UNIT 7 Assignment “Electrochemistry” (Chap 5-pg 193-229 & Chap 19-pg 845-899)
7.1. Assign Oxidation numbers to the elements in bold print.
1) Mg
2) P4
3) Cl
8) AlF3
9) P2O3
15) CO
16) CO3
22) H2O2
23) H2S
2–
–
4) HCl
5) CaO
6) Na2O
7) Fe2O3
10) PF3
11) SO2
12) PCl5
13) ThO2
14) CO2
17) SnS2
18) SO4
20) AlN
21) Fe3O4
–
24) ClO
2–
2–
19) PO4
3–
25) NaClO2
26) KClO3
27) Mg(ClO4)2
28) LiMnO4
32) NO
33) NO2
34) N2O3
35) N2O5
39) NH3
40) N2O
41) Ca(VO3)2
42) MnO2
47) NaHCO3
48) BrCl
49) SCl2
29) K2CrO4 30) Na2Cr2O7
31) SO3
36) N2H4
37) N2
38) NH2OH
43) CHCl3
44) C2H5OH
45) C12H22O11 46) CaCO3
7.2. Balance the following ionic eq'ns taking place in acid solution using oxidation #'s
–
2+
+
2+
+
H
→
Mn
+
H
+
→
Mn
Fe
+
H
+
→
Cr
+
Cu
+
H
+
→
Cu
+
Cu
+
H
+
→
Cu
+
Zn
+
Fe
+
H2C2O4 +
Fe3O4
+
H2S
10)
CeO2
+
Cl
11)
NO3
+
Cu
12)
NO3
+
Fe
13)
NO3
+
Zn
1)
MnO4
2)
MnO4
3)
Cr2O7
4)
NO3
5)
SO4
6)
NO3
7)
H2SO3
8)
MnO4
9)
–
2–
–
2–
–
–
–
–
–
+
Fe
+
Sn
+
2+
2+
3+
–
2+
2+
3+
2+
2+
+
H
+
→
Zn
+
H2O
→
Fe
+
→
Mn
→
H
FeS
+
2+
+
+
→
Cu
+
→
Fe
+
→
Zn
+
H
+
H
H
2–
3+
Ce
H
2+
SO4
→
+
+
2+
2+
3+
2+
3+
+
Fe
+
Sn
+
+
4+
H2O
+
H2O
Fe
+
H2O
+
NO
+
H2O
+
SO2
+
NH4
+
SO4
+
CO2
+
H
+
Cl2
+
H2O
+
NO2
+
H2O
+
NO
+
H2O
+
N2
3+
H2O
+
H2O
+
2–
+
+
+
+
+
+
H
H2O
H2O
+
H2O
14)
NO3
15)
–
+
H2S
Fe
+
NH3OH
16)
HNO2
+
C2O4
17)
MnO4
+
Br
18)
C2H3OCl
+
Cr2O7
19)
CHCl3
+
MnO4
20)
SO4
+
I
21)
MnO4
+
Fe3O4
+
22)
CrO2
+
H2O2
+
23)
CrO4
+
HSnO2
24)
CrO2
+
S2O8
3+
–
2–
–
–
2–
–
+
+
2–
–
2–
+
H
+
→
→
S
+
Fe
+
N2O
+
→
CO2
+
NO
+
H2O
+
→
Br2
+
MnO2
+
H2O
+
→
Cr
+
CO2
+
Cl
+
→
Cl2
+
CO2
+
Mn
+
→
I2
+H2S
+
H2O
+
→
Fe
+
Mn
→
CrO4
H
+
H
+
H
+
H
+
H
–
–
2–
2+
–
H
–
OH
3+
H2O →
+
+
3+
–
CrO2
→
OH
2–
CrO4
NO2
+
+
+
H
2+
H2O
+
HSnO3
2–
+
SO4
+
+
+
–
H2O
2–
–
H2O
–
+
2+
+
H2O
H2O
H2O
–
+
OH
+
H2O
7.3: Balance the following stoichiometric equations using oxidation numbers
1)
Cu
+
HNO3
→
Cu(NO3) 2
+
NO
+
H2O
2)
Cu
+
HNO3
→
Cu(NO3) 2
+
NO2
+
H2O
3)
MnO2
→
MnCl2
+
Cl2
+
H2O
4)
Pb3O4
+
HCl
→
PbCl2
Cl2
+
H2O
5)
KMnO4
+
HCl
→
KCl
6)
K2CrO4
+
HBr
→
KBr
7)
H2S
+
HNO3 →
8)
HNO3
+
HCl
9)
Sb2 (SO4) 3
+
+
HCl
KMnO4
+
→
H2O →
+
+
+
NO2
NO
MnCl2
CrBr3
+
S
+
H3SbO4 +
+
+
Br2
+
Cl2
K2SO4 +
Cl2
+
+
H2O
H2O
H2O
+
H2O
MnSO4
+
H2SO4
10)
CaC2O4 +
KMnO4 +
H2SO4
→
CaSO4
11)
FeSO4 +
KMnO4 +
H2SO4
→
Fe2 (SO4) 3 +
12)
KIO3
+
→
KCl
13)
KI +
KMnO4 +
14)
I2
HNO3
15)
K2Cr2O7
+
KI +
+
HCl
H2SO4
→
HCl
HIO3
→
→
I2
+
K2SO4 +
+
+
NO2
KCl
+
K2SO4 +
I2
+
K2SO4
+
CrCl3
MnSO4 +
H2O +
MnSO4
+
+
CO2
H2O
H2O
+
MnSO4
+
H2O
H2O
+
Cl2
+
H2O
7.4. Drawing electrochemical cells. (19.5→
→pg 866) On each diagram a) label the anode & cathode b) show the
electron flow c) show the movement of all ions in the reactions & the salt bridge.
Ag / Ag+ //sb// Zn2+ / Zn
v
A
Ag / Ag+ //sb// Au3+ / Au
v
B
A
B
1/2 rx(A) ......................................................................E°=
........v
1/2 rx(A) ..............................................................................E°=
........v
1/2 rx(B) ......................................................................E°=
........v
1/2 rx(B) ..............................................................................E°=
........v
.......................................................................E°=
........v
cell rx .............................................................................. E°=
........v
cell rx
Al / Al3+ //sb// Mg2+ / Mg
v
A
Cd / Cd2+ //sb// Ni2+ / Ni
v
B
A
B
1/2 rx(A) ......................................................................E°=
........v
1/2 rx(A) .............................................................................E°=
........v
1/2 rx(B) ......................................................................E°=
........v
1/2 rx(B) .............................................................................E°=
........v
......................................................................E°=
........v
cell rx ............................................................................. E°=
........v
cell rx
–
–
Ag / Ag+ //sb// Cl / Cl2
v
A
Cr / Cr3+ //sb// I / I2
v
B
A
B
1/2 rx(A) ......................................................................E°=
........v
1/2 rx(A) ................................................................................E°=
........v
1/2 rx(B) ......................................................................E°=
........v
1/2 rx(B) ...............................................................................E°=
........v
.......................................................................E°=
........v
cell rx ........................................................................... E°=
........v
cell rx
–
–
–
F2 / F //sb// Li+ / Li
v
I2 / I //sb// Cl / Cl2
v
A
B
A
B
1/2 rx(A) ......................................................................E°=
........v
1/2 rx(A) ...................................................................................E°=
........v
1/2 rx(B) ......................................................................E°=
........v
1/2 rx(B) ...................................................................................E°=
........v
.......................................................................E°=
........v
cell rx
*Practical applications of EC Cells – Batteries
7.5. Attempt questions a) 19.45
cell rx ..................................................................................
E°= ........v
(19.9→pg 885)
b) 19.48
& c) 19.51 on pg 893
* Calculating Cell Potentials / E° values
7.6. Attempt questions a) 19.35
b) 19.36 on pg 893
–
7.7. Which of the following substances will oxidize Br to Br2 ?
–
7.8. Which of the following substances will reduce Br2 to Br ?
7.9. Attempt questions a) 19.72
7.10 Attempt questions a) 19.74
h) 19.84
+
a) Cl2 b) H
a) Cl
–
c) Ni
b) H2 c) Ni
2+
d) MnO4
–
2+
d) Mn
b) 19.73 on pg 894
b) 19.75 c) 19.76
d) 19.78 e) 19.79
i) 19.85
k) 19.87
j) 19.86
f) 19.82
g) 19.83
on pg 894-895
3+
7.11. If a piece of copper metal is dipped into a 1 M Cr sol'n , what will happen?
7.12. What will happen if an aluminum spoon is used to stir a sol'n of Fe(NO3)2? What kind of spoon would be better?
7.13. Can a 1 M Fe2(SO4)3 sol'n be stored in a container made of nickel ?
7.14. For each of the following write the equation for the reaction that is most likely to occur and include E°
( all sol'n are 1 M )
–
–
a) A mixture of powdered I2 & liquid Br2 are placed in a sol'n containing both I & Br .
b) A tin strip is immersed in HNO3 (aq)
c) A mixture of powdered Al and Fe is added to a sol'n of Cr(NO3)3
d) A copper rod is immersed in HCl (aq) through which is bubbled O2 (g)
*The Effect of Concentration on Cell Potentials
7.15. Attempt questions a) 19.96 b) 19.97 c) 19.98 & d) 19.99 on page 895-96
*Cell Potentials and Thermodynamics
7.16. Attempt questions
a) 19.90 b) 19.91 & c) 19.92 d) 19.93 e) 19.94 f) 19.95 on page 895
* Electrolysis
7.17. Attempt questions a) 19.10 b) 19.54 c) 19.55
d) 19.56
e) 19.57 f) 19.58
g) 19.59
h) 19.62 i) 19.65 j) 19.103 on page 893-94
7.18 ** Summary question - 19.110 on pg 897
Lab 7 - 1.
A REDOX Titration
Purpose: To determine the Molarity of an unknown ferrous solution using a REDOX titration then
determine the molar mass of the hydrated ferrous salt and determine its complete formula.
1. Prepare 250 mL of approximately 0.01 M KMnO4 solution. Record the conc'n
2. Transfer 10 mL of the ferrous salt sol'n to an erlenmeyer flask. Add 5 mL of sulfuric acid /85%
phosphoric acid mixture. Wash this sol'n with 10 mL of distilled water
–
3.
Add the KMnO4 sol'n to the burette and begin to titrate. ** The MnO4 ion will be
discolourized by the ferrous ion so it should be added only as fast as the ion can bleach out the
–
2+
colour. The correct end-point is a light pink colour of MnO4 as the Fe is all used up.
4. Repeat as necessary
5. a) Write an equation for the reaction
–
–
b) Using
# mol e created = # mol e consumed
Mox * Vox * nox = Mred * Vred * nred
Calculate the molarity of the unknown ferrous sol'n
6. Given that the sol'n was made by
a) dissolving ……………. of ferrous diammonium disulfate.x hydrate in ……..…. of water, or
b) dissolving ……………. of ferrous sulfate .x hydrate in ……..…. of water,
calculate the molar mass of the ferrous salt , then the # moles of water on the hydrate
7. Write a complete lab report with 2 sets of data & do a % error calculation
Lab questions..
Attempt questions
a) 5.62
h) 5.69
b) 5.63
i) 5.82
c) 5.84 d) 5.58
on page 225-227
e) 5.64
f) 5.65
g) 5.67
Lab 7 -2. A REDOX Titration
Determining the % sodium hypochlorite in 2 commercial chlorine bleaches by redox titration
Javex bleach is a descendant of Eau de Javelle which was first prepared in France. It is made by
the following chemical reaction
Cl2 + 2 NaOH → NaOCl + NaCl + H2O
The amount of sodium hypochlorite, the active ingredient in Javex bleach, can be determined by
carrying out a redox titration. The titration involves measuring out an aliquot of bleach (diluted),
treating it with an excess of KI in the presence of acetic acid, and titrating the iodine set free with
standardized sodium thiosulfate solution. The following reactions are involved:
+
–
–
–
2 H + OCl + 2 e
→ Cl + H2O
reduction
–
–
2I
→ I2 + 2 e
oxidation
net reaction
The iodine that is produced (the yellow-brown colour) is then titrated against sodium thiosulfate
until the I2 is reduced back to I– which is colourless.
2–
–
2–
I2 + 2 S2O3
→ 2I
+ S4O6
THE EXPERIMENT:
1. Make up 250 mL of 0.1 M sodium thiosulfate pentahydrate.
2. Carefully measure out 10 mL of bleach and dilute to 100 mL in a volumetric flask
3. Pipet 10 mL of diluted bleach into your titration flask and add 20 mL distilled water.
4. Add 5 mL of 0.60 M KI solution then swirl in 5 mL of 40% (v/v) acetic acid sol’n
5. Immediately titrate the contents of the flask against the thiosulfate solution.
6. Repeat at least twice for accuracy !!
Calculate
a) the # moles of sodium thiosulfate used in the titration
b) the # moles of I2 produced in the redox reaction
c) the # moles of hypochlorite ion that was in the diluted bleach
d) the mass of sodium hypochlorite that was in the undiluted bleach
e) If the density of the bleach is 1.086 g/mL, calculate the mass/mass % of sodium
hypochlorite in the bottle
7. Write a complete lab report with 2 sets of data
Lab question: Bordeaux mixture is tradionally prepared by mixing copper(II) sulfate with calcium hydroxide
in water. The resulting sol’n is then sprayed on trees & shrubs to fight fungal diseases. In the
analysis of one commercial preparation, a sample weighing 0.238 g was dissolved in HCl.
Excess KI(aq) was then added and iodine was formed thusly: 2 Cu2+ + 4 I2 CuI + I2
The iodine was then titrated with 0.01669 M Na2S2O3 (aq) using starch as the indicator (see rx
equation above). The titration required 28.62 mL of the thiosulfate sol’n.
What was the % (by weight) of copper in the Bordeaux mixture?
Electrochemistry Assignment
1. An unknown metal M forms a soluble
3. 10 Br– + 2 MnO4– + 16 H+
compound, M(NO3)2.
(a) A solution of M(NO3)2 is electrolyzed. When a
constant current of 2.50 amperes is applied for
35.0 minutes, 3.06 grams of the metal M is
deposited. Calculate the molar mass of M and
identify the metal.
(b) The metal identified in (a) is used with zinc to
construct a galvanic cell, as shown below.
Write the net ionic equation for the cell
reaction and calculate the cell potential, E˚ .
K
Metal
M
+
Cl
5 Br2 (l) + 2 Mn2+ +8 H2O
The above rx is set up as a working
electrochemical cell with a 0.64 M sol’n of KBr
acting as the anode and an acidified 0.05 M
KMnO4 sol’n in a 2x10–4 M MnSO4 sol’n
acting as the cathode. The voltage was measured
at 0.35 v. What is the pH of the acidified
cathodic sol’n??
4.
-
Br2 (l) + 2 Fe2+ (aq)
2 Br– (aq) + 2 Fe3+ (aq)
For the rx above, the following data are available:
2 Br– (aq) Br2 (l) + 2e–
E˚ =–1.06 volts
2+
3+
Fe (aq) Fe (aq) + eE˚ =–0.77 volts
Zinc
Br2 (l) 245.4
Br– (aq) 82.0
1.0 M ZnSO4
1.0 M M(NO3)2
(c) Calculate the value of the standard free energy
change, G˚ , at 25˚ C for the reaction in (b).
(a) Determine
(c) Determine
(d) Calculate the cell potential, E, for the cell
shown in (b) if the initial concentration of
ZnSO4 is 0.004 molar, and the concentration of
the M(NO3)2 solution is 650 molar.
S˚ (J/mole.K)
Fe2+ (aq) –113.4
Fe3+ (aq) –293.3
S˚ (b) Determine
H˚
5. a) How many coulombs are required to plate a
2. A direct current of 0.125 ampere was passed
through 200 mL of a 0.25 M solution of Fe2(SO4)3
between platinum electrodes for a period of 1.10
hours. Oxygen gas was produced at the anode.
The only visible change at the cathode was a
change in the color of the sol’n from dirty yellow
to greenish.
*At the end of the electrolysis, the electrolyte was
acidified with sulfuric acid and was titrated with
an aqueous solution of potassium permanganate.
The volume of the KMnO4 solution required to
reach the end-point was 24.65 mL.
(a) How many faradays were passed through the sol’n?
(b) How many moles of ferric ion were reduced ?
(c) Write a balanced net ionic equation for the rx
that occurred during the titration with
potassium permanganate.
(d) Calculate the molarity of the KMnO4 solution.
layer of chromium metal 0.23 mm thick on an
auto bumper with a total area of 0.32 m2 from
a sol’n containing CrO42- ??
( the density of chromium metal is 7.2 g/cm3 )
b) What current flow is required for this
electroplate if the bumper is to be plated in 1
min ??
6. Write a balanced net ionic rx equation for the
following. Include the E° value.
a) a sol’n of potassium dichromate is added to
an acidified sol’n of ferrous chloride.
b) A concentrated sol’n of hydrochloric acid is
added to solid potassium permanganate.
c) Nitrogen dioxide gas is bubbled into an
aqueous solution of bromine.
d) A solution of silver sulfate is electrolyzed
Bonus: Balance the following using oxidation numbers
[Cr(N2H4CO)6]4[Cr(CN)6]3 + KMnO4 + H2SO4
G˚
K2CrO7 + MnSO4 + CO2 + KNO3 + K2SO4 + H2O
SCH 4U - QUIZ A - BALANCING WITH OXIDATION NUMBERS
Balance the following reaction equations showing all relevant oxidation numbers, indicate which
species has been oxidized and reduced as well indicate the number of moles of electrons transferred
(n).
NaNO3 +
HCl +
S
→
SO2 +
NO +
NaCl +
H2O
n = ________
–
OH
+
Sb +
3–
H2O →
HSb6O17
+
H2
n = ________
2–
Cr2O7
+
C2H3OCl +
+
H
→
–
Cl +
CO2 +
Cr
3+
+
H2O
n = ________
KMnO4 +
Bi2(SO4)3 +
H2O →
K2SO4 +
H3BiO4 +
MnSO4 +
H2SO4
n = _________
SCH 4U - QUIZ B - BALANCING WITH OXIDATION NUMBERS
Balance the following reaction equations showing all relevant oxidation numbers, indicate which
species has been oxidized and reduced as well indicate the number of moles of electrons transferred
(n).
–
Cr +
MnO4 +
+
H
2+
→
Mn
+ Cr
3+
+
H2O
n = ___________
HIO3 +
FeI2
+
HCl →
FeCl3
+
ICl
+
H2O
n = ___________
3+
Sb
+
–
MnO4 +
H2O
→
SbO4
3–
+
2+
Mn
+
+
H
n = ___________
K2Cr2O7 +
H2O +
S →
SO2 +
Cr2O3 +
KOH
n = ___________
SCH 4U - QUIZ C - BALANCING WITH OXIDATION NUMBERS
Balance the following reaction equations showing all relevant oxidation numbers, indicate which
species has been oxidized and reduced as well indicate the number of moles of electrons transferred
(n).
2–
Cr2O7
+
2+
Mn
+
H →
+
MnO4
–
+
Cr
3+
+
H2O
n = ___________
NO3
–
+
Fe
+
+
+
H
→
+
NH4 +
Fe
3+
+
H2O
n = ___________
MgC2O4 + NaMnO4 +
H2SO4 → MgSO4 + CO2 + Na2SO4 + MnSO4 +
H2O
n = ___________
K2Cr2O7 +
C2H3OCl +
HCl →
CO2 +
CrCl3 +
KCl +
H2O
n = ___________