rest of chapter 9

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)

9-36
a) 2 K(s) + 2 H₂0(I) H₂(g)+ 2 KOH(aq)
b) NaH(s) + H₂0(I) NaOH(aq) + H₂(g)
c) 2Ag+(aq) + Cu(s) --> Cu²⁺ (aq) + 2Ag(s)
d) Zn(s) + CuSO₄(aq) ZnSO₄(aq) + Cu(s)

9-37
In the reaction 4 C(s) + Se(I)--> 4 CS₂(I)The oxidation state of carbon increases from 0 in C(s) to +4 in CS₂(I_). Carbon is therefore oxidized. The sulfur changes from an oxidation state of 0 in S₈ to -2 in CS_2. Sulfur is therefore reduced.

9-38
The reaction with cyanide involves an exchange of next nearest neighbors (coordinated ligands) about the silver ion. The second reaction involves the exchange of electrons between the silver ions in the "tarnish" and the aluminum metal. Therefore the second reaction with aluminum involves oxidation.

9-39
a) This is a combustion reaction.
b) This is an oxidation-reduction reaction. Iron is reduced from +3 in Fe₂0₃ to 0 in Fe(s). Carbon is oxidized from +2 in CO to +4 in CO_2.
c) This is an oxidation-reduction reaction. Silicon is reduced from +4 in SiO₂ to 0 in SiC. Carbon is oxidized from 0 in C(s) to +2 in CO.
d) This is an oxidation-reduction reaction. Carbon is reduced from +4 in CO₂ to +2 in CO. Hydrogen is oxidized from 0 in H₂ to +1 in H₂0.
e) This is an oxidation-reduction reaction. Carbon is reduced from +2 in CO to -2 in CH₃OH. Hydrogen is oxidized from 0 in H₂ to +1 in CH₃OH.

9-40
a) This is an oxidation-reduction reaction. Magnesium is oxidized from 0 in Mg(s) to +2 in MgCI_2. Hydrogen is reduced from +1 in HCI to 0 in H_2.
b) This is an oxidation-reduction reaction. Iodine is oxidized from 0 in I₂ to +3 in ICI_3. Chlorine is reduced from 0 in CI₂ to -1 in ICI_3.
c) This is an acid-base reaction.
d) This is an oxidation-reduction reaction. Sodium is oxidized from 0 in Na(s) to +1 in NaOH. Hydrogen is reduced from +1 in H₂0 to 0 in H_2.

9-41
a) This is a double displacement reaction not involving electron transfer.
b) This is an oxidation-reduction reaction. The phosphorus is oxidized from a -3 in PH₃ to a +5 in H₃PO_4. The oxygen is reduced from 0 in 0₂ to -2 in H₃PO_4.
c) This is an adduct-forming reaction.
d) This is an oxidation-reduction reaction. The phosphorus is oxidized from 0 in P₄ to +5 in P₄Olo^. The oxygen is reduced from 0 in 0₂ to -2 in P₄O₁₀.

9-42
An oxidizing agent is a substance which accepts electrons and is thereby reduced. A reducing agent is a substance which loses electrons and is thereby oxidized.

9-43
3S₈(s) + 16KClO₃(s) 24SO₂(g) + 16 KCl(s)
Reducing agent: S₈ Oxidizing agent: KClO₃

9-44
H₂O₂(aq) + 2HI(aq) 2H₂O(l) + I₂
Reducing agent: HI Oxidizing agent: H₂O₂

9-45
Cu(s) + 2Ag⁺(aq) Cu²⁺(aq) + 2Ag(s)
Reducing agent: Cu Oxidizing agent: Ag⁺

9-46
a) Al is the reducing agent, Cr₂O₃ the oxidiaing agent
b) Al is the reducing agent, H⁺ the oxidizing agent
c) Al is the reducing agent, I₂ the oxidizing agent

9-47
a) Mg is the reducing agent, CO₂ the oxidizing agent.
b) Mg is the reducing agent, H¢I the oxidizing agent.
c) Mg is the reducing agent, H₂0 the oxidizing agent.
d) Mg is the reducing agent, N₂ the oxidizing agent.
e) Mg is the reducing agent, NH₃ the oxidizing agent.

9-48
(a) Na Na⁺
(b) Zn Zn²⁺
(c) H₂ 2 H⁺
(d) Sn²⁺ Sn⁴⁺
(e) H^-

9-49
(a) Al³⁺ Al
(b) Hg²⁺ Hg
(c) H⁺ H
(d) H₂ 2 H^-
(e) Sn²⁺ Sn

9-50
Every reducing agent is coupled to an oxidizing agent and vice versa. When an oxidizing agent gains electrons, it forms a reducing agent. When a reducing agent loses electrons it forms an oxidizing agent. The oxidizing agent and the reducing agent are linked together and so are called conjugate oxidizing agent and conjugate reducing agent. For example, the metal Na(s) is a reducing agent. It can lose an electron and form its conjugate oxidizing agent Na^+. Na⁺ is an oxidizing agent and it can gain an electron and form its conjugate reducing agent Na(s).

9-51
Strong reducing agents such as potassium or sodium metal lose electrons more easily than other elements. Weak reducing agents such as silver or gold have greater difficulty in losing electrons.

9-52
a) Mg: reducing agent, relatively strong
b) MgO: oxidation agent, relatively weak
c) AgNO₃: oxidation agent, relatively strong
d) Cu: reducing agent, relatively weak

9-53
The fact that sodium metal is a stronger reducing agent than aluminum metal implies that the AI³⁺ ion must be a stronger oxidizing agent than the Na⁺ ion. This is a result of the relationship between a conjugate reducing agent/oxidizing agent pair. The stronger reducing agent Na has a weaker conjugate oxidizing agent, Na⁺.

9-54
Although iron is a stronger reducing agent than copper, in an absolute sense iron is not considered a strong reducing agent, since its ability to donate electrons is low compared to other reducing agents.

9-55
The fact that intact samples of copper metal dating back 5000 years have been recovered demonstrates that copper is less reactive compared to other metals such as iron, aluminum, sodium, and magnesium which are not found in elemental form under similar circumstances.

9-56
Metals above Fe³⁺ in Table 9.4 will serve as reducing agents toward Fe^3+. These include (a) Na, (b) Mg, (c) Al

9-57
To be reduced by H₂, the metal oxide must be of a metal which falls below the hydrogen reaction in Table 9.4. The only metal of the group of oxides listed which can qualify is memury. Therefore only HgO, is predicted to be susceptible to reduction using hydrogen.

9-58
(a) Na, (b) Mg, (c) Al, (d) Fe should all be able to reduce Sn²⁺ to tin metal. Only (e) Hg should not be able to accomplish the reduction.

9-59
Sodium metal is a stronger reducing agent than is hydrogen.

9-60
The less active the metal, the more easily it is reduced. Due to the relative ease of removal of oxygen described in the problem, the three metals arranged in order of increasing activity are Hg < Cu < Al.

9-61
The thermite reaction demonstrates that aluminum is a much stronger reducing agent than is iron.

9-62
Because chromium metal falls below aluminum metal in the table of relative strengths of the common metals as reducing agents, powdered aluminum should be able to reduce chromium(Ill) oxide to chromium metal.

9-63
The fact that titanium metal can be produced by the reaction of titanium(IV) chloride with magnesium metal tells us that magnesium is a stronger reducing agent than is titanium metal.

9-64
(a) The reverse reaction is expected to occur.
(b) The reverse reaction is expected to occur.
(c) The reaction is expected to occur as written.
(d) The reaction is expected to occur as written.

9-65
The first reaction indicates that the reactivity of cadmium must be above that of tin. The second reaction indicates that the reactivity of cadmium is below that of iron. The third reaction indicates that the reactivity of cadmium is below that of Zn. The fourth reaction indicates that the activity of cadmium is below that of chromium. Whether the reduction of Cd⁺² to Cd(s) should be placed above or below that of nickel cannot be judged with the information given.

9-66
The strongest reducing agents are those substances which most easily lose valence electrons. These elements are associated with the lower left corner of the periodic table.

9-67
Although the first bulk amounts of magnesium metal were prepared by the reaction of sodium or potassium metal with MgCI_2, the metal is presently made by the electrolysis of the salt obtained from the concentration of salt brines.

9-68
Iron, of the listed metals, is not manufactured by electrolysis. Carbon has sufficient activity to reduce iron oxides at higher temperature.

9-69
The "roasting" of sulfide ores is an oxidation-reduction process. The formal oxidation number of oxygen changes from 0 of the element to -2 of the oxide while the oxidation number of sulfur changes from -2 of the sulfide to +4 of the sulfur dioxide.

9-70
The metals of Table 9.4, which are prepared by electrolysis, represent those that are most difficult to reduce. At the higher extreme, no more active reducing agents exist other than the "naked" electron of the electrolysis cell. Therefore the elements K, Ba, Sr, Ca, Na, Mg, AI are all commercially prepared by electrolysis of the corresponding salts. The particular salts selected generally reflect consideration of relative melting temperatures. Other metals are also prepared by electrolysis as a means to improve the purity of the product over that obtainable via reduction with carbon and other readily available, inexpensive reducing agents. Copper and chromium come to mind in this context.

9-71
Oxidation occurs at the anode and reduction occurs at the cathode of an electrolysis cell. The reactions for MgCI₂ are:

Cathode: Mg²⁺ + 2 e^- Mg(s)

Anode: 2 CI^-+ Cl₂(g)+ 2 e^-

9-72
It is true that aluminum is a metal of relatively low density (2.70 g/cm³). This property alone would not explain why an aluminum can is able to float on water, since water (1.00 g/cm³⁾ is less dense than aluminum. The beer contains varying amounts of dissolved gases (such as CO₂ , and some alcohol, and possible some free gas at the top) which cause the over all density of some of the cans of beer to be less than that of water.

9-73
One experiment which could be performed to determine the percent by weight of sodium in the alloy would take advantage of sodium's higher reactivity. Addition of water to a weighed amount of alloy would result in reaction of sodium to generate sodium hydroxide, which would dissolve in the water and be washed away from the remaining lead. The lead could then be weighed, and the difference from the alloy weight would give the weight of the sodium. The percentage by weight for sodium could then be calculated (see equations below).

2 Na(s) + 2 H₂0(I) 2 NaOH(aq) + H₂(g)

Pb(s) + H₂0 no reaction

wt of alloy - wt of Pb remaining = wt of Na