Hess's Law
85. Calculate the standard enthalpy change, Ho, for the
formation of 1 mol of strontium carbonate (the material that
gives the red color in fireworks) from its elements.
Sr(s) + C(graphite) + 3/2 O2(g)  SrCO3(s)
The information available is
Sr(s) + ½ O2(g)  SrO(s)
Ho = -592 kJ
SrO(s) + CO2(g)  SrCO3(s)
Ho = -234 kJ
C(graphite) + O2(g)  CO2(g)
Ho = -394 kJ
Sr(s) +
1
O2(g)
2
SrO(s) + CO2(g)
+
C(graphite) + O2(g)
SrO(s)
H° = –592 kJ
SrCO3(s)
H° = –234 kJ
CO2(g)
+
H° = –394 kJ
88. Three reactions very important to the semiconductor
industry are
(a) the reduction of silicon dioxide to crude silicon,
SiO2 (s) + 2 C(s)  Si(s) + 2 CO(g)
Ho = 689.9 kJ
(b) the formation of silicon tetrachloride from crude silicon,
Si(s) + 2 Cl2(g)  SiCl4(g) 


Ho = -657.0 kJ
(c) and the reduction of silicon tetrachloride to pure silicon
with magnesium,
SiCl4 (g) + 2 Mg(s)  2 MgCl2(s) + Si(s)
Ho = -625.6 kJ
What is the overall enthalpy change for changing 1.00 mol of
sand (SiO2) into very pure silicon?
H o 
 moles of product  H
o
f
products    moles of reactants H fo reac tan ts 
91. For each compound below, write a balanced thermochemical equation depicting the formation of 1 mol of the
compound. Standard molar enthalpies of formation are found
in Appendix J.
(a) Al2O3(s)
(b) TiCl4( g)
(c) NH4NO3(s)
92. The standard molar enthalpy of formation of glucose,
C6H12O6(S), is -1274.4 kJ/mol.
(a) Is the formation of glucose from its elements exothermic
or endothermic?
(b) Write a balanced equation depicting the formation of
glucose from its elements and for which the enthalpy of
reaction is -1274.4 kJ.
94. In photosynthesis, the sun's energy brings about the combination of CO2 and H2O to form O2 and a carbon-containing
compound such as a sugar. In its simplest form, the reaction
could be written
6 CO2(g) + 6 H2O(l)  6O2(g) + C6H12O6(s)
Using the enthalpies of formation in Table 6.2,
(a) calculate the enthalpy of reaction and
(b) decide whether the reaction is exothermic or
endothermic.
99. Iron can react with oxygen to give iron(III) oxide. If 5.58
g of Fe is heated in pure O2 to give Fe2O3(s), how much
thermal energy is transferred out of this system (at constant
pressure)?
100. The formation of aluminum oxide from its elements is
highly exothermic. If 2.70 g of AI metal is burned in pure O2
to give Al2O3 how much thermal energy is evolved in the
process (at constant pressure)?
105. M&M candies consist of 70% carbohydrate, 21% fat, and
4.6% protein as well as other ingredients that do not have
caloric value. What quantity of energy transfer would occur if
34.5 g of M&Ms were burned in a bomb calorimeter?
Chapter 7
5. Tell what happens to atomic size and ionization energy
across a period and down a group.
6. Why is the radius of Na+ so much smaller than the radius of
Na? Why is the radius of Cl- so much larger than the radius of
Cl?
14. Assume that a microwave oven operates at a frequency of
1.00 X 1011 S-1.
(a) What is the wavelength of this radiation in meters?
(b) What is the energy in joules per photon?
(c) What is the energy per mole of photons?
15. IBM scientists have developed a prototype computer chip
that operates at 350 GHz (1 GHz = 109 Hz).
(a) What is its wavelength in meters? in nanometers (nm)?
(b) Calculate its energy in joules.
26. Calculate the energy of one photon of x-radiation having a
wavelength of 2.36 nm, and compare it with the energy of one
photon of orange light (3.18 X 10-19 J).
32. To eject electrons from the surface of potassium metal
requires a minimum energy of 3.69 X 10-19 J. When 600 nm
photons shine on a potassium surface, will they cause the
photoelectric effect?
37. Which transition involves the emission of less energy in
the H atom, an electron moving from n = 4 to n = 3 or an
electron moving from n = 3 to n = 1? (See Figure 7.8.)
38. For which of these transitions in a hydrogen atom is energy absorbed? emitted?
(a) n = 1 to n = 3
(b) n = 5 to n = 1
(c) n = 2 to n = 4
(d) n = 5 to n = 4
39. For the transitions in Question 38:
(a) Which ones involve the ground state?
(b) Which one involves the greatest energy change?
(c) Which one absorbs the most energy?