SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 12

Oxidation-Reduction and Electrochemistry

Practice Exercises

1. Which of the following when dissolved in water and placed in the conductivity apparatus would cause the light to glow?

(A) table salt

(B) ethyl alcohol

(C) sugar

(D) glycerine

2. In question 1, the reason that a current could flow is that

(A) ions combine to form molecules

(B) molecules migrate to the charge plates

(C) ions migrate to the charge plates

(D) sparks cross the gap

3. The extent of ionization depends on the

(A) nature of the solvent

(B) nature of the solute

(C) concentration of the solution

(D) temperature of the solution

(E) all of the above

4. Which of the following is TRUE?

(A) The number of positive ions in solution equals the number of negative ions.

(B) The positive ions are called anions.

(C) The positive ions are called cathodes.

(D) The total positive charge equals the total negative charge in solution.

(E) None of the above

5. The hydronium ion is represented as

(A) H2O+

(B) H3O+

(C) HOH+

(D) H

6. In the electrolysis of copper chloride, the substance liberated at the anode is

(A) copper

(B) chlorine

(C) hydrogen

(D) copper chloride

7. Ions are particles that exist

(A) only in water solutions

(B) in some crystals

(C) in polar covalent compounds

(D) in covalent compounds that are not polar

8. Ionic compounds will conduct an electric current when they are

(A) solidified

(B) melted

(C) frozen

(D) dehydrated

9. The cathode in an electrochemical cell is the electrode that is

(A) always negative

(B) always positive

(C) always neutral

(D) the electrode at which reduction takes place

10. Electrolysis of a dilute solution of sodium chloride results in the cathode product

  (A) sodium

  (B) chlorine

  (C) hydrogen

  (D) oxygen

11. Electrode potentials are:

Zn0 → Zn2+ + 2e   E0 = + 0.76 V

Au0 → Au3+ + 3e   E0 = -1.42 V

If a gold foil were placed in a solution containing zinc ions, the reaction potential would be calculated to be

(A) −1.34 V (no reaction)

(B) −2.18 V (no reaction)

(C) −0.66 V (no reaction)

(D) +2.18 V (reaction)

(E) +1.34 V (reaction)

12. A positive reaction potential indicates that

(A) the reaction will not occur

(B) the reaction will occur and give off energy

(C) the reaction will occur if heat or energy is added

(D) the reaction will power an outside alternating electric current

Questions 13–15

The following elements are listed in order of decreasing reactivity as they appear in the electrochemical series.

Ca, Na, Mg, Zn, Fe, H, Cu, Hg, Ag, Au

13. The element that is the best reducing agent and the easiest to oxidize is

(A) Ca

(B) Au

(C) H

(D) Fe

(E) Cu

14. Of the following, the element that does NOT react with hydrochloric acid to produce hydrogen gas is

(A) Zn

(B) Fe

(C) Hg

(D) Ca

(E) Mg

15. In the electrochemical cell shown above, which of the following half-reactions occurs at the anode?

(A)Cu2+ + e → Cu+

(B)Zn(s) → Zn2+ + 2e

(C)Zn2+ + 2e → Zn(s)

(D)Cu(s) → Cu2++2e

(E)Cu2+ + 2e → Cu(s)

Answers and Explanations

1. (A) Table salt is the only substance listed that has ions when dissolved in its water solution and will show a current is moving through the system by lighting the conductivity apparatus.

2. (C) The current is carried through the solution by ions migrating to the charged plates that act as the anode and the cathode.

3. (E) The extent of ionization of a substance in solvent depends on all four of the listed factors.

4. (D) The only statement that is true is that the total positive charge in solution equals the total negative charge. The positive ions migrate to the negative pole called the cathode so they are called cations.

5. (B) The hydronium ion is written as H3O+.

6. (B) Because the anode is positively charged, it attracts the negatively charged Cl- ions that are released from copper chloride. When it is oxidized by losing its electon, the chlorine atom then forms a Cl2 molecule with another atom of chlorine to be released as chlorine gas. The anode reaction would be:

2Cl(aq) → Cl2(g) + 2e

7. (B) Ions exist not only in some water solutions but also in ionic crystal lattices like sodium chloride.

8. (B) When an ionic compound is melted, the ions become free to migrate in the liquid and thereby carry an electric current.

9. (D) The cathode in an electrochemical cell is the electrode at which reduction occurs. The other pole at which oxidation exists is called the anode.

10. (C) The electrolysis of a dilute water solution of sodium chloride contains chloride ions, which are attracted to the anode and are set free as chlorine molecules. The cathode reaction is somewhat more complicated. Although the sodium ions are attracted to the cathode, they are not set free as atoms. Because water can ionize to some extent and the standard electrode chart shows that the hydrogen ion is reduced more easily than the sodium ion, it is hydrogen, not sodium, that is set free at the cathode. The reactions are:

      

11. (B) Because zinc is higher than gold in the standard reduction potentials listing, it will remain in the ionic form, whereas gold will favor the nonionized form. The two half-reactions that would have to occur with the electrode potentials for each are:

      

This negative cell potential indicates that these reactions would not occur.

* Note: The standard potentials (E0) are not multiplied by the coefficients in calculating the E0 for the reaction.

12. (B) A positive reaction potential indicates that the reaction will occur and that energy is given off.

13. (A) Ca is the most active, therefore, it loses electrons so that reduction can occur. Reducing agents are oxidized.

14. (C) Because Hg is below H in activity, Hg cannot displace H.

15. (B) Zn acts as the anode in this setup because it is higher on the activity series than copper.