Chapter 12: Electrochemistry

Question 1

Three types of electrochemical cells?

Answer 1

1. Galvanic cells (voltaic)
2. Electrolytic cells
3. Concentration cells

Question 2

Which cells house spontaneous reactions? Which house nonspntaneous reactions?

Answer 2

• galvanic and concentration cells

- electrolytic cells

Question 3

What do all of the cells contain? Two types? What occurs at each and which way do electrons flow and which way does current flow?

Answer 3

Electrodes: anode, where oxidation takes place and cathode, where reduction takes place. electrons move from anode to cathode, current runs from cathode to anode

Question 4

What does electromotive force respond to? If it’s positive? Negative?

Answer 4

voltage.
If emf is positive, the cell is able to release energy (spontaneous). If negative, the cell must absorb energy, so nonspontaneous

Question 5

What type of cells are typical non-rechargeable batteries? Delta G of reaction? EMF of the cell?

Answer 5

Galvanic cells. negative delta G (spontaneous) and positive EMF (as it always has the opposite sign of delta G

Question 6

Makeup of a galvanic cell?

Answer 6

Two electrodes placed in separate compartments (half-cells) connected by a conductive material (which may contain resistors/capacitors). Electrodes surrounded by aq electrolyte solution (in which cation can be same element as electrode) which are connected by an inert salt bridge

Question 7

What happens once the electrodes are connected by a conductive material? How can this be used?

Answer 7

Charge begins to flow spontaneously as a result of redox reaction between two half-cells. Proceeds towards equilibrium, converting electrical potential into kinetic energy, which can be harnessed to do work by connecting devices into circuit between electrodes

Question 8

Components of a Daniell cell?

Answer 8

Galvanic cell where zinc is the anode, copper is the cathode and each electrode is in an electrolyte solution containing each respective cation and sulfate

Question 9

What is plating/galvanization?

Answer 9

When the electrolyte precipitates out of solution onto the electrode eg. copper plating the cathode in a Daniell cell

Question 10

What does a salt bridge do?

Answer 10

Anions in the salt bridge flow toward the anode and cations flow toward the cathode to balance the charge in the electrolyte solutions, which dissipates the charge gradient that would otherwise stop the reaction

Question 11

Cell diagram notation rues?

Answer 11

anode | anode solution (concentration) || cathode solution (concentration) | cathode

single vertical line is phase boundary, double is a salt bridge or some other barrier

Question 12

Free energy change for an electrolytic cell? What drives the redox reaction?

Answer 12

Positive - a nonspontaneous reaction. Driven by an external voltage source (electrolysis) in which chemical compounds are decomposed eg water into oxygen and hydrogen or NaCl (l) into Na(l) and Cl2(g)

Question 13

Do the half reactions in an electrolytic cell need to separated? Why?

Answer 13

No, they are nonspontaneous

Question 14

General half-reaction equation for a reduction?

Answer 14

M^n+ + ne- –> M(s), whereby one mole of metal will be produced if n moles of electrons are supplied to M^n+

Question 15

What is the Faraday constant?

Answer 15

the amount of charge in one mole of electrons.

1F = 96485 C. Use 10^5 on MCAT

Question 16

Electrodeposition equation?

Answer 16

mol M = It/nF, where mol M is the amount of metal ion being deposited at a specific electrode, I is current, t is time in seconds, n is the number of electron equivalents for a specific metal ion and F is the Faraday constant

Question 17

What is a concentration cell? How is current generated?

Answer 17

A special type of galvanic cell in which the electrodes are chemically identical and current is generated as a function of a concentration gradient between the two electrolytic solutions. Similar to resting membrane potential of a cell

Question 18

How can the voltage of a concentration cell be calculated?

Answer 18

The voltage, as a function of concentrations, can be calculated by the Nernst equation

Question 19

A rechargeable battery can function as what types of cells? Two examples?

Answer 19

Both a galvanic cell and an electrolytic cell. Lead-acid batteries and nickel-cadmium batteries

Question 20

What does a lead-acid battery consist of when fully charged? When discharged?

Answer 20

Two half cells - a Pb anode and a porous PbO2 cathode connected by a conductive material (4M H2SO4). Two PbSO4 electroplated lead electrodes with a dilute H2SO4 solution

Question 21

What do the two electrodes attract and form in a lead-acid battery? Mechanism in case of cathode?

Answer 21

Negatively charged lead anode attracts anionic bisulfate. Porous cathode allows sulfuric acid in, solvates into lead and oxygen, which react to form water and remaining sulfate reacts with lead. Both electrodes form lead sulfate

Question 22

Net equation for discharging a lead-acid battery?

Answer 22

Pb(s) + PbO2(s) + 2H2SO4 -> 2PbSO4(s) + 2H2O

Question 23

How do lead acid batteries charge?

Answer 23

An external source reverses the electroplating process and concentrates the acid solution

Question 24

How is battery energy density measured? How is lead-acid battery energy density?

Answer 24

Energy to battery weight ratio. Very low

Question 25

Electrodes and conductive material in a nickel-cadmium battery? (AA, AAA batteries)

Answer 25

Composed of two half-cells made of solid cadmium (anode) and nickel(III) oxide-hydroxide (cathode) connective by a conductive material (usually potassium hydroxide).

Question 26

Nickel cadmium oxidation at anode and reduction at cathode?

Answer 26

Cadmium reacts with hydroxide to form cadmium hydroxide. Nickel oxide-hydroxide reacts with water to form form nickel(II) hydroxide and free OH-

Question 27

Nickel-cadmium battery net equation?

Answer 27

2NiO(OH) (s) + Cd(s) +2H2O -> 2Ni(OH)2(s) + Cd(OH)2 (s)

Question 28

How does a nickel cadmium battery charge?

Answer 28

External source reverses the electrolytic cell potenticals (as with lead-acid batteries)

Question 29

Difference in performance with a Ni-Cd battery? Ideal for?

Answer 29

Higher energy density and higher surge current (period of large current early in discharge) than Pb-acid. Ideal for appliances that require a rapid response, like remote controls

Question 30

What have Ni-Cd batteries been replaced with recently?

Answer 30

More efficient nickel-metal hydride batteries, which are more energy dense, cost effective and less toxic

Question 31

Electrode charges of a galvanic cell? Of an electrolytic cell?

Answer 31

- anode is negative (source of electrons) and cathode is positive - anode is considered positive as it is connected to the positive pole of he external voltage source and attracts ions from solution. Cathode is negative (for same reason) - -however red at cathode and ox at anode for both

Question 32

What happens in isoelectric focusing?

Answer 32

positively charged amino acids migrate towards the cathode, negatively charged amino acids travel towards anode, separating based on pH on the gel

Question 33

What is reduction potential?

Answer 33

Measured in volts, and defined relative to the standard hydrogen electrode, which is given a potential of 0 V. When compared, allows you to determine which species will be oxidized and which will be reduced. The more positive the potential, more likely to be reduced

Question 34

Conditions for standard reduction potential?

Answer 34

25 degrees celcius, 1 atm of pressure and 1M concentrations

Question 35

For galvanic cells, the electrode with the more positive reduction potential is? less positive is? For an electrolytic cell?

Answer 35

Cathode. Anode. Electrode with more positive reduction potential is forced by external source to be oxidized and is anode. Electrode with less positive reduction potential is forced to be reduced - nonspontaneous

Question 36

How do you get oxidation potential?

Answer 36

Flip sign of reduction potential

Question 37

How is electromotive force calculated?

Answer 37

Is is the difference in potential between two half cells under standard conditions. Equals reduction potential of the cathode minus the reduction potential of the anode

Question 38

How is delta G related to EMF? Units of F and delta G?

Answer 38

delta G = -nF(E^o_cell), where n is the moles of electrons exchanged, F is the faraday constant, and E^o_cell is the standard emf of the cell. F is in J/V, so delta G must be in J

Question 39

How can you calculate EMF when a cell isn't at standard conditions?

Answer 39

E_cell = E^o_cell - (RT)/(nF) * lnQ Where Ecell is the emf of the cell under nonstandard conditions, E^o_cell is the emf of the cell under standard conditions, R is the ideal gas constant, T is the temp in kelvin, n is the number of moles of electrons, F is the faraday constant, Q is the rxn quotient at a given time. Only aq species included in Q calculation

Question 40

Simplified version of the nonstandard emf calculation?

Answer 40

Ecell = E^o_cell - (0.06/n) * logQ

Question 41

Other equation for delta G? By eliminating delta G, how else can you solve for standard EMF?

Answer 41

delta G = -RTlnKeq. nFE^o_cell = RTlnKeq

Question 42

When Keq is less than one, what is the sign of E^o_cell? When Keq is greater than one? Equal to one? These are characteristics of what cells?

Answer 42

Negative, as the natural log of a number between 0 and 1 is negative. Characteristic of electrolytic cells. Positive: galvanic cells. Equals zero: a concentration cell with equimolar concentrations in both half cells, therefore no net ionic equation.