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Name:__________________________________________________
Date:___________
Class:_________
Mr. Wolin (Science)
Review:Redox Reactions and Electrochemistry
What is a redox reaction
Example of redox
Define oxidation
Define reduction
Define redox
Reducing agents
Oxidizing agents
Rules for assigning oxidation numbers
Recognizing redox reactions
free metals that become combined
change of oxidation states
double replacement reactions are not redox
Determine most reactive metal
Substance oxidized is higher on the table
Half reactions
Writing half-reactions
Balancing half-reactions
Spontaneous redox reactions
voltaic cells
Identifying anode
Identifying cathode
Site of oxidation
Site of reduction
Identify direction of electron flow
Non-spontaneous redox reactions
electrolytic cells
electrolysis of salt
Electroplating
Site of oxidation
Site of reduction
Compare and contrast voltaic and electrolytic cells
PART 1: Multiple Choice [68%]
Zn(s) + 2 HCl(aq)  ZnCl2(aq) + H2(g)
Which statement correctly describes what occurs when this reaction takes place in a closed system?
(A) Atoms of Zn(s) lose electrons and are oxidized.
(B) Atoms of Zn(s) gain electrons and are reduced.
(C) There is a net loss of mass.
(D) There is a net gain of mass.
1)
Given the reaction:
2)
What happens during reduction?
(A) an atom loses an electron (B) an atom gains an electron
3)
(C) oxidation
(D) cathode
Given the reaction:
Which species undergoes reduction?
(A) Al (B) Fe (C) Al+3 (D) Fe+3
4)
In the reaction: 4 NH3 + 5 O2  4 NO + 6 H2O
the oxidation number of nitrogen changes from
(A) –2 to –3 (B) –2 to +3 (C) –3 to –2 (D) –3 to +2
5)
Which equation represents an oxidation- reduction reaction?
(A) CH4 + 2O2  CO2 + 2H2O
(B) H2SO4 + Ca(OH)2  CaSO4 + 2H2O
(C) MgCrO4 + BaCl2  MgCl2 + BaCrO4
(D) Zn(NO3)2 + Na2CO3  2NaNO3 + ZnCO3
6)
Which reaction is an example of an oxidation-reduction reaction?
(A) AgNO3 + KI  AgI + KNO3
(B) Cu + 2 AgNO3  Cu(NO3)2 + 2 Ag
(C) 2 KOH + H2SO4  K2SO4 + 2 H2O
(D) Ba(OH)2 + 2 HCl  BaCl2 + 2 H2O
7)
Given the redox reaction:
Cr3+ + Al  Cr + Al3+
Which species is the reducing agent?
(A) Cr3+ (B) Al0 (C) Cr0 (D) Al3+
8)
Which is the most reactive metal?
(A) Cu (B) Mg (C) Zn (D) Au
9)
Which metal will be oxidized by the other three?
(A) Ba (B) Na (C) Zn (D) Pb
10)
When an equation is correctly balanced, it must show conservation of
(A) charge but not of mass
(C) both charge and mass
(B) mass but not of charge
(D) neither charge nor mass
11)
Which half-reaction equation represents the reduction of a potassium ion
(A) K+ + e–  K
(B) K + e–  K+
(C) K+  K + e–
(D) K  K+ + e–
12)
Given the redox reaction: Cr3+ + Al  Cr + Al3+
As the reaction takes place, there is a transfer of
(A) electrons from Al to Cr3+
(C) protons from Al to Cr3+
(B) electrons from Cr3+ to Al
(D) protons from Cr3+ to Al
13)
Which half-reaction shows conservation of charge?
(A) Cu + e–  Cu+
(B) Cu2+ + 2e–  Cu
(C) Cu+  Cu + e–
(D) Cu2+  Cu + 2e–
14)
How many moles of electrons are needed to reduce 1 mole of Fe3+ to Fe2+?
(A) 1 (B) 2 (C) 3 (D) 5
15)
What two things must be balanced in a half reaction?
(A) anode and cathode (B) electrons and protons (C) An Ox and the Red Cat
(D) mass and charge
16)
Which statement describes the redox reaction that occurs when an object is electroplated?
(A) It is spontaneous and requires an electric current.
(B) It is spontaneous and produces an electric current.
(C) It is non-spontaneous and requires an electric current.
(D) It is non-spontaneous and produces an electric current.
17)
In an electrolytic cell, the positive electrode is the
(A) anode, where oxidation occurs
(C) cathode, where oxidation occurs
(B) anode, where reduction occurs
(D) cathode, where reduction occurs
18)
The diagram below shows a key being plated with copper in an electrolytic cell
Given the reduction reaction for this cell:
Cu2+(aq) + 2e-  Cu(s)
This reduction occurs at
(A) A, which is the anode
(B) A, which is the cathode
(C) B, which is the anode
(D) B, which is the cathode
19)
Which process occurs at the anode in an electrochemical cell?
(A) the loss of protons (B) the loss of electrons (C) the gain of protons
(D) the gain of electrons
20)
Which type of electrochemical cell does not require an external power source to carry out the redox reaction?
(A) voltaic cell
(C) electroplating cell
(B) electrolytic cell
(D) all electrochemical cells require a power source
21)
An electrolytic cell differs from a chemical (or voltaic) cell in that the electrolytic cell
(A) produces an electric current
(B) uses an applied electric current
(C) involves oxidation and reduction reactions
(D) occurs spontaneously
22)
Electroplating always takes place
(A) in the wire (B) at the anode
(C) at the cathode
(D) at the salt bridge
(E) in the voltage source
23)
Which of the following reactions is redox?
(A) KCl + AgNO3  AgCl + KNO3
(B) 2 HCl + CaCO3  CaCl2 + H2O + CO2
(C) 2 KMnO4 + 3 Na2SO3 + H2O → 2 MnO2 + 3 Na2SO4 + 2 KOH
(D) HCl + KOH  KCl + H2O
(E) BaCl2 + K2SO4  2 KCl + BaSO4
24)
Which metal is more reactive?
(A) Mg (B) Zn (C) Cu (D) Au
(E) Na
25)
Which element will replace H in HCl?
(A) Au (B) F2 (C) Cu (D) Br2 (E) Mg
26)
LEO the lion says…
(A) GRR (B) GUR
(C) GYR
(D) GER
27)
What is the oxidation number of Chlorine in HCl?
(A) +1 (B) -1 (C) -3 (D) +5
28)
What is the oxidation number of Sulfur in H2SO4?
(A) +1 (B) -2 (C) -3 (D) +6
29)
Given the reaction:
3 Sn4+ (aq) + 2 Cr(s)  3 Sn2+(aq) + 2 Cr3+(aq)
Which half-reaction correctly represents the reduction that occurs?
(A) Sn4+ (aq) + 2e–  Sn2+(aq)
(B) Sn2+(aq)  Sn4+ (aq) + 2e–
(C) Cr(s)  Cr3+(aq) + 3e–
(D) Cr3+(aq) + 3e–  Cr(s)
30)
Given the unbalanced ionic equation: 3Mg + __Fe 3+  3Mg2+ + __Fe
When this equation is balanced, both Fe3+ and Fe have a coefficient of
(A) 1, because a total of 6 electrons is transferred
(B) 2, because a total of 6 electrons is transferred
(C) 1, because a total of 3 electrons is transferred
(D) 2, because a total of 3 electrons is transferred
31)
The overall reaction in a electrochemical cell is Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s).
As the reaction in this cell takes place, the
(A)
(B)
(C)
(D)
mass of the Zn(s) electrode decreases
mass of the Cu(s) electrode decreases
Cu2+(aq) concentration remains the same
Zn2+(aq) concentration remains the same
32)
Base your answer to the following question on the diagram of the chemical cell at 298 K and on the equation
below.
In the given reaction, the Ag+ ions
(A) gain electrons
(B) lose electrons
(C) gain protons
(D) lose protons
33)
A student collects the materials and equipment below to construct a voltaic cell:
• two 250-mL beakers
• wire and a switch
• one strip of magnesium
• one strip of copper
• 125 mL of 0.20 M Mg(NO3)2(aq)
• 125 mL of 0.20 M Cu(NO3)2(aq)
Which additional item is required for the construction of the voltaic cell?
(A) an anode (B) a battery (C) a cathode (D) a salt bridge
34)
The diagram below represents a chemical cell at 298 K.
When the switch is closed, electrons flow from
(A) Al(s) to Ni(s)
(B) Ni(s) to Al(s)
(C) Al3+(aq) to Ni2+(aq)
(D) Ni2+(aq) to Al3+(aq)
For Teacher
Use Only
PART 2: Show all work (no work, no credit) [32%]
1) Assign the oxidation numbers for each element in Na2Cr2O7:
Na: +1
Cr: +6 O: -2
[2]
2) Write the balanced half reactions for the following equation: 3Mg(s) + 2Al+3  3Mg+2 + 2Al(s)
Oxidation half reaction: 3Mg0  3Mg+2 + 6e-
[2]
Reduction half reaction: 2Al+3 +6e-  2Al0
[2]
3) Fill in the missing part(s) of the half reaction, and write whether it is oxidation or reduction.
Sn0
 Sn2+ + 2e-
reaction type: OXIDATION
[2]
4) For each electrochemical cell shown:
a) Indication the type of electrochemical cell shown.
b) Indicate if the reaction that takes place is spontaneous or non-spontaneous.
a) Voltaic cell
a) Electroplating or Electrolytic
a) Electrolytic
b) spontaneous
b) non-spontaneous
b) non-spontaneous
[3]
For Teacher
Use Only
5) Base your answers to questions a - g on the diagram of the electrochemical cell shown below.
a) Which metal is the more reactive?
[2]
Pb
b) When the switch is closed, in which half-cell does oxidation occur? Half-cell 1
[2]
c) When the switch is closed, state the direction that electrons will flow through the wire. To the right
[2]
d) Which metal is the anode?
Pb
[2]
e) Which metal is the cathode?
Ag
[2]
f) Based on the equation above,
1) write the balanced half-reaction that occurs in half-cell 1: Pb0  Pb+2 + 2e-
[2]
2) write the balanced half-reaction that occurs in half-cell 2: 2Ag+1 +2e-  2Ag0
[2]
g) Explain the function of the salt bridge.
The salt bridge maintains balance of charge in the half-cells.
[2]
7) Use the diagram below and your knowledge of chemistry to answer parts a – g.
C
D
B
Fe(s)
A
E
a) Which metal is the most reactive?
Fe
[1]
b) Use the letters to indicate the
direction and path that electrons
will follow in this setup.
ABCDE
[1]
c) Which letter represents the anode?
A
[1]
d) Which letter represents the cathode?
E
[1]
g) What is this process called? Electroplating
[1]