Ch 20: Electrochemistry

Question 1

A voltaic (or galvanic) cell is an electrochemical cell that produces electrical current from a ___________ chemical reaction.

Answer 1

SPONTANEOUS

Question 2

What is the purpose of the salt bridge in a voltaic cell?

Answer 2

To complete the circuit. Flowing ions within the salt bridge neutralizes the charge buildup, allowing the reaction to continue.

Question 3

Electrons in a voltaic cell flow spontaneously from the _____ to the ______.
They do this because the anode is more negatively charged, and therefore, ______ electrons.

Answer 3

anode to the cathode
repels

Question 4

Electrical current is measured in…

Answer 4

….amperes (A).

Question 5

In all electrochemical cells, what do we call the electrode where oxidation occurs?

Answer 5

the anode

Question 6

What do we call the electrode where reduction occurs?

Answer 6

the cathode

Question 7

The anode and cathode need not actually be negatively charged, however the _______ is the electrode with the relatively MORE NEGATIVE charge.

Answer 7

anode

Question 8

As electrons flow out of the anode, ________ ions form in oxidation half-cell, resulting in a buildup of ________ charge in the solution.

Answer 8

positive
positive

Question 9

As electrons flow into the cathode, _______ ions are reduced at the reduction half-cell, resulting in a buildup of ________ charge in the solution.

Answer 9

positive

negative

Question 10

The salt bridge completes the circuit, allowing electrical current to flow.
How does the salt bridge work to neutralize the charge buildup in the solution?

Answer 10

The negative ions flow to neutralize the accumulation of positive charge at the anode, and the positive ions flow to neutralize the accumulation of negative ions at the cathode.

Question 11

The half-cell electrode that is arbitrarily assigned to have an electrode potential of zero is the _______ ________ ________. This half-cell consists of an inert platinum electrode immersed in 1 M HCl with hydrogen gas at 1 atm bubbling through the solution. When the SHE act as the cathode, the following half-reaction occurs. Thus, we define E of this cell to be 0.00 V.

Answer 11

standard hydrogen electrode

Question 12

E°_{cell =} E°_final - E°_{initial =}

Answer 12

E°_cathode - E°_anode

Question 13

The more negative the electrode potential is, the greater the ________ ______ of an electron at that electrode (because negative charge repels electrons).

Answer 13

potential energy

Question 14

E°_cell is ________ for spontaneous reactions and ________ for nonspontaneous reactions

Answer 14

positive

negative

(Remember that this is the OPPOSITE trend compared to ΔG of a reaction)

Question 15

Does multiplying a reaction by some constant to balance a redox reaction affect the value of the E°_cell ?

Answer 15

No! E°_cell is an intensive property, not an extensive one.

Question 16

What mnemonic can help you to predict the spontaneous direction of a redox reaction?

Answer 16

N.I.O → more Negative Is Oxidation

P.I.R. → more Positive Is Reduction

Question 17

What equation relates ΔG° to K (the equilibrium constant)?

Answer 17

ΔG° = - RT ln K

Question 18

Since we know that ΔG° = - RT ln K

and that ΔG° = -nFE°_cell

The equation:

-nFE°_cell = -RT ln K

ties in all the related entities (E°_cell, K, and ΔG°). What equation can we derive from this one?

Answer 18

• E°_cell* = (RT/nF) ln K
• …*which when simplified, gives us….
• E°_cell* = (0.0592 V/n) * log K

Question 19

Question 20

A redox reaction has an equilibrium constant of K = 1.2 × 10³. What is true regarding ΔG°_rxn and E°_cell for this reaction?

Answer 20

E°_cell is positive and ΔG°_rxn is negative.

Question 21

What equation relates the cell potential of an electrochemical cell to the standard cell potential and the reaction quotient?

Answer 21

The Nernst equation!

E_cell = E°_cell - (0.0592 V/n) log Q

Question 22

When a redox reaction reaches equilibrium Q = K. The redox reaction has no tendency to occur in either direction and E_cell = 0. This last point illustrates why _________ do not last forever.

Answer 22

batteries

Question 23

When a redox reaction occurs within a voltaic cell under conditions in which Q < 1, the greater concentration of reactant relative to products drives the reaction to the right, resulting in E_{cell __} E°_cell

Answer 23

E_cell > E°_cell

Question 24

When a redox reaction occurs within a voltaic cell under conditions in which Q > 1, the greater concentration of products relative to reactants drives the reaction to the left, resulting in E_{cell __} E°_cell

Answer 24

E_cell <E°_cell

Question 25

In an electrochemical cell, Q = 0.0010 and K = 0.10. What can you conclude about E_cell and E°_cell?

a. ) E_cell is positive and E°_cell is negative.
b. ) E_cell is negative and E°_cell is positive.
c. ) Both E_cell and E°_cell are positive.
d. ) Both E_cell and E°_cell are negative.

Answer 25

a.) E_cell is positive and E°_cell is negative.

Since K < 1, E°_cell is negative (under standard conditions, the reaction is not spontaneous). Since Q < K, E_cell is positive (the reaction is spontaneous under the nonstandard conditions of the cell).

Question 26

In a theoretical concentration cell, electrons flow spontaneously from the half-cell with the ______ copper ion concentration to the half cell with the ______ copper ion concentration.

While this sounds like something is flowing against its concentration gradient, the flow of electrons has the effect of increasing the concentration of Cu²⁺ in the dilute cell, while decreasing the concentration of Cu²⁺ in the concentrated half-cell.

Answer 26

lower

higher

Question 27

Because of the different concentration in the two half-cells of a concentration cell, the cell potential must be calculated using the….

Answer 27

….Nernst equation.

Question 28

The more positive the standard potential (E°_cell) is, the stronger the _________ agent, and the more likely _________ is to occur.

Answer 28

oxidizing agent

reduction

Question 29

The more negative the standard potential (E°_cell) is, the stronger the _________ agent, and the more likely _________ is to occur. (Note, looking at the products side of the equation.)

Answer 29

reducing

oxidation

Question 30

In an electrolytic cell, electrical current drives an otherwise _____________ redox reaction through a process called electrolysis.

Answer 30

nonspontaneous

Question 31

In an electrolytic cell, the anode has become the cathode, and the cathode has become the anode. Electrons still flow toward the cathode, but the reaction is nonspontaneous, and the signs of charge have flipped.

The cathode is __, and the anode is __.

Answer 31

(-)

(+)

Question 32

In a voltaic cell, the anode is the source of electrons and is therefore labeled with a ________ charge. The cathode draws electrons and is therefore labeled with a positive charge. In an electrolytic cell, however, the source of the electrons is the ________ _____ source. The external power source must draw electrons away from the anode; thus, the anode must be connected to the ________ terminal of the battery. Similarly, the power source drives electrons toward the cathode (where they are used in reduction), so the cathode must be connected to the negative terminal of the battery.

Answer 32

negative

external power

positive

Question 33

A dead battery is one that has…

Answer 33

….reached equilibrium.

Question 34

In an electrolysis reaction, the cation that is most ______ _______ (the one with the more positive electrode potential) is reduced first.

Answer 34

easily reduced

Question 35

The anion that is most easily oxidized (the one with the ____ ________ electrode potential) is oxidized first.

Answer 35

more negative

Question 36

In general, the cations of active metals – those that are not easily reduced, such as Li+, Na+, K+, Mg2+, Ca2+, Al3+ – cannot be reduced from aqueous solutions by electrolysis because water is reduced at a _____ ______.

When multiple half-reactions are possible, the one that will actually occur is the one that occurs most _______.

Answer 36

lower voltage

easily

Question 37

A sacrificial electrode must be composed of a metal that ________ ____ _______ than iron, that is, it must be below iron on table 20.1 - (the list of standard reduction potentials in decreasing order).

Answer 37

oxidizes more easily