the heat capacity
(c) H° depends on temperature. (d) The reaction N 3H →
2 2 of the bomb contents. The bomb has an interior volume of 380
2NH has n 2. 3
3 i i cm and is initially filled with O (g) at 30 atm pressure.
2
5.3 For 2H S(g) 3O (g) → 2H O(l) 2SO (g), express
2 2 2 2 5.13 When 0.6018 g of naphthalene, C H (s), was burned in
H° in terms of standard-state molar enthalpies H° of the 10 8
T m,i an adiabatic bomb calorimeter, a temperature rise of 2.035 K
species involved.
was observed and 0.0142 g of fuse wire (used to ignite the
1
5.4 For Na(s) HCl(g) → NaCl(s) H (g), H° 298 is sample) was burned. In the same calorimeter, combustion of
2
2
1
319 kJ mol . Find H° for: 0.5742 g of benzoic acid produced a temperature rise of 1.270 K,
298
(a) 2Na(s) 2HCl(g) → 2NaCl(s) H (g) and 0.0121 g of fuse wire was burned. The U for combustion
2
(b) 4Na(s) 4HCl(g) → 4NaCl(s) 2H (g) of benzoic acid under typical bomb conditions is known to
2
1
(c) NaCl(s) H (g) → Na(s) HCl(g) be 26.434 kJ/g, and the U for combustion of the wire
2
2
is 6.28 kJ/g. (a) Find the average heat capacity of the calorime-
Section 5.3 ter and its contents. Neglect the difference in heat capacity be-
5.5 True or false? (a) H° is zero for O(g). (b) H° 298 is tween the chemicals in the two experiments. (b)Neglecting the
298
f
f
zero for O (g). (c) H° is zero for O (g). changes in thermodynamic functions that occur when species
2
400
f
2
are brought from their standard states to those that occur in the
5.6 For each of the following, write the reaction of formation
calorimeter, find U° and H°of naphthalene.
from reference-form elements at room temperature: (a) CCl (l); c c
4
(b) NH CH COOH(s); (c) H(g); (d) N (g). 5.14 The reaction 2A(g) 3B(l) → 5C(g) D(g) is carried
2 2 2
out in an adiabatic bomb calorimeter. An excess of A is added
5.7 For which elements is the reference form at 25°C (a) a
liquid; (b) a gas? to 1.450 g of B. The molecular weight of B is 168.1. The reac-
tion goes essentially to completion. The initial temperature is
25.000°C. After the reaction, the temperature is 27.913°C. A
Section 5.4
direct current of 12.62 mA (milliamperes) flowing through the
5.8 Write balanced reactions for the combustion of one mole
calorimeter heater for 812 s is needed to bring the product mix-
of each of the following to CO (g) and H O(l). (a) C H (g);
2 2 4 10 ture from 25.000°C to 27.913°C, the potential drop across the
(b) C H OH(l).
2 5 heater being 8.412 V. Neglecting the changes in thermody-
5.9 True or false? (a) When sucrose is burned in an adiabatic namic functions that occur when the reactants and products
constant-volume calorimeter, U 0 for the combustion are brought from their standard states to the states that occur in
process, where the system is the calorimeter contents. (b) The the calorimeter, estimate U° 298 and H° 298 for this reaction.
reaction N (g) 3H (g) → 2NH (g) has H° U°. (c) The (One watt one volt one ampere one joule per second.)
3
2
T
T
2
reaction N (g) → 2N(g) is endothermic. (d) When an exother- 5.15 For H (g) O (g) → H O(l), find H° U°
1
2
2
2
2
mic reaction is carried out in an adiabatic container, the prod- (a) neglecting V° 2 ; (b) not neglecting V° 298 298
.
ucts are at a higher temperature than the reactants. (e) For m,H 2 O(l) m,H 2 O(l)
CH OH(l), H° 298 U° 298 equals H° 298 U° 298 [where 5.16 The standard enthalpy of combustion at 25°C of liquid
c
f
f
3
c
H O(l) is formed in the combustion reaction]. acetone (CH ) CO to CO (g) and H O(l) is 1790 kJ/mol. Find
3 2
2
2
2
H° and U° of (CH ) CO(l).
5.10 Use data in the Appendix to find H° for: f 298 f 298 3 2
298
(a) 2H S(g) 3O (g) → 2H O(l) 2SO (g) 5.17 The standard enthalpy of combustion of the solid amino
2 2 2 2
(b) 2H S(g) 3O (g) → 2H O(g) 2SO (g) acid alanine, NH CH(CH )COOH, to CO (g), H O(l), and
2 2 2 2 2 3 2 2
(c) 2HN (g) 2NO(g) → H O (l) 4N (g) N (g) at 25°C is 1623 kJ/mol. Find H° and U° of
3 2 2 2 2 f 298 f 298
solid alanine. Use data in the Appendix.
5.11 (a) Use Appendix data to find H° 298 and U° 298 of
c
c
a-D-glucose(c), C H O , to CO (g) and H O(l). (b) 0.7805 g of 5.18 Given the following H° values in kcal/mol, where gr
2
298
2
12
6
6
a-D-glucose is burned in the adiabatic bomb calorimeter of stands for graphite,
Fig. 5.4. The bomb is surrounded by 2.500 L of H O at
2
24.030°C. The bomb is made of steel and weighs 14.05 kg. Fe 2 O 3 1s2 3C1gr2 S 2Fe1s2 3CO1g2 117
Specific heats at constant pressure of water and steel at 24°C FeO1s2 C1gr2 S Fe1s2 CO1g2 37