Fundamentals of Electrochemistry - W.H. Freeman

1504T_ch14_270-297 1/23/06 11:35 Page 293
molecules. For ATP synthesis, G 34.5 kJ/mol. This energy is
then made available to the cell when ATP is hydrolyzed to ADP
(adenosine diphosphate). In animals, ATP is synthesized when protons
pass through a complex enzyme in the mitochondrial membrane.
14 Two factors account for the movement of protons through
this enzyme into the mitochondrion (see the figure): (1) [H ] is
higher outside the mitochondrion than inside because protons are
pumped out of the mitochondrion by enzymes that catalyze the oxidation
of food. (2) The inside of the mitochondrion is negatively
charged with respect to the outside.
(a) The synthesis of one ATP molecule requires 2H to pass
through the phosphorylation enzyme. The difference in free energy
when a molecule travels from a region of high activity to a region of
low activity is
A high
G RT ln
A low
How big must the pH difference be (at 298 K) if the passage of
two protons is to provide enough energy to synthesize one ATP
molecule?
(b) pH differences this large have not been observed in mitochondria.
How great an electric potential difference between inside and
outside is necessary for the movement of two protons to provide
energy to synthesize ATP? In answering this question, neglect any
contribution from the pH difference.
(c) The energy for ATP synthesis is thought to be provided by
both the pH difference and the electric potential. If the pH difference
is 1.00 pH unit, what is the magnitude of the potential
difference?
Positive electric
potential
+
+
–
+
Mitochondrial membrane
–
–
Negative electric
potential
Low H +
concentration
ADP + HPO 2– 2H +
2H +
High H +
concentration
+
+
+
–
4
ATP
–
–
Phosphorylation
enzyme
H + H +
H + driven
out of
mitochondrion
during
respiration
Problems
Basic Concepts
14-5. Consider the redox reaction
14-1. Explain the difference between electric charge (q, coulombs),
electric current (I, amperes), and electric potential (E, volts).
Thiosulfate
14-2. (a) How many electrons are in one coulomb?
(b) How many coulombs are in one mole of charge?
14-3. The basal rate of consumption of O 2 by a 70-kg human is
about 16 mol of O 2 per day. This O 2 oxidizes food and is reduced to
H 2 O, providing energy for the organism:
O 2 4H 4e T 2H 2 O
(a) To what current (in amperes C/s) does this respiration rate
correspond? (Current is defined by the flow of electrons from food
to O 2 .)
(b) Compare your answer in part (a) with the current drawn by a
refrigerator using 5.00 10 2 W at 115 V. Remember that power
(in watts) work/s E I.
(c) If the electrons flow from nicotinamide adenine dinucleotide
(NADH) to O 2 , they experience a potential drop of 1.1 V. What is
the power output (in watts) of our human friend?
14-4. A 6.00-V battery is connected across a 2.00-k resistor.
(a) How many electrons per second flow through the circuit?
(b) How many joules of heat are produced for each electron?
(c) If the circuit operates for 30.0 min, how many moles of electrons
will have flowed through the resistor?
(d) What voltage would the battery need to deliver for the power to
be 1.00 10 2 W?
I 2 2S 2 O 2 3
T 2I S 4 O 2 6
Tetrathionate
(a) Identify the oxidizing agent on the left side of the reaction and
write a balanced oxidation half-reaction.
(b) Identify the reducing agent of the left side of the reaction and
write a balanced reduction half-reaction.
(c) How many coulombs of charge are passed from reductant to
oxidant when 1.00 g of thiosulfate reacts?
(d) If the rate of reaction is 1.00 g of thiosulfate consumed per
minute, what current (in amperes) flows from reductant to
oxidant?
14-6. The space shuttle’s expendable booster engines derive their
power from solid reactants:
6NH4 ClO 4 (s) 10Al(s) S 3N 2 (g) 9H 2 O(g)
FM 117.49
5Al 2 O 3 (s) 6HCl(g)
(a) Find the oxidation numbers of the elements N, Cl, and Al in
reactants and products. Which reactants act as reducing agents and
which act as oxidants?
(b) The heat of reaction is 9 334 kJ for every 10 mol of Al consumed.
Express this as heat released per gram of total reactants.
Galvanic Cells
14-7. Explain how a galvanic cell uses a spontaneous chemical
reaction to generate electricity.
Problems 293