Energetics and Bonding page 1
Problem Set
1.
a. State Hess's law and explain what is meant by the enthalpy change of a reaction.
b. Calculate the enthalpy change of the reaction
CO(g) + 2H2(g)  CH3OH(l)
at 298 K, from the following data at 298 K:
kJ mol-1
H = -283.0
H = -285.8
H = -715.0
CO(g) + 1/2 O2(g)  CO2(g)
H2(g) + 1/2 O2(g)  H2O(l)
CH3OH(l)+ 1 1/2 O2(g)  CO2(g)+ 2H2O(l)
[Ans. b. -136.9 kJ mol-1]
2.
a. Describe, with the aid of a diagram, the lattice structure of crystalline sodium chloride and
show how it accounts for the characteristic physical properties of the substance.
b. Define
(i) enthalpy change of formation,
(ii) lattice energy
What factors determine the magnitude of lattice energy?
c. Draw a Born-Haber cycle for the formation of caesium chloride and use it to calculate the
value for the lattice energy of this compound.
Data:
Enthalpy change of atomization of caesium:
Cs(s)  Cs(g)
H = +79 kJ mol-1
Enthalpy change of atomization of chlorine:
1/2 Cl2(g)  Cl(g)
H = +121 kJ mol-1
First ionization energy of caesium:
Cs(g)  Cs+(g) + eH = +376 kJ mol-1
Electron affinity of chlorine:
Cl(g) + e-  Cl-(g)
H = -364 kJ mol-1
Enthalpy change of formation of caesium chloride:
Cs(s) + 1/2 Cl2(g)  Cs+Cl-(s)
H = -433 kJ mol-1
[Ans. b. -645 kJ mol-1]
3. Use the following data to determine the enthalpy change for the reaction
1/2 I2(s)  I-(aq)
All steps in your calculation must be clearly shown.
H /kJ mol-1
I2(s)  I2(g)
30
1/2 I2(g)  I(g)
76
I(g)  I-(g)
-297
I-(g)  I-(aq)
-305
[Ans. -511 kJ mol-1]
Energetics and Bonding page 2
4. Some bond energy terms are listed below:
Bond
H-H
C-H
C-Br
C-C
C=C
Br-Br
Bond energy term (kJ mol-1)
435
415
284
356
598
193
a. What do you understand by the bond energy term?
b. Use the data given, calculate the enthalpy change of formation, from gaseous atoms, of
(i) gaseous propene,
(ii) gaseous 1,2-dibromopropane.
c. Calculate the enthalpy change, H, for the following reaction:
CH2=CH-CH3(g) + Br2  CH2BrCHBrCH3(g)
[Ans. b. (i) -3444 kJ mol-1 (ii) -3770 kJ mol-1 (c) -133 kJ mol-1]
5.
a. Define standard enthalpy change of formation, illustrating your answer by reference to
MgCO3(s).
b. When 0.20 g of magnesium ribbon was dissolved in an excess of hydrochloric acid in a
vacuum flask, the temperature rose by 8.6oC. Separate experiments showed that the vacuum
flask and its contents required 500 J to raise the temperature by 1oC.
(i) Write an equation for this reaction.
(ii) Calculate the heat released in this experiment.
(iii) Hence calculate the enthalpy change per mole of magnesium.
c. In a similar experiment, solid magnesium carbonate reacted with an excess of hydrochloric
acid and the enthalpy change was found to be -90 kJ mol-1 of magnesium carbonate. The
enthalpy change of formation of H2O(l) is -285 kJ mol-1 and of CO2(g) is -393 kJ mol-1. Use
these data and your result from b. (iii) to calculate the enthalpy change of formation of
MgCO3(s).
d. Would you expect the enthalpy change of formation of calcium carbonate, CaCO3(s), to be
numerically greater or less than that of MgCO3(s)?
Explain your answer.
[Ans. b. (ii) -4.3 kJ (iii) -516 kJ mol-1 c. -1104 kJ mol-1
d. Hoionization (Ca) < Hoionization (Mg) as Ca > Mg in size.
Ho lattice (CaCO3) < Holattice (MgCO3)
Hoformation (CaCO3) and Hoformation (MgCO3) are therefore similar in value.]
Energetics and Bonding page 3
6.
a. State Hess's law of constant heat summation (law of conservation of energy).
b. Define precisely the terms:
(i) enthalpy of formation;
(ii) enthalpy of neutralization
c. Using the following data, collected at 25oC and standard atmospheric pressure, in which the
negative sign indicates heat evolved, calculate the enthalpy of formation of potassium
chloride, KCl(s):
H(298 K)/kJ mol-1
(i) KOH (aq) + HCl(aq)  KCl(aq) + H2O(l)
-57.3
(ii) H2(g) + 1/2 O2(g)  H2O(l)
-285.9
(iii) 1/2 H2(g) + 1/2 Cl2(g) + aq  HCl(aq)
-164.2
(iv) K(s) +1/2 O2(g)+1/2 H2(g)+aqKOH(aq)
-487.0
(v) KCl(s) + aq  KCl(aq)
+18.4
d. When solid potassium chloride is dissolved in water, heat is absorbed.
KCl(s) + aq  KCl(aq)
H = +18.4 kJ mol-1
Use the ideas formulated in the kinetic-molecular theory of matter to discuss what happens
during the dissolution of a crystalline salt in water, and explain why heat is absorbed in the
above reaction.
[Ans. c. -441.0 kJ mol-1]