Chapter 10

A battery is constructed from tin and copper by dipping strips of each metal into a solution of its ions \(\left(\mathrm{Sn}^{2+}\right.\) and \(\mathrm{Cu}^{2+}\), respectively). As the battery operates, the \(\mathrm{Sn}^{2+}\) concentration increases and the \(\mathrm{Cu}^{2+}\) concentration decreases. (a) What is getting oxidized? (b) What is getting reduced? (c) Draw a battery similar to the one you drew for WorkPatch 10.7. Make sure you label which way the electrons flow, and also label the cathode, \(+\), and anode, \(=\)

A student claims that in his battery, the electrons flow from the positive to the negative electrode. In fact, this is not true for any battery. Explain why electrons would never flow in this direction and how the direction of electron flow tells you that the cathode is where reduction occurs.

How can the words cation and anion help you remember if \(+\) or \(-\) is associated with the cathode and anode of a battery?

Suppose you have two metals, \(A\) and \(B\), and solutions of their ions, \(A^{+}\) and \(B^{+}\). Metal \(B\) is more active than metal \(\mathrm{A}\). (a) Write the spontaneous redox reaction for this situation. (b) What is the oxidizing agent? (c) What is the reducing agent? (d) Describe which way the electrons flow.

You are trapped on an island with lots of different metals, chemicals, and supplies but no EMF series table. How would you go about deriving an EMF series, and how would it be useful?

What happens when you place an active metal in a solution of ions of a less active metal?

What happens when you place a less active metal in a solution of ions of a more active metal?

Using the EMF series on page 391 , decide which of the following redox reactions is spontaneous. Explain your answer. (a) \(3 \mathrm{~K}+\mathrm{Al}^{3+} \rightarrow \mathrm{Al}+3 \mathrm{~K}^{+}\) (b) \(\mathrm{Al}+3 \mathrm{~K}^{+} \rightarrow 3 \mathrm{~K}+\mathrm{Al}^{3+}\)

Using the EMF series on page 391, decide which of the following redox reactions is spontaneous. Explain your answer. (a) \(3 \mathrm{Ag}+\mathrm{Au}^{3+} \rightarrow \mathrm{Au}+3 \mathrm{Ag}^{+}\) (b) \(\mathrm{Au}+3 \mathrm{Ag}^{+} \rightarrow 3 \mathrm{Ag}+\mathrm{Au}^{3+}\)

You are trapped on a desert island with plenty of water (both fresh and salt), a drinking glass, some wire, a radio, and no batteries. You do have a tin cup, a tube of toothpaste containing stannous fluoride \(\left(\mathrm{Sn} \mathrm{F}_{2}\right.\), a source of \(\mathrm{Sn}^{2+}\) ions), a silver pendant, and undeveloped black-andwhite film (such film has silver bromide, \(\mathrm{AgBr}\), in it, a source of \(\mathrm{Ag}^{+}\) ions). (a) How would you use the above materials to construct a battery? Show how with a diagram, including an arrow over the wire to show which way the electrons flow. (You can make a salt bridge by soaking a sock in salt water and then dipping one end of the sock in one cell and the other end in the other cell.) (b) Which metal would be eaten away? Explain. (c) Which is the oxidizing agent? (d) Which is the reducing agent?

Recharging a battery means forcing a spontaneous redox reaction to run backwards - in the nonspontaneous direction - once all the reactants have been used up. In Practice Problem \(10.24\), you considered a battery made from lead and copper. (a) Write the spontaneous redox reaction for this battery. (b) Write the recharging reaction for this battery.

Lithium is a very active metal, so active that it reacts with water to make flammable hydrogen gas. \(\mathrm{Li}(s)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{LiOH}(s)+1 / 2 \mathrm{H}_{2}(g)\) (a) What metal would you use in combination with Li to make a battery having the highest voltage? (Consult the EMF series.) (b) Too rapid a discharge of a lithium battery (which can occur if there is a short circuit) can overheat a lithium battery and cause it to explode and catch fire. Why is trying to put out such a fire with water a bad idea? (c) Would lithium be the anode or cathode in a lithium ion battery? Explain.

There are two definitions of oxidation, one involving oxygen, the other not. (a) State the two definitions of oxidation. (b) Are they compatible? Explain. (c) When we speak of corrosion of metals, what chemical reaction are we usually talking about? Use Fe as an example.

A piece of magnesium can be attached to the iron hull of a boat to prevent it from rusting. (a) Why is the magnesium called a sacrificial metal? (b) How does the Mg keep the iron hull from rusting? (c) Which can be thought of as the positive cathode, the \(\mathrm{Mg}\) or the steel hull? Explain. (d) Putting a block of \(\mathrm{Mg}\) on a steel boat hull does not create a battery. Why not?

What is the oxidation state of the iodine atom in \(\mathrm{H}_{2} \mathrm{IO}_{6}{\underline{\phantom{xx}}}^{3-} ?\)

Identify the oxidizing and reducing agents in the reaction \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \rightarrow \mathrm{N}_{2}+\mathrm{Cr}_{2} \mathrm{O}_{3}+4 \mathrm{H}_{2} \mathrm{O}\)

Burning octane, \(\mathrm{C}_{8} \mathrm{H}_{18}\), in your car engine forms water and carbon dioxide: \(2 \mathrm{C}_{8} \mathrm{H}_{18}+25 \mathrm{O}_{2} \rightarrow 18 \mathrm{H}_{2} \mathrm{O}+16 \mathrm{CO}_{2}\) What gets oxidized and what gets reduced?

Which metal is most easily oxidized: \(\mathrm{Pt}, \mathrm{Hg}, \mathrm{Fe}\), \(\mathrm{Mg}, \mathrm{Zn} ?\)

Metal strips are immersed in aqueous solutions of various salts. In which combinations do you expect a spontaneous redox reaction: (a) Silver strip in \(\operatorname{CuBr}_{2}(a q)\) (b) Copper strip in \(\mathrm{ZnSO}_{4}(a q)\) (c) Zinc strip in \(\operatorname{AgNO}_{3}(a q)\) (d) Gold strip in \(\mathrm{FeCl}_{2}(a q)\) (e) Copper strip in \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}(a q)\)

Can you protect a steel (predominantly iron) structure from corrosion by connecting it by wire to a small plate made of nickel? What elements (if any) would make better sacrificial metals?

Which ion is most difficult to reduce: \(\mathrm{Mn}^{2+}, \mathrm{Hg}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Mg}^{2+}, \mathrm{Li}^{+} ?\)

The blood alcohol level of a person can be detected by reacting a sample of blood plasma with dichromate ion, \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\), which takes part in an electron-transfer reaction with ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), in the blood: \(2 \mathrm{Cr}_{2} \mathrm{O}_{7}{\underline{\phantom{xx}}}^{2-}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}+16 \mathrm{H}^{+} \rightarrow\) \(4 \mathrm{Cr}^{3+}+2 \mathrm{CO}_{2}+11 \mathrm{H}_{2} \mathrm{O}\) Assign an oxidation state to each atom in this reaction and indicate the oxidizing agent and the reducing agent.

A battery was produced using copper metal in a solution of Cu \(^{2+}\) ions connected to rhodium metal in a solution of \(\mathrm{Rh}^{3+}\) ions. Copper is the anode and rhodium is the cathode. Is rhodium higher or lower than copper in the EMF series?

Assign an oxidation state for each nitrogen atom in \(\mathrm{N}_{3}^{-} .\) (Hint: Begin with a dot diagram for the ion.)

Which of the following are electron-transfer reactions? For those that are, indicate which reactant is the reducing agent and which reactant is the oxidizing agent. (a) \(\mathrm{SeO}_{3}^{2-}+4 \mathrm{I}^{-}+6 \mathrm{H}^{+} \rightarrow \mathrm{Se}+2 \mathrm{I}_{2}+3 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{HI}+\mathrm{KOH} \rightarrow \mathrm{KI}+\mathrm{H}_{2} \mathrm{O}\) (c) \(4 \mathrm{HCl}+\mathrm{O}_{2} \rightarrow 2 \mathrm{Cl}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{SiO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{4} \mathrm{SiO}_{4}\)

Tin cans are actually iron plated with tin. What is the advantage of the tin coating?

Fluorouracil is a compound administered to cancer patients as a part of chemotherapy. Assign an oxidation state to every atom:

Which is a better reducing agent, \(\mathrm{Zn}\) or \(\mathrm{Pb}\) ?

Assign an oxidation state to each carbon in: (a) \(\mathrm{H}_{3} \mathrm{CCH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{CCH}_{2}\) (c) HCCH

The spontaneous redox reaction \(\mathrm{Mn}+\mathrm{Cd}^{2+} \rightarrow \mathrm{Mn}^{2+}+\mathrm{Cd}\) takes place in a battery. (a) What is the oxidizing agent? (b) What is the reducing agent? (c) Which metal is the cathode? (d) Which metal is the anode?

Assign an oxidation number to each atom and identify the oxidizing agent and reducing agent: \(3 \mathrm{Na}_{2} \mathrm{SO}_{3}+2 \mathrm{KMnO}_{4}+\mathrm{H}_{2} \mathrm{O} \rightarrow\) \(3 \mathrm{Na}_{2} \mathrm{SO}_{4}+2 \mathrm{MnO}_{2}+2 \mathrm{KOH}\)

You are trying to determine the identity of an unknown metal. You place a strip of it in a solution of \(\mathrm{Mg}^{2+}\), and no reaction occurs. You then place a strip of it in a solution of \(\mathrm{Zn}^{2+}\), and zinc metal plates out on the strip. Name one possibility for the identity of the unknown metal.

Use the shortcut rules to assign an oxidation state to each atom: (a) \(\mathrm{PF}_{6}\) (b) \(\mathrm{Mo}_{2} \mathrm{O}_{7}^{2-}\) (c) \(\mathrm{HPbO}_{2}^{-}\) (d) \(\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\)

When you turn on an electrical appliance, are you consuming electrons?

You create a battery using a zinc anode in a solution of \(\mathrm{Zn}^{2+}\) ions connected to a nickel cathode in a solution of \(\mathrm{Ni}^{2+}\) ions. The two beakers are also connected with a salt bridge containing \(\mathrm{K}^{+}\) ions and \(\mathrm{Cl}^{-}\) ions. (a) Toward which electrode do \(\mathrm{K}^{+}\) ions flow? (b) Toward which electrode do \(\mathrm{Cl}^{-}\) ions flow?

Assign an oxidation state to each atom in the amino acid glycine:

Identify the oxidizing agent and reducing agent in the reaction \(\mathrm{IO}_{4}^{-}+7 \mathrm{I}^{-}+8 \mathrm{H}^{+} \rightarrow 4 \mathrm{I}_{2}+4 \mathrm{H}_{2} \mathrm{O}\)

The black tarnish that forms on silver is silver sulfide, \(\mathrm{Ag}_{2} \mathrm{~S}\). The tarnish can be removed by wrapping the silver with a piece of aluminum foil and placing it in a solution of \(\mathrm{NaHCO}_{3}\) (baking soda) to allow for the flow of ions. Explain how the aluminum foil aids in removing the tarnish.

Under what circumstances does fluorine not have an oxidation state of \(-1 ?\)

Which of the following are electron-transfer reactions? For those that are, indicate which reactant is the reducing agent and which reactant is the oxidizing agent. (a) \(3 \mathrm{H}_{2} \mathrm{SO}_{3}+2 \mathrm{HNO}_{3} \rightarrow 3 \mathrm{H}_{2} \mathrm{SO}_{4}+2 \mathrm{NO}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Mg}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}+\mathrm{H}_{2}\) (c) \(\mathrm{SO}_{3}{\underline{\phantom{xx}}}^{2-}+2 \mathrm{H}^{+} \rightarrow \mathrm{SO}_{2}+\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{PbO}+\mathrm{CO} \rightarrow \mathrm{Pb}+\mathrm{CO}_{2}\)

Assign an oxidation state to each atom in HCN. (Hint: Begin with a dot diagram.)

A battery is made by connecting strips of lead and palladium and dipping each metal into a solution of its ions \(\left(\mathrm{Pb}^{2+}\right.\) and \(\mathrm{Pd}^{2+}\), respectively). Over time, the mass of the lead strip decreases and the mass of the palladium strip increases. (a) Which metal is the anode? (b) Which metal is the cathode? (c) Write the spontaneous redox reaction going on in this battery.

What is the range of oxidation states possible for carbon? (Hint: Consider that carbon bonds first to atoms that are all higher in electronegativity and then to atoms all lower in electronegativity.)

Identify the oxidizing agent and reducing agent in the reaction \(\mathrm{NO}_{3}^{-}+4 \mathrm{Zn}+7 \mathrm{OH}^{-}+6 \mathrm{H}_{2} \mathrm{O} \rightarrow\) \(4 \mathrm{Zn}(\mathrm{OH})_{4}{\underline{\phantom{xx}}}^{2-}+\mathrm{NH}_{3}\)

Galvanized steel is steel coated with zinc. Why is galvanized steel less likely to rust than uncoated steel? (Hint: Remember, steel is mainly iron.)

Which of the following are electron-transfer reactions? For those that are, indicate which reactant is the reducing agent and which reactant is the oxidizing agent. (a) \(\mathrm{PF}_{3}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{3} \mathrm{PO}_{3}+3 \mathrm{HF}\) (b) \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}\) (c) \(2 \mathrm{Cr}_{2} \mathrm{O}_{3}+3 \mathrm{Si} \rightarrow 4 \mathrm{Cr}+3 \mathrm{SiO}_{2}\) (d) \(\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}\)

A street vendor sells you a ring he claims is pure gold. However, when you place the ring in a solution of \(\mathrm{Cu}^{2+}\) ions, copper plates out on the ring. How do you know the ring is a fake? Could it be made of silver?

Use the shortcut rules to assign an oxidation state to each atom: (a) \(\mathrm{NaPO}_{3}\) (b) \(\mathrm{B}(\mathrm{OH})_{3}\) (c) \(\mathrm{V}_{2} \mathrm{O}_{5}\) (d) \(\mathrm{K}_{2} \mathrm{TiF}_{6}\)

The reaction \(\mathrm{Cu}^{2+}+\mathrm{I}_{2} \rightarrow \mathrm{Cu}^{+}+2 \mathrm{I}^{-}\) is not possible as written. Assign an oxidation number to each atom and explain what is wrong with this reaction.

A battery consists of a strip of titanium metal in a solution of \(\mathrm{Ti}^{2+}\) ions connected to a strip of zinc metal in a solution of \(\mathrm{Zn}^{2+}\) ions. Over time, the concentration of \(\mathrm{Zn}^{2+}\) ions decreases and the concentration of \(\mathrm{Ti}^{2+}\) ions increases. (a) Which metal is the anode? (b) Which metal is the cathode? (c) Where must titanium be on the EMF scale relative to zinc? (d) Write an equation that describes the spontaneous electron-transfer reaction occurring in this battery.