12: Electrochemistry Flashcards Preview

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Flashcards in 12: Electrochemistry Deck (32):
1

electrochemical cells

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

2

anode

electrode where oxidation occurs

3

cathode

electrode where reduction occurs *AN OX & RED CAT

4

electromotive force (emf)

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

5

movement of electrons

  • 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

6

galvanic/voltaic cells

nonrechargeable batteries 

  • -ΔG and +Ecell 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

7

Daniell cell

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

8

plating/galvanization

precipitation process onto the cathode itself of reduced ions

9

salt bridge

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 

10

cell diagram

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) 

11

electrolytic cells 

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

12

Faraday's law

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

13

Faraday's constant

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- 

14

electrodeposition equation

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-

15

concentration cell

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

16

cell membrane of a neuron

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

17

rechargeable cell/battery

one that can function as both a galvanic and electrolytic cell

ex. lead-acid battery

18

lead acid/storage battery

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

19

Nickel-Cadium batteries

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)

20

electrode charge designations

  • 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 

21

isolectric focusing 

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

22

standard hydrogen electdoe (SHE)

0 V 

used as a reference point for reduction potential

23

reduction potential

tendency of species to gain electrons and to be reduced...

  • + means more likely to be reduced 

24

standard reduction potential (Eºred

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

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

25

reduction potentials for galvanic cells

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

26

reduction potentials for electrolytic cells

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)

27

standard electromotive force (emf/ E°cell)

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

cell = Eºred, cathode - Eºred, anode

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

28

Gibbs free energy (ΔGº)

standard change in free energy

ΔGº=-nFEºcell

  • 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
  • cell=standard emf of cell
  • galvanic cells have -ΔGº and + Eºcell
  • electrolytic cells have + ΔGº and - Eºcell

29

nerst equation

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

A image thumb
30

potentiometer

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

31

equilibria

ΔGº = -nFEºcell = -RTlnKeq

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

32

Gibbs free energy of an electrochemical cell with varying concentrations

ΔG = ΔGº + RTlnQ