Ch 12 - Electrochemistry

Question 1

What is an electrochemical cell?

Answer 1

describes any cell in which oxidation-reduction reactions take place

Question 2

What are electrodes?

Answer 2

strips of metal or other conductive materials placed in an electrolyte solution

Question 3

What is an anode v a cathode? How do electrons and current flow between the 2?

Answer 3

AN Ox and a RED CAT

• anode: always the site of oxidation (attracts anions)
• cathode: always the site of reduction (attracts cations)
• electrons flow from anode to cathode
• current flows from cathode to anode

Question 4

What are cell diagrams?

Answer 4

shorthand notation that represent the reactions taking place in an electrochemical cell

• written from anode to cathode with electrolytes (the solution) in between
• a vertical line represents a phase boundary, and a double vertical line represents a salt bridge or other physical boundary

Question 5

What are galvanic (voltaic) cells?

Answer 5

house spontaneous reactions (G < 0) with a positive electromotive force (anode has negative designation)

Question 6

What are electrolytic cells?

Answer 6

house nonspontaneous reactions (G > 0) with a negative electromotive force

• can be used to create useful products through electrolysis
• cathode as negative designation

Question 7

What are concentration cells?

Answer 7

specialized form of a galvanic cell in which both electrodes are made of the same material
- rather than a potential difference causing the same movement of charge, it is the concentration gradient between the 2 solutions

Question 8

What does the charge on an electrode depend on?

Answer 8

the type of electrochemical cell one is studying:

• for galvanic cells, the anode is negatively charged and the cathode is positively charged
• for electrolytic cells, the anode is positively charged and the cathode is negatively charged

Question 9

What are rechargeable batteries? How are they ranked?

Answer 9

• electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states
• often ranked by energy density - the amount of a cell can produce relative to the mass of battery material

Question 10

What cathode/anode are used when discharging and charging lead-acid batteries? What is their energy density?

Answer 10

• when discharging (galvanic), consist of Pb (-) anode and PbO2 (+) cathode in a concentrated sulfuric acid solution
• when charging (electrolytic), the PbSO4-plated electrodes (- cathode; +anode) are dissociated to restore the original Pb and PbO2 electrodes and concentrate the electrolyte
• these cells have low energy density

Question 11

What cathode/anode are used when discharging and charging nickel-cadmium batteries? What is their energy density?

Answer 11

• when discharging (galvanic), consists of a Cd anode (-) and a NiO(OH) cathode (+) in a concentrated KOH solution
• when charging (electrolytic), the Ni(OH)2 (- cathode) and Cd(OH)2 (+ anode) plated electrodes are dissociated to restore the original Cd and NiO(OH) electrodes and concentrate the electrolyte
• these cells have a higher energy density than lead-acid batteries

Question 12

What are nickel-metal hydride batteries?

Answer 12

have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective, and are significantly less toxic

Question 13

What is a surge current?

Answer 13

an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved

Question 14

What does a reduction potential quantify?

Answer 14

the tendency for a species to gain electrons and be reduced

- the higher the reduction potential, the more a given species wants to be reduced

Question 15

How are standard reduction potentials (Ered) calculated?

Answer 15

• by comparison to the standard hydrogen electrode (SHE) under standard conditions of 298 K, 1 atm, and 1 M

Question 16

What is the standard reduction potential of standard hydrogen electrode (SHE)?

Answer 16

0 V

Question 17

What is the standard electromotive force (Ecell)?

Answer 17

the difference in standard reduction potential between the 2 half-cells
Ecell = Ered,cathode - Ered,anode

Question 18

What is the difference between the reduction potentials of 2 half reactions for galvanic and electrolytic cells?

Answer 18

• galvanic is positive

- electrolytic is negative

Question 19

How do electromotive force and change in free energy relate to each other?

Answer 19

• they always have opposite signs
• when Ecell (+), G (-) –> galvanic cells
• when Ecell (-), G(+) –> electrolytic cells
• when Ecell (0), G(0) –> concentration cells
  deltaG = -nFEcell
  n is number of moles exchanged

Question 20

What does the Nernst equation describe?

Answer 20

the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force
Ecell = Eocell - 0.0592/n x logQ

Question 21

What is the relationship between the equilibrium constant (Keq) and electromotive force (Ecell)?

Answer 21

• when Keq > 1, Ecell (+)
• when Keq < 1, Ecell (-)
• when Keq = 1, Ecell (0)
  Keq is the ratio of products’ concentrations at equilibrium over reactants’, raised to their stoichiometric coefficients

Question 22

What is the purpose of the salt bridge?

Answer 22

to exchange anions and cations to balance, or dissipate, newly generated changes
- contain inert electrolytes (ionic compounds are strong electrolytes)

Question 23

What is Faraday’s law?

Answer 23

• states that the liberation of gas and deposition of elements on electrodes is directly proportional to the number of electrons being transferred during oxidation-reduction reaction
• here, normality or gram equivalent weight is used
• these observations are proxy measurements of the amount of current flowing in a circuit

Question 24

What is one Faraday (F) equivalent too?

Answer 24

the amount of charge contained in one mole of electrons (1F = 96,485 C)

Question 25

What is the electrodeposition equation?

Answer 25

• summarizes the process and helps determine the number of moles of element being deposited on a plate
  Moles of Metal It is Not Fun
  mol M = It/nF
  mol M is the amount of metal ion being deposited at a specific electrode
  I is current
  n is number of electrons equivalents for a specific metal ion
  F = 96485 C/mol e-
  t in seconds

Question 26

What can alter the emf of an electrochemical cell?

Answer 26

temperature of the solutions in the half cells

Eocell is dependent on the change in free energy of the system (RT ln Keq = nFEocell)