Page 127 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions 1 1 \

Step 6. Adding Reactions (6.9g) and (6.9h) yields
-
30Cl ( aq) + 3H 20(1) + 2Cr(0Hh(s) + lO{OH - (aq)} + 6e - -
3CJ - (aq) + 6oH - (aq) + 2Cro� - (aq) + 8H20(1) + 6e -
Canceling 3 H 20(1), 60H - (aq), and 6e - from both sides of the equa­
tion gives

2Cr(0Hh(s) + 30Cl - (aq) + 40H - (aq)-
2Cr0� - ( aq) + 3CI ( aq) + 5H20(1) (6. 1 0 )
-
Step 7 . Inspection shows that Reaction (6. 1 0 ) conserves the various
atoms and electric charge.
Photosynthesis is probably the most important redox reaction. Dur­
ing photosynthesis in green plants, light energy is used to convert C0 2
and H 20 into oxygen and energy-rich organic compounds called carbo­
hydrates (e.g. , glucose, C H 6 1 20 ) . Without photosynthesis the Earth' s
6
atmosphere would have n o oxygen. I f photosynthesis stopped, most
Ii ving things would die in a few years . The reverse of photosynthesis is
called respiration. This occurs primarily at night when carbohydrates in
i
plants react with oxygen to release C02 and H2 0 back n to the atmo­
sphere. In prehistoric times green plants, growing in warm climates n
i
a n atmosphere much richer in C02 than the present atmosphere, "tied
up" vast quantities of carbon in the form of glucose. As a result, enor­
mous deposits of fossil fuels (i.e . , coal, oil , and gas) were deposited in
the ground . The combustion of these fuels in modern times is releasing
this carbon back into the atmosphere in the form of C02 gas. Carbon
dioxide is a "greenhouse gas" that contributes to global warming.
Exercise 6.4. The unbalanced equation for photosynthesis is
(6. 1 1 )

Balance the equation for this reaction.
Solution.
Step 1 . The unbalanced reaction is (6. 1 1 ) .
Step 2 . The oxidant is C0 (g) because it contains carbon that under­
2
goes a decrease in oxidation number [from + 4 in COz(g) to 0 in
C H 20 ] . Therefore, the unbalanced reduction half-reaction is
6
6 1
(6. 1 la)
1
The reductant is H20( ) , because it contains oxygen which undergoes
an increase in oxidation number [from -2 in H20(1) to 0 in 02(g)] . The
unbalanced oxidation half-reaction is