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CHEM1602 > Chapter 19: Electrochemistry > Flashcards

Chapter 19: Electrochemistry Flashcards

1
Q

Give the six steps for balancing redox reactions in acidic solutions.

A
  1. Assign oxidation numbers
  2. Half-rxns
  3. Balance non H/O, balance O by adding H2O, balance H by adding H+
  4. Balance charge
  5. Equate # electrons
  6. Add equations together
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2
Q

What is the additional step for balancing redox reactions in basic solution?

A

After balancing H by adding H+, neutralize H+ with OH-.

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3
Q

What is a voltaic (or galvanic cell)?

A

An electrochemical cell that spontaneously converts chemical energy to electrical energy.

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4
Q

What happens at the anode?

A

The anode is negative. Electrons are made at the anode, and oxidation takes place there.

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5
Q

What happens at the cathode?

A

The cathode is positive. Reduction takes place there.

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6
Q

What is the purpose of a salt bridge in an electrochemical cell?

A

The salt bridge neutralizes charge by “completing the circuit”; it allows ions to flow from one cell to the other.

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7
Q

What is a Coulomb (C)?

A

A unit of charge. qe = -1.602 x 10^-19 C

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8
Q

What is an Ampere (A)?

A

A unit of current. 1 A = 1 C/s

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9
Q

What is the electromotive force?

A

The force that causes electrons to move through wire, causing electrons to go from areas of high potential energy to low potential energy.

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10
Q

What is cell potential (Ecell)?

A

The potential difference between two electrodes.

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11
Q

What is standard cell potential (E˚cell)?

A

The cell potential when all species are in their standard state. Concentrations are 1.0 M and gases have P = 1.0 atm.

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12
Q

What shorthand notation is used to depict an electrochemical cell?

A

Anode information on left, cathode on right. Single lines indicate a phase boundary, double lines indicate a salt bridge.

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13
Q

What is the standard hydrogen electrode?

A

A half-cell with a potential energy of zero. Used as a reference point against which the potentials of other half-cells can be measured.

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14
Q

How is E˚cell calculated?

A

E˚cell = E˚cathode - E˚anode

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15
Q

What direction do electrons flow in a voltaic cell?

A

Electrons flow from the cell with the more negative reduction potential to the cell with the more positive reduction potential.

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16
Q

What is reduced in chemical reactions?

A

Oxidizing agents.

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17
Q

What does the magnitude and charge of E˚ indicate about a reduction half-reaction?

A

Half-cells with more positive E˚, the more likely the species on the left will be reduced. More negative values of E˚, the less likely the half-reaction will occur as reduction and more likely the reverse reaction will occur.

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18
Q

What do electrochemical potentials depend on?

A

Concentrations (but not stoichiometric coefficients).

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19
Q

When looking at the chart of reduction half-reactions, how must the reactions be paired to be spontaneous?

A

The reaction between any substance on the left with any substance lower and on the right is spontaneous and thus product favored.

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20
Q

What type of reactants are at the top of the reduction half-reaction chart?

A

Strong oxidizing agents.

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21
Q

What type of reactants are at the bottom of the reduction half-reaction chart?

A

Weak oxidizing agents

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22
Q

What type of products are at the top of the reduction half-reaction chart?

A

Weak reducing agents

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23
Q

What type of products are at the bottom of the reduction half-reaction chart?

A

Strong reducing agents

24
Q

How do you know which metals will dissolve in strong acid?

A

Strong acids create H+ in water, so the two paired half-reactions are the metal reduction and SHE rxn (0 V). Half-reactions with negative reduction potentials will spontaneously dissolve in strong acid.

25
Q

Which three metals will not dissolve in strong acid?

A

Cu, Ag, AU

26
Q

What are the exceptions to the three metals that will not dissolve in strong acid?

A

Ag and Cu will dissolve in HNO3.

27
Q

What does ∆G˚cell represent?

A

The maximum amount of work a system can do. ∆G˚cell = w electrical

28
Q

How is ∆G˚cell related to K?

A

∆G˚cell = -RTlnK

29
Q

How is ∆G˚cell related to E˚cell?

A

∆G˚cell = -QE˚cell = -nFE˚cell

30
Q

What does Q represent?

A

The magnitude of charge moved.

31
Q

What is F?

A

Faraday’s constant: 96,485 C/mol

32
Q

What does n represent?

A

mol electrons transferred per mol of rxn as written

33
Q

What is the Nernst equation?

A

Ecell = E˚cell - (RT/nF)lnQ

34
Q

What is electrolysis?

A

A process in which electricity drives a non-spontaneous rxn.

35
Q

What does current represent?

A

The amount of charge per unit time (C/s).

36
Q

What is corrosion?

A

The deterioration of metals by a product-favored oxidation rxn.

37
Q

What’s the difference between an electrolytic cell and a voltaic (galvanic) cell?

A

An electrolytic cell consumes electrical current to drive a nonspontaneous chemical rxn. A voltaic cell is an electrochemical cell that produces electrical current from a spontaneous chemical rxn.

38
Q

What are electrodes?

A

Conductive surfaces through which electrons can enter or leave the half-cells.

39
Q

Describe the role of both negative and positive ions within the salt bridge.

A

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

40
Q

What does the value of the electrode potential tell you about the potential energy of an electron at that electrode?

A

The more negative the electrode potential is, the greater the potential energy of an electron at that electrode.

41
Q

What type of pairing with the SHE creates a spontaneous reaction?

A

An electrode in which the electron has more negative potential than the SHE. This repels electrons toward the SHE.

42
Q

What type of pairing with the SHE creates a nonspontaneous reaction?

A

An electrode in which the electron has more positive potential than the SHE. This draws electrons away from the SHE.

43
Q

When looking at the reduction half-potentials table, which pairings create a spontaneous rxn?

A

Any reduction half-rxn listed is spontaneous when paired with the reverse of a half-rxn that appears below it in the table.

44
Q

What is the process by which acids dissolve metals?

A

Most acids dissolve metals by the reduction of H+ ions to hydrogen gas and the corresponding oxidation of the metal to its ion.

45
Q

Give the relative values of ∆G˚, E˚cell, and K for a spontaneous rxn.

A

∆G˚ < 0
E˚cell > 0
K > 1

46
Q

Give the relative values of ∆G˚, E˚cell, and K for a nonspontaneous rxn.

A

∆G˚ > 0
E˚cell < 0
K < 1

47
Q

What happens when a redox rxn within a voltaic cell occurs under conditions in which Q < 1?

A

The greater concentration of reactants relative to products drives the rxn to the right, resulting in Ecell > E˚cell.

48
Q

What happens when a redox rxn within a voltaic cell occurs under conditions in which Q > 1?

A

The greater concentration of products relative to reactants drives the rxn to the left, resulting in Ecell < E˚cell.

49
Q

In a concentration cell (an electrochemical cell with nonstandard concentrations), in what direction do the electrons flow?

A

Electrons spontaneously flow from the half-cell with the lower ion concentration to the half-cell with the higher ion concentration. The flow of electrons has the effect of increasing the concentration of ions in the dilute cell and decreasing the concentration of ions in the concentrated half-cell.

50
Q

In an electrolytic cell, the cathode is connected to which side of the voltage source?

A

The negative side. This causes electrons to be driven toward the cathode.

51
Q

In an electrolytic cell, the anode is connected to which side of the voltage source?

A

The positive side. This causes electrons to be drawn away from the anode.

52
Q

In the electrolysis of a pure molten salt, what happens?

A

The anion is oxidized and the cation is reduced.

53
Q

Why can’t the cations of active metals be reduced in aqueous solutions?

A

The cations of active metals–those that are not easily reduced, such as Li+, K+, Na+, Mg2+, Ca2+, and Al3+–cannot be reduced from aqueous solutions by electrolysis because water is reduced at a lower voltage.

54
Q

What is overvoltage?

A

An additional voltage that must be applied in order to get some nonspontaneous rxns to occur.

55
Q

What is a sacrificial electrode?

A

A metal that oxidizes more easily than iron that protects iron from rusting.

CHEM1602 (8 decks)

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