ANSWERS TO END-OF-CHAPTER QUESTIONS

. Average bond energies are based on bonds within molecules in the gaseous state. In the
given chemical equation, the H2O formed is present as a liquid rather than as a gas.
Additional energy is released when gaseous water condenses to the liquid state, so the stated
value of 286 kJ is greater than the 249 kJ calculated in part a.
25. Every year, 5.6 × 10 21 kJ of energy comes to Earth from the Sun. Why can’t this energy be
used to meet all of our energy needs?
Answer:
Solar energy is distributed over Earth’s surface. Therefore, it is not very easy to capture,
store, and transform solar energy to meet our energy needs.
26. This unbalanced equation represents the last step in the production of pure silicon for use in
solar cells.
__Mg(s) + __SiCl4(l) ⎯→ __MgCl2(l) + __Si(s)
a. How many electrons are transferred per atom of pure silicon formed?
b. Is the production of pure silicon an oxidation or a reduction reaction? Why do you think
so?
Answer:
a. This is the balanced equation: 2 Mg(s) + SiCl4(l) ⎯→ 2 MgCl2(l) + Si(s)
Each magnesium atom loses 2 electrons, and there are 2 magnesium atoms providing
electrons for one silicon atom. Silicon in SiCl4 picks up the four electrons.
b. The silicon atoms in SiCl4 gain four electrons to form elemental silicon, a reduction
process. The two magnesium atoms each lose two electrons, an oxidation process. Overall,
the production of pure silicon is an oxidation-reduction reaction.
27. The symbol represents an electron and the symbol represents a silicon atom. Does this
diagram represent a gallium-doped p-type silicon semiconductor, or does it represent an
arsenic-doped n-type silicon semiconductor? Explain your answer.
Answer:
Each Si atom is surrounded by 8 electrons, but the atom in the center (the one that is doping
the semiconductor) is surrounded by 9 electrons. Silicon is in Group 4A and has 4 outer
electrons. Thus central atom in the figure must have 5 outer electrons. This is consistent with
an element in Group 5A such as arsenic, so this is an n-type silicon semiconductor.
28. Describe the main reasons why solar cells have solar energy conversion efficiencies
significantly less than the theoretical value of 31%.
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