Download Unit_6_Review_Guide_ANSWERS

CHEMISTRY – Mr. Channell
Name: __________________________
Unit 6- Redox Reactions and Electrochemistry Review Guide ANSWERS
1. What is an oxidation reduction reaction? What happens during these types of reactions?
Oxidation is a loss of electrons, while reduction is a gain of electrons. Electrons are
transferred between elements/compounds.
2. Determine the oxidation numbers for the element named in each of the following cases.
a. Copper in a copper penny
0
b. Copper in the compound Cu(SO4)
+2
c. Hydrogen in NaH2PO4
+1
d. Oxygen in N2O5
-2
e. Nitrogen in NH4+
-3
f. Rubidium in any compound
+1 (transition metal…use common ion
sheet!)
3. For each of the following reactions, tell whether it is a redox reaction, and if it is, name
the element oxidized, the element reduced, the oxidizing agent and the reducing agent.
+2 -2 +1
+1-2
+2 -2
+1 -2
a. Fe(OH)2 + H2S -----> FeS + 2H2O
+2 -4 +2 +2 -2
+2 -2
+2 -2
No change in oxidation numbers = no electron transfer = NO REDOX!
(double replacement reactions are never redox)
+4 -2
-3 +1 +1 -2
-3 +1 +4 -2 +1 -2
b. SnS2 + 2(NH4)HS -----> (NH4)2SnS3 + H2S
+4 -2
-3 +4 +1 -2
-6 +8 +4 -6 +2 -2
No change in oxidation numbers = no electron transfer = NO REDOX!
(double replacement reactions are never redox)
+1 -2 0
0
+1 -1
c. H2S + Br2 -----> S + 2HBr
+2 -2 0
0
+1 -1
Redox
Oxidized element = S
Reduded element = Br2
Reducing agent = H2S
Oxidizing agent = Br2
+2 -2
+1 +5 -2
+2 +5 -2
0
+2 -2
+1 -2
d. 3 CuS + 8 HNO3 -----> 3Cu(NO3)2 + 3S + 2NO + 4H2O
+2 -2
+1 +5 -6
+2 +10 -12
0
+2 -2
+2 -2
Redox
Oxidized element = S
Reduded element = N
Reducing agent = CuS
Oxidizing agent = HNO3
4. In the half-reaction method of balancing redox equations, what does each half-reaction
represent and how does it differ from the traditional balancing method?
Each half reaction represents either the oxidation (loss) or the reduction (gain) of
electrons.
5. Alkali metals displace hydrogen from water according to the following general reaction:
0
+1 -2
+1 -2 +1 0
2X + 2H2O -----> 2XOH + H2
0
+2 -2
+1 -2 +1 0
Write half-reactions and a balanced equation for the reaction.
Oxidation = (2X  2X + 1e-)2
Reduction = H2 + 2e-  H2
6. Balance the following equation using half-reactions.
0
+3 +5 -2
+1 +5 -2
0
3Ag + Zr(NO3)3 -----> 3Ag(NO3) + Zr
0
+3 +15 -18
+1 +5 -6
0
Oxidation = (Ag  Ag+1 + 1e-)3
Reduction = Zr+3 + 3e-  Zr
7. At which electrode does oxidation occur? At which electrode does reduction occur?
Red Cat, An Ox
Reduction = cathode
Oxidation = anode
8. Write the two half-reactions for each of the following voltaic cells. Identify the anode and
cathode reactions. What is the direction of electron movement in the circuit?
a. Mg | Mg2+ || Hg2+ | Hg
Anode (Oxidation…loss of electrons) = Mg  Mg+2 + 2eCathode (Reduction…gain of electrons) = Hg+2 + 2e-  Hg
Electrons move from Mg (loss) to Hg (gain)
b.
Cd | Cd2+ || Br2 | Br1-
Anode (Oxidation…loss of electrons) = Cd  Cd+2 + 2eCathode (Reduction…gain of electrons) = 2Br-1 + 2e-  Br2
Electrons move from Cd (loss) to Br (gain)
9. Calculate the cell potential for the following reaction. Is the reaction spontaneous? Why
or why not?
Fe + Ni+2 <-----> Fe+2 + Ni
Ni = reduced Ni+2 + 2e-  Ni
Fe = oxidized Fe  Fe+2 + 2eEv = Er – Eo
(-.25) – (-.44) = +.19 V
Positive Ev = SPONTANEOUS
10. What is the maximum voltage that you could expect from a cell that consisted of Al | Al3+
and Ag | Ag1+ half cells? Write the electrode reactions and the overall reaction.
Ag = reduced (Ag+1 + 1e-  Ag) 3
Al = oxidized Al  Al+3 + 3eOverall reaction = Al + 3Ag+1  Al+3 + 3Ag
Ev = Er – Eo
(+.80) – (-1.66) = +2.46 V
11. Calculate the cell potential and overall equation for the following reaction given the
following two standard reduction potentials.
Ba2+ + 2e- -----> Ba
Cu1+ + e- -----> Cu
Ev = Er – Eo
Eo= -2.92
Eo= 0.52
(+.52) – (-2.92) = +3.44 V
Overall equation = Ba+2 + 2Cu+1  Ba + 2Cu
Note: You must double your number of Copper atoms to balance the
charges (You need 2Cu+1 to equal Ba+2)
12. Match the definition with the correct term on the right.
a..
electrochemical cells that are used to
convert chemical energy into electrical energy
b
salt bridge
b.
tube containing a conducting solution
h
half cell
c.
electrode at which reduction occurs
d
cell potential
a
voltaic cell
difference between the reduction potentials
of the two half-cells
c
cathode
measurement of the tendency of a given
half-reaction to occur as a reduction
g
anode
f.
ability of the cell to produce an electric current
f
electrical potential
g.
electrode at which oxidation occurs
e
reduction potential
d.
e.
h. one part of a voltaic cell in which either
oxidation or reduction occurs
13.) If given two metals, draw a picture of an electrochemical cell and include the following
in your picture:
Two half cells, electrodes, conducting solutions, salt bridge and compound, labels for anode
and cathode, the oxidized and reduced, the half cell reactions, the overall reaction (balanced
using the half reaction method). Also find the voltage potential. Practice with Cobalt and
Magnesium.
Cathode: Co+2 + 2e-  Co
Anode: Mg  Mg+2 + 2eOverall: Co+2 + Mg  Co + Mg+2
Ev = Er – Eo
(-.28) – (-2.37) = + 2.09 V
Co
Co(SO4) – Cathode (reduced)
Mg
Mg(SO4) – Anode (oxidized)