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Key Concepts in Thermochemistry: Chapter 5 - Prof. Zelda Ziegler, Assignments of Chemistry

The key concepts of chapter 5 in a college-level thermochemistry course. Class period assignments, problem sets, and answers. Topics covered include relative enthalpies, thermochemical equations, heat flow, specific heat, and calculating heat for a reaction. Students are expected to skim certain sections for background knowledge but will not be tested on them.

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Pre 2010

Uploaded on 08/18/2009

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CH 221--Ziegler
Chapter 5
KEY CONCEPTS
Class period 1 -- Reading pp. 153-155(b); 155b-156m; 156m-158 (m); 158m- 164.
1. Relate the following: relative enthalpies of reactants and products, sign of H, heat flow
between system and surroundings. Problem 5.30
Note: There is much in the chapter which we not emphasize, particularly P-V work.
This is why I want you to skim the parts indicated above. They give useful
background material, but you will not be examined on them.
Class period 2 -- Reading pp. 166 – 171.
2. Use a thermochemical equation to relate H to mass of reactant or product and/or to
H of the reverse reaction. Problems 5.33, 5.34
3. Relate heat flow to specific heat, mass, and temperature change. Problems 5.44, 5.46,
5.47
4. Calculate q for a reaction from calorimeter data. Problems 5.49, 5.50
Class period 3 -- Reading pp. 174-180; 300(m) to 303(b).
5. Use Hess’ law to calculate H. Problems 5.58, 5.60
6. Relate H to enthalpies of formation. Problems 5.63, 5.68, 5.72 a)
7. Define the term “bond energy,” and relate bond energy to bond strength. Problems 8.61
a) and b)
Skim the pages marked with highlighter, for meaning, but don’t worry about being tested on these
topics.
pf2

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CH 221--Ziegler

Chapter 5

KEY CONCEPTS

Class period 1 -- Reading pp. 153-155(b); 155b-156m; 156m-158 (m); 158m- 164.

  1. Relate the following: relative enthalpies of reactants and products, sign of ∆H, heat flow between system and surroundings. Problem 5. Note: There is much in the chapter which we not emphasize, particularly P-V work. This is why I want you to skim the parts indicated above. They give useful background material, but you will not be examined on them.

Class period 2 -- Reading pp. 166 – 171.

  1. Use a thermochemical equation to relate ∆H to mass of reactant or product and/or to ∆H of the reverse reaction. Problems 5.33, 5.
  2. Relate heat flow to specific heat, mass, and temperature change. Problems 5.44, 5.46,
  3. Calculate q for a reaction from calorimeter data. Problems 5.49, 5.

Class period 3 -- Reading pp. 174-180; 300(m) to 303(b).

  1. Use Hess’ law to calculate ∆H. Problems 5.58, 5.
  2. Relate ∆H to enthalpies of formation. Problems 5.63, 5.68, 5.72 a)
  3. Define the term “bond energy,” and relate bond energy to bond strength. Problems 8. a) and b)

Skim the pages marked with highlighter, for meaning, but don’t worry about being tested on these topics.

Answers to problems:

E (^) ZnCO 3 (s)

ZnO (s) + CO 2 (g)

5.30 ZnCO 3 (s) Æ ZnO(s) + CO 2 (g) ∆H = 71.5 kJ 71.5 kJ 5.33 See Text A- 5.34 a) heat is absorbed. b) 4.54 x 10^3 kJ c) 2.82 g H 2 d) ∆H = -90.7 kJ for the reverse reaction; and when 32.0 g CO reacts by this reverse reaction, the change in heat is -104 kJ (heat is released).

5.44 a) A transferred more heat to the water than B did, so its capacity to hold heat is greater, and its heat capacity is greater. b) Unless you know how much A and B weighed, you can’t express its specific heat capacity, which is usually given in units of J/g°C or J/gK.

5.46 a) 75.4 J/mol°C b) 634.9 J/°C, 506 kJ

5.47 3.47 x 10^4 Joules

5.49 -45.7 kJ/ mol NaOH

5.50 25 kJ/mol NH 4 NO 3

5.58 a) 125 kJ b) See energy diagram.

c) No, we are tracking heat and if the products are at an elevated temperature, then the amount of heat required to get them to that temperature must be accounted for as well. (Do this by taking into consideration the specific heat of the material and the temperature difference. Specific heat is a characteristic of a material. It is defined as the amount of heat required to raise the temperature of 1 gram of material by 1 °C.)

E

Y

X

Z

90 kJ

35 kJ

5.60 -867.7 kJ

5.63 See text A-

5.68 a) ½ H 2 (g)+ ½ Br 2 (g) Æ HBr (g) ∆H (^) f°(HBr(g)) = -36.23 kJ/mol ; ∆Hrxn=-36.23 kJ

b) Ag (s) + ½ N 2 (g)+ 3/2 O 2 (g) Æ AgNO 3 (s) ∆H (^) f°( AgNO 3 (s)) = -124.4 kJ/mol ∆Hrxn = -124.4 kJ c) 2Hg (l) + Cl 2 (g) Æ Hg 2 Cl 2 (s) ∆H (^) f°(Hg 2 Cl 2 (s)) = -264.9 kJ/mol ∆Hrxn = -264.9 kJ

d) 2 C (s, graph)+ 3H 2 (g) + ½ O 2 (g)Æ C 2 H 5 OH (l) ∆Hf° (C 2 H 5 OH (l)) = -277.7 kJ/mol ∆Hrxn = -277.7 kJ 5.72 a) -976.94 kJ/mol (yes, the answer book made a mistake on this one)

8.61 a) 304 kJ/mol b) 82 kJ/mol