Cracking the SAT Chemistry Subject Test

CHAPTER 12 REDOX AND

ELECTROCHEMISTRY

Drill 1

1. A During a reduction half-reaction, electrons are gained.


2. E The oxidation number on Ag is +1 on both sides, and the oxidation
   number of Cl is −1 on both sides. Thus, no electrons are transferred.


3. E All of the other options have an element in its pure form, which
   always has an oxidation state of 0.


4. C Two moles of electrons are needed to turn Cu2+ into Cu (and to
   turn Mg into Mg2+). Reaction A has 3 moles, B has 1 mole, D has 4 moles
   (two oxygen atoms changing oxidation state from 0 to −2), and E has no
   electron transfer.


5. T, T, CE

Divide  and conquer.    Look    at  the first   statement,  and

evaluate    it  on  its own.    Is  it  true?   Yes.    Wherever    there’s

oxidation,  there   must    be  reduction.

Look    at  the second  statement   by  itself. Is  it  true?   Yes.    If

one species gives   up  electrons,  something   else    must

accept  them.   Now see whether the whole   sentence

makes   sense:  “Any    reaction    in  which   one atom    is

oxidized    requires    that    another atom    be  reduced because

if  one species donates electrons,  another must    acquire

them.”  Does    it? Absolutely. Oxidation   means   losing

electrons,  and reduction   means   gaining electrons.

Oxidation   must    accompany   reduction   because if

electrons   are lost    from    one place,  they    must    be  gained

by  another.    The second  statement   explains    the first,  so

fill    in  oval    CE.