1. The standard reduction potential for M2+ + 2e- --> M is +0.34 V for Cu2+/Cu and -0.44 V for Fe2+/Fe. It follows that
1. Cu is a better reducing agent than Fe
2. Fe2+ is a better oxidizing agent than Cu2+
3. Fe is more easily oxidized than Cu
4. a pipeline connected to a block of copper will not rust (from Fe2+)
2. A researcher constructs battery based on silver and chromium, as shown below
Cr | Cr3+ | | Ag+ | Ag
Cr3+ + 3e- --> Cr Eo = -0.74 V
Ag+ + e- --> Ag Eo = +0.80 V
a. What is the overall balanced cell reaction under standard conditions?
b. Calculate the cell potential, [[Delta]]Eo
c. Calculate the equilibrium constant (Keq) at 298K
d. What is [[Delta]]E at 298K when [Cr3+] = 1.0 * 10-4M and [Ag+] = 1.0M?
e. If the battery generates a current of 0.30 amp for 2.0 hrs, what mass of metal is deposited at the cathode? (Atomic weights: Cr=52; Ag=108.)
3. In order to plate a copper object with silver, a solution of AgNO3 is electrolyzed with the copper object serving as the cathode. A current of 0.50 ampere (0.50 coul/s) flows for 1.0 hour. What mass of silver (atomic weight = 108 g/mol) is plated out?
4. Use the cell indicated to answer the following questions.
Ag | Ag+ (1M) | | Ni2+ (1M) | Ni
Ag+ + e- --> Ag Eo = +0.80 V
Ni2+ + 2e- --> Ni Eo = -0.25V
a. What is [[Delta]]Eo for the cell?
b. Write the balanced overall cell reaction.
c. Calculate the equilibrium constant for the cell reaction.
d. Calculate the maximum useful work that can be obtained from this cell.
5. In order to determine the Ksp for Cu(OH)2, a chemist constructed the cell shown below. The left half contains a buffer solution (pH=9.0) saturated with Cu(OH)2 while the right half cell contains Cu2+ at 1.0M. The measured cell voltage was 0.265 V.
Cu | Cu(OH)2(s), Buffer (pH=9.0) | | Cu2+ (1M) | Cu
a. Write the overall cell reaction.
b. Calculate the Cu2+ concentration in the left half cell.
c. Calculate Ksp for Cu(OH)2
6. CaF2 (fluorite) is an important mineral. The solubility of CaF2 in pure water is 3.5 * 10-4M.
a. Calculate Ksp for CaF2.
b. CaF2 is a potential fluoridation agent for water supplies. Calculate the maximum concentration of fluoride that can be obtained in hard water having [Ca2+] = 0.01M.
7. The cell illustrated below was constructed to measure the standard reduction potential of Br2. Use the data given to calculate EBr2o. All the reactions are in water.
Br2 | Br- | | Cl- | Cl2(g) [[Delta]]Eo = 0.29 V
Cl2(g) + 2e- --> 2Cl- Eo =1.36 V
Br2 + 2e- --> 2Br- Eo = ???
8. Myoglobin is an oxygen storage protein containing a heme group. Heme consists of an iron(II) ion surrounded by a porphyrin molecule, which serves as a chelating ligand. One function of the protein is to protect the heme group from oxidation to Fe(III), which will not bind oxygen. In the absence of the protein, free iron(III) heme [abbreviated as Fe(III) has a reduction potential of +0.14V:
Fe(III) + e- --> Fe(II) Eo =+0.14 V
The reduction potential of oxygen in water is:
O2(g) + 2H2O + 4e- --> 4OH- Eo =+0.40 V
Show by calculation of the relevant [[Delta]]E that a neutral (pH=7) aqueous solution containing equal concentrations of Fe(II) heme and Fe(III) heme is unstable with respect to oxidation by oxygen in air (PO2 = 0.2 atm) at 298K.
9. The cell shown below generates a potential of 0.65V at 298K. Calculate Ksp for AgBr.
Ag | AgBr(s), NaBr (0.10M) | | AgNO3 (0.10M) | Ag
[[Delta]]E = 0.65V
10. An electrochemical cell with inert platinum electrodes is shown below:
The half cell reactions are:
Fe3+ + e- --> Fe2+ Eo =+0.77 V
VO2+ + 2H+ + e- --> V3+ + H2O Eo =+0.34V
a. What is [[Delta]]Eo for the cell reaction?
b. Write the balanced cell reaction.
c. Assuming all species are 1M, circle the correct answer:
1. The cathode is on the Right or Left ?
2. The electrons flow R to L or L to R ?
d. Calculate the equilibrium constant for the reaction at 25C.
e. Calculate [[Delta]]E when all species are 1.0M except H+, which is 0.010M.
f. What is the maximum useful work that can be obtained from this cell under standard conditions?
11. A student wishes to plate a silver coin with copper. The coin is used as the cathode in an electrolysis cell containing aqueous CuSO4. A constant current is applied for 30 minutes. What must the current be in amperes if 1.50 g of copper is plated out? (The density of copper is 10.6 g/cm3).
Prof. Baird's old #3 exams also contained a number of questions from Chapter 21.[*]
1) For each of the following pairs which state has the higher entropy.
a) a mole of liquid water b) a mole of water vapor at 1 atm b b) a packaged deck of cards b) or the same deck shuffled b c) 1 gram of liquid H2O and 1 gram of b) the 2 gram mixture of the two b liquid D2O in separate beakers
2) For each of the following changes, tell whether [[Delta]]S is positive(+), negative (-), or
zero(0) for the system, the surroundings, and total (the "universe").
Process [[Delta]]S [[Delta]]Ssurr [[Delta]]S
system oundings total
a) The expansion, in an isolated + 0 +
system, of a gas into an evacuated
system
b) N2(g) + 2O2(g) --> 2NO2(g), Q < 0 - + +
c) water at 50C cooled reversibly to - + 0
water at 25C, 1 atm
d) CaCl2(s) spontaneously dissolves -44.7 +44.7 0
in water at 25C. The
[[Delta]]So=-44.7J/mol*K for the
reaction. This is carried out
reversibly. Give values for the
various entropy changes in the table
3. Methyl alcohol, CH3OH, is of interest as a source of methane from the decomposition reaction shown in the table. Using thermodynamic tables answer the questions below.
CH3OH(l) --> CH4(g) + 1/2O2(g)
a) Calculate the entropy change, 162.033 1/2*(205.138)+186.264-126
[[Delta]]So, for this reaction .8
b) Compute the free energy for the 115.55 -50.72-(-166.27)
reaction
c) Is the reaction "spontaneous" at NO
25C
4. Using thermodynamic tables for the reaction: C6H12O6 <--> C6H12O6(aq,1M) answer the questions in the table.
a) Compute the free energy for [[Delta]]Go=-917.5-(-910.5)=7kJ/mol glucose (s) dissolving in water to glucose (aq,1M) b) Does glucose dissolve in water at Y 298K? Y/N
5. Civilization has bees devastated, as predicted by the Branch Dividians, we have taken to the hills and are trying to create a modern world from scratch. Fortunately, we have saved our chemistry text. We have found some magnetite, Fe3O4, and we have produced coke, C(s), by partially burning wood and intend to make metallic iron by the reaction:
Fe3O4(s) + 2C(s) --> 3Fe(s) + 2CO2(g)
The enthalpy change for this reaction at 298K is [[Delta]]Ho=331.49kJ/mole and the free energy is [[Delta]]Go = 226.68kJ/mole.
a) Will any substantial quantity of No, [[Delta]]Go>0 metallic iron by formed at 298K. Y/N b) At what temperature can we expect 943K the equil. const. = 1, [[Delta]]Go=0?6. A current of 2amps is used for 60min. to electrolyze copper from a solution of Cu2+ at a potential of 0.337volts.
a) How many Coulombs are used 2 amps * 60min * 60min/sec = 7200C b) How many moles Cu(s) are plated (7200/2)*(1/96500)=0.0373moles out? c) What weight Cu(s) in grams is 0.0373 * 63.5 = 2.35g plated out? d) What power in watts was used? 0.337V * 2amp = 0.674W
7. The electrolysis of magnesium chloride proceeds according to
Mg2+ + 2Cl- --> Mg(s) + Cl2(g)
a) Write a half reaction occurring at the anode 2Cl- --> Cl2(g) + 2e- b) Write a half reaction occurring at the cathode Mg++ + 2e- --> Mg(s)
8. An aqueous solution of Na2SO4 was electrolyzed for 30 min. During this process 25mL of O2(g) at 1atm pressure and 25C was collected at the anode. Find the current used to produce the oxygen gas.
Current used to produce 25mL of O2(g) 0.22 amps
9. The first use of electrochemistry in organic synthesis was by Kirman in 1801 when he oxidized ethanol to form acetaldehyde. Complete the following table concerned with this system. [Ask Prof. Steim for the historical details!]
CH3CH2OH(aq) + 1/2O2(g) --> CH3CH3CHO(aq) + H2O(l)
a) Calculate the Gibbs Free energy, [[Delta]]Go=-134.06-237.129-(-181.56)
[[Delta]]Go for this reaction. = -189.63kJ/mol
b) Calculate the equilibrium constant K = 2*1033
for the reaction.
c) Using the half reaction for water: CH3CH2OH(aq)+1/2O2 ->CH3CHO(aq)
02(g)+4H+(aq)+4e- <--> 2H2O(l) H2O(l) H2O(l) <--> 1/2O2(g) + 2H+(aq)
[[Epsilon]][[omicron]]=1.23V find a + 2e- CH3CH2OH(aq)-->CH3CHO(aq) +
half reaction for the oxidation of 2H+(aq) + 2e-
ethanol, CH3CH2OH(aq)
d) Calculate the overall Eo for the E = 0.983V
reaction
e) Calculate the Eo for the reaction E = -0.247V
half reaction for the oxidation of
ethanol using the above information.
10. A titanium chloride solution was electrolyzed for 500seconds using a 0.120 amp current. 15mg of titanium metal was deposited. What is the oxidation number of titanium in the titanium chloride? The atomic weight of Ti is 47.88 g/mol
Oxidation number of Ti 2
11. A micro electrode specific for chloride is placed inside a cell with a reference electrode outside the cell. The chloride concentration [Cl-]outside = 0.560M. An electrochemical potential between the two electrodes is measured to be Eo = -0.067V. What is the concentration of chloride within the cell?
Concentration of Cl- in the cell 0.041M12. For each of the following changes, tell whether [[Delta]]S is positive(+), negative(-), or zero(0) for the system, the surroundings, and total (the "universe").
[[Delta]]S system surroundings total
adiabatic (Q=0) reversible gas 0 0 0
expansion
C6H12O6+6O2(g) ->6CO2(g)+6H2O(l), Q<O + + +
water frozen reversibly to ice at - + 0
273K, 1 atm
13. Using the concept of order-disorder for entropy, what is the sign of the entropy change for the following reactions.
reaction [[Delta]]S+/-,0
Mg(s) + 1/2O2(g) --> MgO(s) -
Pb(s) + Cl2(g) --> PbCl2(s) -
2ICl(g) --> I2(s) + Cl2(g) -
NaCl(s) --> NaCl(aq) +
14. In each of the following reactions indicate which element is oxidized and which is reduced.
indicate element is ox or red or blank below chemical
2Al(s) + 3Cl2(g) --> 2AlCl3(s)
Al-ox Cl-red
Fe (s) + HCl(g) --> FeCl2(s) + H2
Fe-ox H-red Cl-blank
Zn + Cu(PO4)2 --> Cu(s) + Zn3(PO4)2
Zn-ox Cu-red (PO4-3)-blank
AsO2 + ClO- --> AsO3- + Cl-
As-ox Cl-red
15. We are brewing beer and someone notices there are two competing processes: 1) oxidation of glucose (C6H12O6) to carbon dioxide and water and 2) the decomposition of glucose to ethanol (C2H5OH).
1) C6H12O6(s) + 6O2(g) --> 6CO2(g) + 6H2O(l)
and
2) C6H12O6(s) --> 2C2H5OH(l) + 2CO2(g)
Using the thermodynamic tables at the end.
a) Calculate the [[Delta]]Go298 for reaction 1)
=-2870 kJ/mol
b) Calculate th [[Delta]]Go298 for reaction 2)
=-219.12 kJ/mol
c) Using these two values predict whether we will produce mostly ethanol (reaction 2), or mostly carbon dioxide and water (reaction 1)
Since [[Delta]]Go298 for reaction 1) << [[Delta]]Go298 for reaction 2) K1>>K2 and carbon dioxide and water will be greatly favored.
d) If O2(g) concentration is greatly reduced how will the Gibbs free energy for reaction 1) be affected?
If oxygen is excluded from the reactions then reaction 1) will be suppressed and reaction 2) favored. This is why oxygen is kept out of the fermentation process so that reaction 2) will dominate.
16. The body uses glucose-6-phosphate (G6P) to form glycogen (formed from glucose units) for its use as an energy source, and the formation of G6P is therefore important.
a) Using the value of the Gibbs free energy, will G6P- be favored by the reaction?
G(aq) + H2PO4-(aq) --> G6P-(aq) + H2O(l) [[Delta]]Go=+16.4kJ/mole
Reactants favored over products because of positive free energy.
b) Another important reaction is the transformation of adenosine triphosphate (HATP3-) into adenosine diphosphate (HADP2-) as follows
HATP3-(aq) + H2O(l) --> HADP2-(aq) + H2PO4-(aq) [[Delta]]Go=32.9kJ/mole
Using the data at hand find [[Delta]]Go for the reaction
HATP3-(aq) + G(aq) --> G6P-(aq) + HADP2-(aq) [[Delta]]Go=?
[[Delta]]Go = 16.4-32.9=-16.5kJ/mol
c) Find the equilibrium constant Kc(298K). Will this coupled reaction favor products or reactants?
[[Delta]]Go = 789.2
17. a) How many Faradays are required to deposit 0.25g of Ni from a solution of Ni2+? Note: the atomic weight of Ni=58.71 g/mol.
N2+ + 2e- --> Ni
= 0.0086 Faradays
b) How much current in amps is required if the plating is to occur in 1.0 minutes
=13.7amps
18. For the overall reaction
Cr2O72-(aq) + 6I-(aq) + 14H+(aq) --> 2Cr3+(aq) + 3I2(s) + 7H2O(l) [[Delta]]Eo=0.795V n=6
a) Calculate [[Delta]]Go.
=-460 kJ/mol
b) At pH=0, [Cr2O72-]=1.5M and [I-]=0.40M the measured cell voltage is E=0.87V. Calculate the [Cr3+] in the cell.
[Cr2+] = 1.21 * 10-5M
19. A fuel cell working on the working reaction
C3H8(g) + 10O2(g) --> 3CO2(g) + 4H2O(l) [[Delta]]Eo=1.022V
may arise from the following half reactions
1) C3H8(g) + 20OH-(aq) --> 3CO2(g) + 14H2O + 20e- Eo1=??V
2) O2(g) + 2H2O(l) + 4e- --> 4OH- Eo2=0.401V
a) calculate E1 for the first half reaction
=0.621V
b) calculate [[Delta]]Go for the overall reaction.
=1972.5 kJ/mol
c) Write the expression for Q in terms of partial pressures. Will an increase in the C3H8(g) and O2(g) pressures cause an increase or decrease in the cell voltage?
p3CO2
Q = _____________________
pC3H8 * p5O2
we know that the cell voltage is
E=Eo-(RT/nF)lnQ
and so an increase of oxygen and C3H8 pressures will decrease Q making the cell voltage greater.
20. People derive energy from the oxidation of glucose.
C6H12O6(s) + 6O2(g) --> 6CO2(g) + 6H2O(l)
a) calculate [[Delta]]Go for this reaction using thermodynamic tables at the end
=2,870.04 kJ/mol
b) Determine the electrochemical potential, E; n=24, for this reaction.
=1.24V
c) Write a half reaction for the oxidation of glucose assuming the oxygen half reaction
O2(g) + 4H+(aq) + 4e- --> 2H2O(l)
Multiply this reaction by 6 and subtract it from the overall reaction.
12H2O(l) --> 6O2(g) + 24H+(aq) + 24e-
C6H12O6(s) + 6O2(g) --> 6CO2(g) + 6H2O(l)
C6H12O6(s) + 6H2O(l) --> 6CO2(g) + 24H+(aq) + 24e-
Or add
3O2(g) + 12H+(aq) + 12e- --> 6H2O(l)
to
C6H12O6(s) + 6O2(g) --> 6CO2(g) + 6H2O(l)
to get
C6H12O6(s) + 3O2(g) --> 6CO2(g) + 12H+ + 12e-
or C6H12O6(s) + 5O2(g) --> 6CO2(g) + 4H2O + 4H+ + 4e-, etc.
d) determine the half cell potential Eglu for the oxidation of glucose.
= 0.01V
21. In the following reactions indicate which are "spontaneous" as written. Place an "S" for a spontaneous reaction in the appropriate box and leave a blank for non-spontaneous.
reaction E(volt) S or -
2Al(s) + 3Cl2(g) --> 2AlCl3(s) 3.02 S
Cd(s) + Fe2+(aq) --> Cd2+(aq) + Fe3+ -0.04 -
glucose(aq)+H2PO4-->glucose-6-phosphate-(aq)+H2O(l) -0.17 -
NaCl(s) --> NaCl(aq) 0.0931 S
22. Cytochrome-c (CytC), important in biological electron transport,
contains the same iron binding group as hemoglobin. The half reaction for the
reduction of the ferric
(Fe3+) compound to the ferrous
(Fe2+) compound is:
CytC(Fe3+) + e- --> CytC(Fe2+) E1=0.22V
Cytochrome-a (CytA) is a similar, higher molecular weight compound having a half reaction:
CytC(Fe3+) + e- --> CytA(Fe2+) E2=0.22V
If these two reactions, occurring in adjacent parts of the membrane, are coupled to give the net reaction
CytA(Fe3+) + CytC(Fe2+) --> CytA(Fe2+) + CytC(Fe3+)
a) calculate the overall electrode potential.
=0.07V
b) calculate [[Delta]]G.
=-6.76Kj/mol
c) calculate the equilibrium constant.
=15.35
23. In rivers and streams the half reaction which converts dissolved oxygen to water is
O2(g) + 4H+(aq) + 4e- --> 2H2O(l) Eo=1.229V
for standard conditions of [H+]=1M and pO2=1atm. or [O2]=1.4*10-4molar. What value of E is expected if the oxygen concentration is standard at 1atm. but the hydrogen ion is [H+]=10-7?
=0.816V