LO 79 - 84

Question 1

Summarize a spontaneous redox reaction that proceeds forward in the standard state

Answer 1

ΔG° < 0
E°cell > 0
K > 1

Question 2

Summarize a nonspontaneous redox reaction that proceeds in reverse in the standard state

Answer 2

ΔG° > 0
E°cell < 0
K < 1

Question 3

Draw the triangle between the equations for ΔG°, E°cell, and K

Answer 3

Question 4

In the equation:
Ecell = E°cell - (0.0592V/n) · log(Q)
what does n represent?

Answer 4

the number of mols e- transferred in the redox reaction

Question 5

What is Faraday’s constant?

Answer 5

F = 96,485 C /mol e-

Question 6

Describe the behavior of a cell in where:
Q < 1
Ecell > E°cell

Answer 6

Voltaic cell where the [reactants] > [products], driving the reaction to the right

Question 7

Describe the behavior of a cell in where:
Q > 1
Ecell < E°cell

Answer 7

Electrochemical cell where the [reactants] < [products], driving the reaction to the left

Question 8

Describe the behavior of a cell in where:
Q = 1
Ecell = E°cell

Answer 8

No tendency in either direction which is why, as reactants are depleted, batteries ‘die’

Question 9

_____________ cells are voltaic cells constructed in which differences in the concentration drives the direction of current flow

Answer 9

concentration

Question 10

E°cell = ____________ for all concentration cells

Answer 10

0

Question 11

When calculating Ecell for a concentration cell using
Ecell = E°cell - (0.0592V/n) · log(Q), what is Q?

Answer 11

Q = dilute / concentrated

Question 12

Explain the relationship between ion concentration and e- movement in a concentration cell at the anode and cathode

Answer 12

e- flow spontaneously from the half-cell with the low ion concentration to the half-cell with the high ion concentration.

Question 13

What are the primary properties of dry cell batteries?

Answer 13

• Dry cell batteries do not contain large amounts of liquid water
• Zinc case acts as the anode:
  Zn(s) → Zn2+(aq) + 2 e–
• Graphite rod immersed in a moist paste of MnO2 with NH4Cl acts as the cathode:
  2 MnO2(s) + 2 NH4+(aq) + 2 e– → Mn2O3(s) + 2 NH3(g) + H2O(l)

Question 14

What are the primary properties of alkaline batteries?

Answer 14

• Alkaline batteries use a basic medium
• Zinc case acts as the anode:
  Zn(s) + 2 OH–(aq) → Zn(OH)2(s) + 2 e–
• Graphite rod immersed in a moist paste of MnO2 with KOH acts as the cathode:
  2 MnO2(s) + 2 H2O(l) + 2 e– →
  2 MnO(OH)(s) + 2 OH–(aq)

Question 15

What are the primary properties of lead-acid storage batteries?

Answer 15

Question 16

In a _________, the reactants (provided from an ___________ source) constantly flow through the battery, generating electrical current as they undergo a _____________

Answer 16

fuel cell
external
redox reaction

Question 17

Draw a hydrogen-oxygen fuel cell converting hydrogen and oxygen gases into water

Answer 17

Question 18

In an ___________ cell, a current drives an otherwise nonspontaneous reaction through ______________. The voltage required to cause electrolysis depends on the specific __________

Answer 18

electrolytic
electrolysis
half-reactions

Question 19

In voltaic cells the ______ is the source of e- and has a negative charge. The ________ draws e- and has a positive charge

Answer 19

anode
cathode

Question 20

In electrolytic cells, e- are __________ from the anode which must be connected to the ______ terminal of the external power source. e- are driven towards the cathode which is connected to the _____________ terminal

Answer 20

drawn away
positive
negative

Question 21

What happens to the anode and cathode when a pure, molten salt undergoes electrolysis?

Answer 21

the anode is oxidized, and the cathode is reduced

Question 22

In a mixture of different cations and anions how do you determine which will react in a cell?

Answer 22

Compare the E°cell of each cation reduction, the more positive E°cell is more easily reduced at the cathode. Similarly for anions, the E°cell that is the least positive is more easily oxidized.

Question 23

When an aqueous solution undergoes electrolysis, it is important to remember that . . .

Answer 23

H2O can also undergo electrolysis and, as such, is treated as a reaction that will occur if it is the one that is reduced or oxidized more easily

Question 24

Give the cations of active metals that are not easily reduced from aqueous solutions by electrolysis because H2O reduces at a lower voltage

Answer 24

Li+, K+, Na+, Mg 2+, Ca 2+, and Al 3+

Question 25

____________ occurs when more energy is needed to get a nonspontaneous reaction to occur

Answer 25

overvoltage

Question 26

What are two useful conversions to relate charges and time back to the mass of a substance?

Answer 26

1 amp = 1 coulomb/ second
F = 96,485 coulomb/ mol e-

Question 27

_____________ is the gradual oxidation of a metal that occurs from exposure to oxidizing agents in the environment

Answer 27

corrosion

Question 28

The ____________ of O2 has a strong tendency to cause the _________________ of other metals

Answer 28

reduction
oxidation

Question 29

Imperfections in metals cause __________________ where oxidation occurs which e- produced then travel to _____________ where they react with . . .

Answer 29

anodic regions
cathodic regions
O2 and H+ ions dissolved in air moisture

Question 30

Explain why moisture must be present for rusting to occur

Answer 30

the presence of water is necessary because water is a reactant in the last reaction and because charge must be free to flow between the anodic and cathodic regions

Question 31

Explain why additional electrolytes promote rusting

Answer 31

the presence of electrolytes on the surface of iron promotes rusting because they enhance current flow

Question 32

Explain why acid promote rusting

Answer 32

since H+ ions are involved in the reduction of oxygen, lower pH enhances the cathodic reaction and leads to faster rusting

Question 33

Sometimes a _________ ___________ made of a more easily oxidized metal is placed in contact to help prevent rusting

Answer 33

sacrificial electrode