electrochemistry Flashcards
electrochemistry real world applications
photosynthesis, conversion of food to energy, combustion, batteries and fuel cells, corrosion and prevention
What is being reduced and oxidized? 2Fe 3+ + HCO2H + 2H2O –> 2Fe2+ + CO2 + 2H30+
rules for balancing redox rxns
- separate into 2 half rxns
- balance all elements except O and H
- Balance O with H2O
- balance H with H+
- Balance charge with e-
- Find multiplier that allows e- to cancel
- combine/add half rxn together
- check to be sure balaced- this is for acidic conditions
- for basic conditions, add OH- to both sides to eliminate H+, cancel residual water, check again
practice C2O4 2- + MnO4- <–> CO2 + Mn2+
galvanic electrochem cell beaker sketch: Fe (s) –> Fe2+ + 2e- , Cu2+ + 2e- –> Cu (s)
Standard Hydrogen Electrode (SHE) 2H+ 9aq) + 2e- –. H2(g) purpose
a standard to compare the reaction potential of all other substances
cell potentials
electrochemical cells generate electricity (e- flow) by physically separating two half rxns, each half cell contributes to overall cell potential (voltage cell can produce)
E° cell equation
E°cell= E°oxidation + E°reduction OR E°cell= E°cathode- E°anode
Standard reduction potentials
always given in reduction format, use to find oxidation potential by multiplying by (-) flipping rxn
manipulating E° potentials
- flip sign when E°red becomes E°ox
- if multip;y rxn by constant DO NOT multiply E°red- intensive property
galvanic cell overview
-balanced redox rxn for an electrochemical cell
- designate cathode and anode
-SRP to determine the direction of e- flow, calculate E° cell for system
for galvanic cell E°cell should be (+) and spontaneous
- consider phases for all species involved, may need chemically inert conductor Pt or graphite as electrode for non-solid species like H2 (g) specify in diagram
1 coulomb (C)
Measure of charge on 6.2x10^18 e-
charge on 1 e-
1.6x10^-19 C
1 Faraday (F)
charge on 1 mol e- = 96,485 C/mol e-
1 Volt (V) = 1 Joule per Coulomb = 1 J/C
Half-reactions are defined by changes in free energy associated with transferring e-
since half rxn potential is determined by free energy change from electron transfer, we can find the free energy delta G carried by 1 Coulomb charge – this is SRP for half rxn
relationship between E and delta G
E has opposite sign as delta G
if energy is released (delta G> 0) then
electrons will be carried through cell, positive E potential
role of Q
accounting for concentrations and pressures that are not standard state
Q, K, and delta G at at equilibrium
Q=K, Delta G= 0
when not at EQ, cell potential shifts
left or right, depending on cell potential
nernst equation
Delta G°= -nFE° – see eq sheet for more examples of derivatives
spontaneous forward direction
delta G° <0, E° cell>0, K >1
spontaneous in the reverse direction
delta G° >0, E° cell<0, K <1
no net rxn: system at eq
delta G° 0, E° cell 0, K 1
what will happen to cell with identical electrodes and different ion concentrations, how will sys shift to reach eq
primary batteries
high power, long-lasting, inexpensive, NON-rechargeable (single use) ex: dry cell battery
secondary batteries
rechargeable for many cycles, ex: lead acid battery, lithium-ion battery
fuel cells
“clean” run on hydrogen-containing fuel, longer lifetime than traditional batteries, ex: Proton Exchange Membrane
corrosion
spontaneous, undesirable redox chemistry
corrosion prevention
- protective later (paint)
- mix in corrosion-resistant metals (alloy to protect vulnerable material with corrosion-resistant species)
- Physically connect the sacrificial anode (donate e- to the system first, protecting another metal from unwanted oxidation (more negative SRP, ex: zinc, mg)
electrolysis definition
Reverse of galvanic cell: input a voltage to run the electrochemical rxn in non-spontaneous rxn
electrolysis applications
- can be used to selectively “plate out” metals that are dissolved in solution
- industrial processes such as aluminum and copper refinement
faraday’s law of electrolysis
n=It/F
Electrolysis practice problem
check discussion worksheet
sketch galvanic cell
galvanic cell characteristics:
- spontaneous
- chemical energy turns into electrical energy
- wide range anode/cathode combinations possible
- cell potential determined from E red of half cells
often two diff half rxns
sketch concentration cell
concentration cell characteristics:
- type of galvanic call
- same half rxn in cathode and anode e
- different concentrations drive cell towards equilibrium (equal concentration)
sketch electrolytic cell
Electrolytic cell characteristics:
- non-spontaneous
- electrical energy turns into chemical energy
- powered with current
- metal ions from solution (or other products) form at electrode
-some similarities to cathode of galvanic cell
- E red determines which product forms if multiple possible
