Cracking the SAT Chemistry Subject Test

•           Hydrogen’s  oxidation   state   doesn’t change  (it’s   +1  on  both    sides).

•    Oxygen’s    oxidation   state   changes     from    0   to  −2.     This    means   that

nitrogen’s  oxidation   state   must    change  to  balance the reduction   of  oxygen.

•           What    is  the change  in  nitrogen’s  oxidation   state?  On  the left    side    of  the

equation,   its oxidation   state   is  −3  (to balance the total   +3  oxidation   state   of

the 3   hydrogen    atoms   to  which   each    is  bonded).    On  the right   side    of  the

equation,   its oxidation   state   is  +2  (to balance the −2  oxidation   of  the oxygen

atom    to  which   each    is  bonded).    So  its oxidation   state   changes from    −3  to

+2; nitrogen    is  oxidized.

Here’s the half-reaction that tells us what’s happening to it.

Oxidation:  4N−3    →   4N+2    +   20e−

1   nitrogen    atom    loses   5   electrons.

You now know that reduction has to balance oxidation. The nitrogen atoms lost a
total of 20 electrons; some other atoms must gain a total of 20 electrons. What
gains electrons and is reduced? Oxygen does. Its oxidation state goes from 0 on
the left to −2 on the right, and here’s that half-reaction.

Reduction:  5O 2    +   20e−    →   10O−2

Of the 10 oxygen atoms that have a −2 oxidation state on the right side, 4 are in
NO and 6 are in H 2 O. The nitrogen atoms lose a total of 20 electrons, and the

oxygen atoms gain a total of 20 electrons.

Many important reactions are redox reactions. In a combustion reaction, a
compound containing carbon and hydrogen reacts with molecular oxygen, O 2 , to

produce CO 2 and H 2 O. Look closely at the combustion of acetylene (C 2 H 2 ), and

you’ll see it’s a redox reaction.

Carbon is oxidized and oxygen is reduced in this reaction. The rusting of iron is
also a redox reaction. Here’s a simplified expression of this process.