CHAPTER 5 Problems:1, 12, 16, 18, 20, 28, 30, 36, 38, 40, 48, 50, 60, 62, 66, 87, 96,
108, 122
1) Define the following terms: system, surroundings, thermal energy, chemical energy,
potential energy, kinetic energy, law of conservation of energy.
12) Explain what is meant by a state function. Give two examples of quantities that are
state functions and two that are not state functions.
16) Calculate q, and determine whether heat is absorbed or released when a system does
work on the surroundings equal to 64. J and U = 213. J.
18) The diagram below (left) shows a system before a process. Determine which of the
diagrams ( i, ii, or iii ) could represent the system after it undergoes a process where
a) work is done on the system and U is negative
b) the system releases heat and U is positive
c) the system absorbs heat and U is positive
20) Define these terms: enthalpy and enthalpy of reaction. Under what conditions is the
heat of a reaction equal to the enthalpy change for the same reaction?
28) Determine the amount of heat (in kJ) given off when 1.26 x 104 g of NO2 are
produced according to the equation
2 NO(g) + O2(g)  2 NO2(g)
H = - 114.6 kJ/mol
29) Consider the reaction
2 H2(g) + 2 Cl2(g)  2 HCl(g)
H = - 184.6 kJ/mol
If 3.00 moles of H2 reacts with 3.00 moles of Cl2 to form HCl, calculate the work done
(in joules) against a pressure of 1.00 atm. What is U for the reaction? Assume the
reaction goes to completion and that V = 0. Note 1 Latm = 101.3 J.
1
36) Calculate the amount of heat liberated (in kJ) from 366. g of mercury when it cools
from 77.0 C to 12.0 C.
38) A 0.1375 g sample of solid magnesium is burned in a constant volume bomb
calorimeter that has a heat capacity of 3024. J/C. The temperature increases by 1.126
C. Calculate the heat given off by the burning Mg in kJ/g and kJ/mol.
40) A 50.75 g sample of water at 75.6 C is added to a sample of water at 24.1 C in a
constant pressure calorimeter. If the final temperature of the combined water is 39.4 C
and the heat capacity of the calorimeter is 26.3 J/C, calculate the mass of water
originally in the calorimeter.
48) From the following data
C(graphite) + O2(g)  CO2(g)
Hrxn = - 393.5 kJ/mol
H2(g) + 1/2 O2(g)  H2O(l)
Hrxn = - 285.8 kJ/mol
2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(l)
Hrxn = - 3119.6 kJ/mol
calculate the enthalpy change for the reaction
2 C(graphite) + 3 H2(g)  C2H6(g)
50) Calculate the standard enthalpy change for the reaction
2 Al(s) + Fe2O3(s)  2 Fe(s) + Al2O3(s)
given that
2 Al(s) + 3/2 O2(g)  Al2O3(s)
Hrxn = - 1669.8 kJ/mol
2 Fe(s) + 3/2 O2(g)  Fe2O3(s)
Hrxn = - 822.2 kJ/mol
2
60) The standard enthalpies of formation of ions in aqueous solutions are obtained by
arbitrarily assigning a value of zero to the enthalpy of formation of H+ ion; that is, by
saying Hf(H+(aq)) = 0.
a) For the reaction
HCl(g)  H+(aq) + Cl-(aq)
Hrxn = - 74.9 kJ/mol
find Hf for the Cl-(aq) ion. Note Hf(HCl(g)) = - 92.3 kJ/mol
b) Given that Hf(OH-(aq)) = -229.6 kJ/mol, find the enthalpy change for the
neutralization reaction of a strong acid and a strong base, such as
HCl(aq) + KOH(aq)  KCl(aq) + H2O(l)
Note Hf(H2O(l)) = - 285.8 kJ/mol. HINT: Start by writing the net ionic equation
corresponding to the above neutralization reaction.
62) Calculate the heat of combustion for the following reactions from the standard
enthalpies of formation given in Appendix 2 of Burdge. Note that reaction b assumes the
combustion product for sulfur is SO2(g).
a) C2H4(g) + 3 O2(g)  2 CO2(g) + 2 H2O(l)
b) 2 H2S(g) + 3 O2(g)  2 H2O(l) +2 SO2(g)
66) Calculate the heat of decomposition for the process below at constant pressure and T
= 25.0 C.
CaCO3(s)  CaO(s) + CO2(g)
(Data are given in Appendix 2 of Burdge.)
87) For which of the following reactions does Hrxn =Hf ?
a) H2(g) + S(rhombic)  H2S(g)
b) C(diamond) + O2(g)  CO2(g)
c) H2(g) + CuO(s)  H2O(l) + Cu(s)
d) O(g) + O2(g)  O3(g)
3
96) Acetylene (C2H2) and benzene (C6H6) have the same empirical formula. In fact,
benzene can be made from acetylene by the process
3 C2H2(g)  C6H6(l)
The enthalpies of combustion for acetylene and benzene are
Hcom(C2H2(g)) = - 1299.4 kJ/mol Hcom(C6H6(l)) = - 3267.4 kJ/mol
Using this information and the enthalpies of formation for carbon dioxide and water
Hf(CO2(g)) = - 393.5 kJ/mol
Hf(H2O(l)) = - 285.8 kJ/mol
find the enthalpies of formation for C2H2(g) and C6H6(l). From those values find the
enthalpy change for the reaction that converts acetylene into benzene.
108) A 3.52 g sample of ammonium nitrate (NH4NO3) was added to 80.0 mL of water in
a constant pressure calorimeter of negligible heat capacity. As a result the temperature of
the solution decreased from 21.6 C to 18.1 C. Calculate the heat of solution for
ammonium nitrate, the enthalpy change corresponding to the process
NH4NO3(s)  NH4+(aq) + NO3-(aq)
Assume that the specific heat of the solution is the same as the specific heat of water.
122) Calculate the standard enthalpy change for the fermentation process, in which
glucose (C6H12O6(s)) is converted to ethanol (C2H5OH(l)) and carbon dioxide. Data are
given in Appendix 2 of Burdge.
4