chapter9

9-14
When considering the advantages of using aluminum to replace steel, one should not refer to aluminum as being "lighter" than steel. One should instead indicate that aluminum is less dense than steel. "Lightness" refers to the amount of matter while density is an intensive property of matter.

9-15
The observed rate at which iron containing components "rust" would seem to indicate that iron is more reactive than aluminum. Just the opposite is the true condition. Aluminum rapidly becomes coated with an oxide layer which forms a barrier to further reaction. The very thin layer of AI₂0₃ stays intact on the surface of the metal. The Fe₂0₃ which forms on the iron surface rapidly falls away leaving the underlying metal surface accessible to future reaction.

9-16
The very thin coating of AI₂0₃ adheres to the metal surface providing a protective barrier though which no reactants can pass. As long as the coating is not disturbed by mechanical or chemical means, the active metal below is protected.

9-17
The existence of the lead-acid storage battery illustrates the property of lead which it shares with only a limited number of other main group metals; unlike the main groups 1, 2, and I₃, lead can exhibit more than one oxidation state in forming compounds with non-metals. The two different oxidation states of lead are readily accessible as a result of the energy difference between them.

9-18
The tin coating of a steel can is designed to protect the iron surface from access by active substances which would corrode the iron. As long as the surface is unbroken, the iron does not react, but if the tin barrier is broken the iron corrodes. If fruit cocktail were packaged in a steel can, the fruit would react with the iron causing rust and spoiling the food. Tin is less reactive than AI and does not have to be coated with plastic.

9-19
Aluminum cans are coated with a plastic liner to keep the acids/bases from having access to the protective AI₂0₃ coating. If this were not done, the acidic components of the soft drinks would tend to dissolve the can.

9-20
Children might believe that metals such as iron loose weight when they rust because the iron oxide coating tends to fall away from the base metal leaving fresh surfaces open to reaction. However, if one were to weigh both rust and iron metal, one would find that rust weighs more than the metal alone. The additional weight represents the added oxygen in the rust (Fe₂0₃).

9-21
(a) Na₂O, NaH
(b) MgO, MgH₂
(c) Al₂O₃, AlH₃
(d)SiO₂, SiH₄
(e) P₂O₅, PH₃
The element best matching the suggested compositions is magnesium.

9-22
With XH₄ and XO2, the main-group metal must belong to group 14 (the carbon group).

9-23
Main group elements matching the reactivities described are to be found in group 13.

9-24
The product of the reaction of aluminum (lose 3 valence electrons) with nitrogen (gain 3 valence electrons) should have the formula AlN.

9-25
The product of the reaction of strontium metal with phosphorus should have the formula Sr₃P₂ .

9-26
Gallium of Group 13 combining with arsenic of Group 15 should give a product with the formula GaAs.

9-27
(a) 2Na(s) + F₂(g)2 NaF(s)
(b) 4Na(s) +0₂(g)2Na₂0(s)
(c) 2Na(s) +H₂(g)2 Nail(s)
(d) 16 Na(s) + S₈(s) 8 Na₂S(s)
(e) 12 Na(s) + P₄(s) -- 4 Na₃P(s)

9-28
(a) Ca(s) + H₂(g) CaH₂(s)
(b) 2 Ca(s) + 0₂(g₎ 2 CaO(s)
(c) 8 Ca(s) + S₈(s₎ 8 CaS(s)
(d) Ca(s) + F₂(g₎-- CaF₂(s)
(e) 3 Ca(s) + N₂(g₎ Ca₃N₂(s)
(f) 6 Ca(s) + P₄(s) 2 Ca₃P₂(s)

9-29
(a) ZnF₂
(b) AIF₃
(c) SnF_2orSnF4
(d) MgF₂
(e) BiF_3orBiF5

9-30
The answer is (a): Ca(s) + 2 H₂0(I)-- Ca²⁺ (aq) + 2 OH- (aq) + H₂(g)

9-31
Magnesium is not reactive enough to react with H₂O(l)at room temperature. The products of the reaction of Mg with steam are MgO(s) and H₂(g). Mg(OH)₂ cannot be a product since there are no liquid water molecules to solvate the Mg⁺² and OH^- ions.

9-32
Unless we allow atmospheric oxygen to gain access to the reaction under consideration, no OH^- can be formed. Because potassium metal acts as a reducing agent (electron source), it seems unlikely that H⁺ will form. H₂ is an expected product as is K^+. At an intermediate stage, PH^2- is also expected. Of the choices given, the following are not formed: H⁺ ions and OH^- ions.

9-33
a) Mg(s) + 2 HCl(aq) MgCl₂(aq) + H₂(g)
b) Cu(s) + HCl(aq) no reaction
c) 2 Cs(s) + 2 H₂0(I) 2 CsOH(aq) + H₂(g)

9-34
Reaction (d) lacks a charge balance, negative on one side and positive on the other side of the equation. The chemistry suggests that magnesium (a reducing agent) oxidizes nitrogen to form nitrate.

9-35
a) Mg(s) + I₂(s) MgI₂(s)
b) 2 AI(s) + 3 H₂SO₄(aq) AI₂(SO₄)3(aq) + 3 H₂(g)
c) Fe₂0₃(s)+ 3 H₂(g) 3 H₂0(I)+ 2 Fe(s)
d) 2 Mg(I) + TiCI₄(g) 2 MgCI₂(s)+ Ti(s)