WEB review answers
1) Which most readily gains electrons?
Cu Cu+2 Fe+2 Zn+2 Au+3 most readily gains electrons
2) Which most readily loses electrons?
Hg(l) Cu+2 Sn+4 Ba most readily loses electrons Al
Calculate the cell potentials or voltages (E0). Indicate spontaneity.
3. Cl2 + 2Br- -----> 2Cl- + Br2 E0= + 0.27 v spontaneous
4. 2MnO4- + 5Pb + 16H+ -----> 2Mn+2 + 8H2O + 5Pb+2 E0= + 1.64 v spontaneous
5. Will AgNO3 react with Zn? Write a balanced redox reaction and calculate E o
2Ag+ + Zn ------> 2Ag + Zn+2 E0= + 1.56 v spontaneous Yes!
6. What would happen if you used an iron spoon to stir a soulution of Al 2(SO4)3(aq) ? Write a balanced
redox reaction and calculate Eo.
3Fe + 2Al+3 ------> 3Fe+2 + 2Al E0= - 1.21 v nonspontaneous No reaction!
7) What are the differences between an electrochemical cell and an electrolytic cell?
electrochemical cell
electrolytic cell
Produces electricity
Consumes electricity
Spontaneous
Nonspontaneous
Salt bridge Two beakers
No salt bridge One beaker
Metal Electrodes
Inert C or Pt Electrodes
8) What are the similarities between an electrochemical cell and an electrolytic cell?
electrochemical cell or electrolytic cell
The Anode is the site of oxidation and the cathode is the site of reduction.
Anions migrate to the anion and cations migrate to the cathode.
Both have redox reactions.
Electrons flow through a wire from the anode to the cathode
9) State how you would determine each of the following in an electrochemical or electrolytic cell.
Electrochemical Cell
Electrolytic Cell
The site of reduction
Highest on Reduction Chart
The Negative Electrode
The site of oxidation
Lowest on Reduction Chart
The Positive Electrode
The +ve electrode
Site of Reduction
Connected to the power supply +ve
The -ve electrode
Site of Oxidation
Connected to the power supply -ve
The anions migrate to the
Anode
Anode
The cations migrate to the
Cathode
Cathode
The electrode that gains mass
Cathode
Neither inert electrodes
The electrode that loses mass
Anode
Neither inert electrodes
The electrons flow from
Anode to Cathode
Anode to Cathode
10.a) Draw an operating electrochemical cell using an Al half-cell and a Mg half-cell. Label the parts of the
electrochemical cell including the anode or cathode, and all reagents and materials used. Write the reactions
and determine the E0.
Cathode Reaction: 2(Al+3 + 3e- ------> Al ) E0= - 1.66
Anode Reaction: 3(Mg -------> Mg+2 + 2e- ) E0= +2.37
Redox Reaction: 2Al+3 + 3Mg -----> 3 Mg+2 + 2 Al E0= +0.71
11. (a) Write the half reaction that occurs at each electrode during the electrolysis of aqueous 1 M NaI.
Anode : 2I- --------> I2 + 2e- E0= -0.54 v
Cathode :
2H2O
+
2e-
------>
H2
+
2OH-
E0= -0.41v
(b) What is the minimum required voltage for this process? E0= +0.95 v
12. (a) Write the half reaction that occurs at each electrode during the electrolysis of molton NaI.
Anode : 2I- --------> I2 + 2e- E0= -0.54 v
Cathode : Na+ + 1e- ------> Na E0= -2.71v
(b) What is the minimum required voltage for this process? E0= +3.25v
13. Aluminum is produced industrially from aluminum oxide, Al 2O3. Demonstrate your understanding of
this process by
(i) describing how the process is carried out: C electrodes are used in the electrolysis of molten Al2O3.
(ii) writing equations of the reactions involved in the process, and
Anode : 2O-2 --------> O2 + 4e- Cathode : Al+3 + 3e- ------> Al
(iii) describing how the problem of the high melting point of Al 2O3 is overcome.
Mixing Al2O3 with cryolyte lowers the melting point from 2000º C to 900ºC, which is suitable for
electrolysis.
14. Consider the following redox data:
3V + 2Ga3+ ----> 3V2+ + 2Ga E=+0.64V
3V2+ + 2Al ----> 3V + 2 Al3+ E=+0.46V
Based on these observations, a student concludes that Ga +3 and Al will react spontaneously. List the
oxidizing agents in order of decreasing strength. Write reduction reactions for each. Determine the
strongest reducing agent. Determine if Ga +3 and Al will react spontaneously.
strongest oxidizing agent Ga3+ + 3e- ----> Ga
V2+ + 2e- ----> 3V
Al3+ + 3e- ----> Al strongest reducing agent
Ga+3 and Al will react spontaneously as Ga+3 is a stronger oxidizing agent than Al+3.
15. Balance the equation for the following half reaction occuring in acid solution:
7 H2O + 2V ----> HV2O7-3 + 13H+ + 10e16. Balance the following redox reaction occuring in basic solution:
4 H2O + 2 MnO4- + 3 C2O4-2 -----> 2 MnO2 + 8 OH- + 6 CO2
17. 250ml .200M MnO4- reacts with excess SO3-2. How many grams of MnO2 are produced?
2MnO4- + 3SO3-2 + H2O -----> 2MnO2 + 3SO4-2 + 2OH- 4.35g MnO2
18. Determine the oxidation number for each bold atom.
MnO2 IO3- Cr2O7-2 C2O4-2 Al(NO3)3 NH4Cl NaH
+4
+5
HOOH
NO3-
-1
+5
+6
+3
H3PO4 Na2C2O4
+5
+3
+3
I2
0
-3
-1
N2O3 Pt(H2O)4+2
+3
+2
19. 250ml of .500M MnO4- are required to titrate a 100ml sample of SO3-2. Calculate the [SO3-2]
2MnO4- + 3SO3-2 + H2O -----> 2MnO2 + 3SO4-2 + 2OH- [SO3-2] = 1.88M
20. How is the breathalyzer reaction used to determine BAC? Write the reaction and describe how it works.
BAC is blood alcohol concentration which is related to the concentration of alcohol in the breath. The
alcohol in a breath sample reacts with Cr2O7-2 reducing the orange color of the solution.
8 H+ + Cr2O7-2 (orange) + 3C2H5OH ------> 2Cr+3 (green) + 3C2H4O + 7H2O
21. 2H+ + Mg-----> Mg+2 +H2
Determine the Oxidizing agent H+ and the Reducing agent Mg
22. Choose a suitable redox reactant to oxidize Cl- to ClO4- in a redox titration. MnO4- in acid
23. Describe as an electrochemical or electrolytic cell:
a) Fuel cell
electrochemical
b) Charging a car battery
electrolytic
c) Discharging a car battery
electrochemical
d) Ni plating
electrolytic
e) Industrial Al production
electrolytic
f) Cl2 production
electrolytic
g) Electrowinning
electrolytic
24) Which of the reactants is gaining electrons? Which of the reactants is the oxidizing agent?
Br2 (oxidizing agent and is gaining electrons) + SO2 +Na2SO4 +H2O --------> 2H2SO4 + 2NaBr
25) A student studied the following reactions and she recorded:
Pd+2 + Cu -------> Pd + Cu+2 spontaneous
Pd+2 + Au -------> no reaction
Pd+2 + Hg -------> no reaction
Au+3 + Hg -------> Au + Hg+2 spontaneous
List the oxidizing agents from strongest to weakest. List the reducing agents from strongest to weakest.
Predict if the reaction will occur.
Strongest Oxidizing agent Au+3 + 3e- -------> Au
Hg+2 + 2e- -------> Hg
Pd+2 + 2e- -------> Pd
Cu+2 + 2e- -------> Cu Weakest Reducing Agent
Au+3 + Cu ------------> Yes Spontaneous Reaction
26) Match each type of electrolytic cell with the example cell.
Electrowinning Pure Al is reduced at the cathode from molten bauxite (Al2O3).
Electroplating A silver anode oxidizes & Ag reduces on a Cu cathode
Electrorefining Pure Pb is reduced at the cathode while impure Pb oxidizes at the anode
27. List the anode, cathode, anode reaction , cathode reaction, and electrolyte for each commercial
electrochemical cell.
Cell
anode
anode reaction
cathode
cathode reaction
+4
-
electrolyte
Leclanche or
Common Dry Cell
Zn
Zn-->Zn+2 + 2e-
C/MnO2
Mn +1e ----->
Mn+3
NH4Cl
Alkaline Cell
Zn
Zn-->Zn+2 + 2e-
C/MnO2
Mn+4 +1e- ----->
Mn+3
KOH
Lead Storage or Car
Pb
Battery
Pb ---> Pb+2+ 2e-
PbO2
PbO2 + SO4-2 +
4OH-1 + 2e- ----->
PbSO4 + 2H2O
H2SO4
Fuel Cell
H2 + 2OH- --->
2H2O + 2e-
C
O2 + 2H2O +4e---KOH
-> 4OH-
C
28. Which of the above cells requires continuous input of O2 and H2 and is produced by Ballard Industries.
Fuel Cell
29. List the anode, cathode, anode reaction , cathode reaction, and electrolyte for each commercial
electrolytic cell.
Cell
anode anode reaction
cathode cathode reaction
electrolyte
Electrolysis of
C
Molten Al2O3
O-2 ---> O2 + 2e-
C
Al+3 + 3e- ---> Al
Molten Al2O3
Electrolysis of
C
Aqueous NaCl
2Cl- ----> Cl2 + 2e-
C
2H2O + 2e-----> H2 +
2OH-
Aqueous NaCl
Silver-plating
a Cu spoon
Ag ---> Ag+ + 1e-
Cu
Ag+ + 1e- ---> Ag
AgNO3(aq)
Pure
Pb
Pb+2 + 2e- ---> Pb
PbSiF6
Ag
Electrorefining
Impure
pure Pb from
Pb ---> Pb+2 + 2ePb
impure Pb
30. Describe each term:
salt bridge
U-tube filled with salt solution connecting two half cells in a electrochemical cell. Allows ions to migrate.
electrolyte
A solution that contains ions and conducts electricity.
anode
Metal electrode that is the site of oxidation.
cathode
Metal electrode that is the site of reduction.
spontaneous
Occurs naturally and has a positive Eo.
electronegativity
The ability to attract electrons.
cation
Positive ion.
anion
Negative ion.
electrochemical cell Two metals in electrolytes joined by a salt bridge using spontaneous redox reaction to produce an electrical c
electrolytic cell
A power supply connected to inert electrodes in an electrolyte which forces nonspontaneous redox reactions
oxidation number
The real or apparent charge of an atom if all bonds are assumed to be ionic.
electrolysis
What occurs in an electrolytic cell.
oxidation
Loss of electrons.
reduction
Gain of electrons.
oxidizing agent
Causes oxidation by undergoing reducing.
reducing agent
Causes reduction by undergoing oxidation.
electrode
Strip of C or Metal used in a cell.
corrosion
The oxidation of a metal often Fe ---> Fe+2 + 2e-
electrowinning
The reduction of a metal from its ore by electrolysis.
electrorefining
The purification of a metal by electrolysis.
overpotential effect
The higher than normal voltage for the oxidation and reduction of water.
fuel cell
An electrochemical cell that converts H2 and O2 into H2O and electricity.
31. Define corrosion of a metal, and illustrate your definition with reference to an example, using
appropriate equations. Give TWO methods by which corrosion can be prevented and describe how each
method works. The two methods must involve different chemical principles.
The corrosion of iron Cathode: Fe ----> Fe+2 + 2eAnode: O2 + 2H2O + 4e- ------> 4OH1) Painting prevents the reactants: O 2 + 2H2O from colliding with the surface of the metal.
2) Galvinizing allows Zn to corrode (anode) instead of Fe (cathode) due to the action of the chemical cell
they produce.
32. Explain why you would choose Zn or Cu to cathodically protect iron? Zn because it oxidizes more
readily than Fe forcing Fe to be the cathode or site of reduction.
33. A+2 does not react with B, while C+2 reacts with B. Rank the oxidizing agents in decreasing order of
strength. Rank the reducing agents in decreasing order of strength. Will A +2 react with C?
C+2 + 2e- --------> C Oxidizing agents: C+2 B+2 A+2
B+2 + 2e- --------> B Reducing Agents: A B C
A+2 + 2e- --------> A
A+2 will not react with C.
34. Write half reactions for each using the reduction table and list the half cell potential.
Half Reaction
Eo
oxidation of water
H2O --------> 1/2 O2 + 2H+ + 2e-
-0.82v
oxidation of water in
acid
2 H2O --------> 1/2O2 + 2H+ + 2e-
-1.23v
reduction of water
2 H2O + 2e- ------------> H2 + 2OH-
-0.41v
reduction of water in
alkaline
2 H2O + 2e- ------------> H2 + 2OH-
-0.83v
oxidation of H2 in water
H2 + 2OH- ------------> 2 H2O + 2e-
+0.41
oxidation of H2 in acid
H2 ------------> 2H+ + 2e-
0.00v
oxidation of H2 in base
H2 + 2OH- ------------> 2 H2O + 2e-
+0.83
reduction of Cr2O7-2 in
acid
Cr2O7-2 + 14 H+ + 6 e- -------> 2Cr3+ + 7 H2O
+1.23v
reduction of HBr
2H+ + 2e- ------------> H2
0.00v
35.Completely analyze the following electrochemical cell.
The anode reaction is:
Zn -----------> Zn+2 + 2e-
The cathode reaction is:
Cu+2 + 2e- -------> Cu
The electrons flow from ___ to ___
anode to cathode
The ions that migrate to the Zn electrode are:
NO3-
The ions that migrate to the Cu electrode are:
Zn+2 Cu+2 K+
The intial voltage of this cell is:
1.10v
The voltage of this cell once equilibrium is reached is:
0.00v
Describe the change in [Cu+2] in the Cu half cell
decreases
Describe the change in [NO3-1] in the Zn half cell
increase
36. Completely analyze the following electrochemical cell.
38.
The anode reaction is:
H2 ------------> 2H+ + 2e-
The cathode reaction is:
Cu+2 + 2e- -------> Cu
The electrons flow from ___ to ___
anode to cathode
The ions that migrate to the Pt electrode are:
NO3- Cl-
The ions that migrate to the Cu electrode are:
Cu+2 K+ H+
The intial voltage of this cell is:
0.34v
The voltage of this cell once equilibrium is reached is:
0.00v
Describe the change in [Cu+2] in the Cu half cell
decreases
Describe the change in [NO3-1] in the H+/H2 half cell
increases
Completely analyze the following electrolytic cell. Note that the electrodes are not inert and because
of that, the anode might oxidize.
Anode Reaction
Cu  Cu2+ + 2ev
E0 = -0.34
Cathode Reaction
2H2O + 2e-  H2 + 2OHv
E0 = -0.41
Chemicals produced at the anode
Cu2+
Chemicals produced at the cathode
H2
OH-
The electrons flow from
Anode (right) to cathode (left)
The MTV
+0.75 v
Which electrode is the anode ?
right
38. Completely analyze the following electrolytic cell.
Anode Reaction
2O-2 ---------> O2 +4e-
Cathode Reaction
Al+3 + 3e- ---------> Al
Chemicals produced at the anode O2
Chemicals produced at the
cathode
Al
The electrons flow from __to __
anode to cathode
The chemical used to lower the
mp is:
cryolite
Which electrode is the anode ?
the one on the right