12: Electrochemistry

Question 1

electrochemical cells

Answer 1

contained systems in which oxidation-reduction reactions occur… 3 types galvanic, electrolytic, concentration

Question 2

anode

Answer 2

electrode where oxidation occurs

Question 3

cathode

Answer 3

electrode where reduction occurs *AN OX & RED CAT

Question 4

electromotive force (emf)

Answer 4

corresponds to the voltage or electrical potential difference of the cell… if emf is +, cell is able to release energy (ΔG<0). if emf is -, cell absorbs energy so its nonspontaneous

Question 5

movement of electrons

Answer 5

• current runs from cathode to anode
• electrons move through an electrode in opposite flow of current from anode to cathode because oxidation (loss) occurs at anode

Question 6

galvanic/voltaic cells

Answer 6

nonrechargeable batteries

• -ΔG and +E_cell meaning the reaction is spontaneous… energy is harnessed by separaed reduction and oxidation half-reactions
• 2 electrodes are placed in half-cells and connected to each other with conductive material
• electrodes are surounding by aqeous electrolyte solution with cations & anions
• solutions are connected by a salt bridge, which is made of an inert salt

Question 7

Daniell cell

Answer 7

galvanic cell in which a zinc electrode is placd in an aqueous ZnSO₄ solution and a copper electrode is placed in an aqueous CuSO₄ solution.

• electrons flow from the zinc anode through the wire to the copper cathode

Question 8

plating/galvanization

Answer 8

precipitation process onto the cathode itself of reduced ions

Question 9

salt bridge

Answer 9

made of an inert electrolyte which contains ions that will not react with electrodes or ions in solution but rather diffuse into solution to balance out charge of newly created ions and charge of leftover ions

Question 10

cell diagram

Answer 10

shorthand notation representing reactions in an electrochemical cell

1. reactants and products are always listed from left to right in form: anode | anode soln (conc) || cathode soln (conc) | cathode​
2. single vertical line indicates a phase boundary
3. double verical line indicates presence of salt bridge

ex. Zn (s) | Zn²⁺ (1 M) || Cu²⁺ (1 M) | Cu (s)

Question 11

electrolytic cells

Answer 11

nonspontaneous reactions that require input of energy (+ ΔG)

• called electrolysis because external voltage source causes chemical compounds to decompose
• anode & cathode can consist of any material
• ex. NaCl is decomposed into Cl₂ (g) and Na (l)
  
  • Na+ ions migrate toward cathode to be reduced to Na and Cl- ions migrate toward anode to be oxidized to Cl₂

Question 12

Faraday’s law

Answer 12

liberation of gas and deposition of elements on electrodes is directly proportional to the number of electrons being transferred ruing the redox reaction… normality or gram equivalent weight is used

Mⁿ⁺ + n e^- → M (s)

1 e^- carries cahrge of 1.6 x 10^-19 C

Question 13

Faraday’s constant

Answer 13

1 F = 96,485 C, which is amount of charge contained in 1 mole of electrons or 1 equivalent

*round up to 10⁵ C/mol e^-

Question 14

electrodeposition equation

Answer 14

sumarizes process of Faraday’s law and helps determine number of moles of element being deposited on a plate

mol M = It/nF

• I is current
• t is time in seconds
• n is number of electron equivalents
• F is 10⁵ C/mol e^-

Question 15

concentration cell

Answer 15

special type of galvanic cell that contains 2 half-cells connected by a conductive material allowing a spontaneous redox reaction to proceed, which generates current and delivers energy

• electrodes are chemically identical… so current is generated as a function of a concentration gradient established between 2 solutions surrounding electrodes
• current stops when the concentrations of ionic species in the half-cells are equal… V=0 when concentrations are equal

Question 16

cell membrane of a neuron

Answer 16

best representation of a concentration cell… value of electrical potential across membrane depends on concentrations and charges of ions… allow Vm (resting membrane potential) to be maintained

Question 17

rechargeable cell/battery

Answer 17

one that can function as both a galvanic and electrolytic cell

ex. lead-acid battery

Question 18

lead acid/storage battery

Answer 18

rechargeable battery

• as a voltaic cell when fully charged consists of 2 half-cells… Ph anode and porous PbO₂ cathode connected by conductive material (H₂SO₄)
• when fully discharged, consists of 2 PbSO₄ electroplated lead electrodes with dilute concentration of H₂SO₄

Question 19

Nickel-Cadium batteries

Answer 19

rechargeable cells that consist of 2 half-cells made of solid cadium (anode) and nickel(III) oxide-hydroxide (cathode) connected by a conductive material

*AA & AAA cells

• charging reverses the electrolytic cell potentials
• higher energy denisty than lead-acid batteries
• higher surge current (period of large current early in discharge cycle)

Question 20

electrode charge designations

Answer 20

• galvanic cell: anode is - and cathode is + because anode is source of electrons which move through conductive material to the cathode
• electrolytic cell: anode is + because it is attached to the positive pole of the external voltage source and attracts anions from solution and cathode is - because external source is used to reverse the charge of an electrolytic cell
• oxidation always occurs at anode
• reduction always occurs at cathode
• cations are always attracted to cathode
• anions are always attracted to anode

Question 21

isolectric focusing

Answer 21

technique used to separate amino acids or polypeptides based on their isolelectric points (pI)

• • charged amino acids migrate toward the cathode
• • charged amino acids migrate toward the anode

Question 22

standard hydrogen electdoe (SHE)

Answer 22

0 V

used as a reference point for reduction potential

Question 23

reduction potential

Answer 23

tendency of species to gain electrons and to be reduced…

• • means more likely to be reduced

Question 24

standard reduction potential (Eº_red)

Answer 24

measured under standard conditions (298 K, 1 atm, 1M)

• more positive Eº_red means greater relative tendecy for reduction to occur
• less positive Eº_red means greater relatvie tendency for oxidation to occur

Question 25

reduction potentials for galvanic cells

Answer 25

electrode with more positive reduction potential is the cathode… electrode with the less positive reduction potential is the anode

• reaction is spontaneous (-ΔG) because species with a stronger tendency to gain electrons is actually doing so

Question 26

reduction potentials for electrolytic cells

Answer 26

electrode with the more positive reduction potential is forced by external voltage to be oxidized and is therefore the anode… electrode with the less positive reduction potential is forced to be reduced as is therefore the cathode

• movement of electrons is in the direction against the tnedency of the the electrochemical species so the reaction is nonspontaneous (+ΔG)

Question 27

standard electromotive force (emf/ E°_cell)

Answer 27

difference in potential (voltage) between 2 half-cells under standard conditions

Eº_cell = Eº_{red, cathode} - Eº_{red, anode}

*potential of each electrode does not depend on the size of the elctrode, but rather the identity of the material… Eº_red only changes if chemical identity of that electrode is changed

Question 28

Gibbs free energy (ΔGº)

Answer 28

standard change in free energy

ΔGº=-nFEº_cell

• n is number of moles of electrons exchanged
• F= faraday constant = 10⁵ if expressed in coulumbs (J/V), then ΔGº expressed in J not kJ
• Eº_cell=standard emf of cell
• galvanic cells have -ΔGº and + Eº_cell
• electrolytic cells have + ΔGº and - Eº_cell

Question 29

nerst equation

Answer 29

used when conditions deviate from standard conditions (298 K, 1 atm, a M)

• assume 298 K
• Q=[C]^c[D]^d/[A]^a[B]^b
  
  • only species in solution are included

Question 30

potentiometer

Answer 30

voltmeter that draws no current and gives a more accurate reading of emf of a cell

Question 31

equilibria

Answer 31

ΔGº = -nFEºcell = -RTlnK_eq

*log/ln is + if K>1, 0 if K=1, and - if K<1

Question 32

Gibbs free energy of an electrochemical cell with varying concentrations

Answer 32

ΔG = ΔGº + RTlnQ