Ch 12 - Electrochemistry Flashcards
What is an electrochemical cell?
describes any cell in which oxidation-reduction reactions take place
What are electrodes?
strips of metal or other conductive materials placed in an electrolyte solution
What is an anode v a cathode? How do electrons and current flow between the 2?
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
What are cell diagrams?
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
What are galvanic (voltaic) cells?
house spontaneous reactions (G < 0) with a positive electromotive force (anode has negative designation)
What are electrolytic cells?
house nonspontaneous reactions (G > 0) with a negative electromotive force
- can be used to create useful products through electrolysis
- cathode as negative designation
What are concentration cells?
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
What does the charge on an electrode depend on?
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
What are rechargeable batteries? How are they ranked?
- 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
What cathode/anode are used when discharging and charging lead-acid batteries? What is their energy density?
- 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
What cathode/anode are used when discharging and charging nickel-cadmium batteries? What is their energy density?
- 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
What are nickel-metal hydride batteries?
have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective, and are significantly less toxic
What is a surge current?
an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved
What does a reduction potential quantify?
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
How are standard reduction potentials (Ered) calculated?
- by comparison to the standard hydrogen electrode (SHE) under standard conditions of 298 K, 1 atm, and 1 M
What is the standard reduction potential of standard hydrogen electrode (SHE)?
0 V
What is the standard electromotive force (Ecell)?
the difference in standard reduction potential between the 2 half-cells
Ecell = Ered,cathode - Ered,anode
What is the difference between the reduction potentials of 2 half reactions for galvanic and electrolytic cells?
- galvanic is positive
- electrolytic is negative
How do electromotive force and change in free energy relate to each other?
- 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
What does the Nernst equation describe?
the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force
Ecell = Eocell - 0.0592/n x logQ
What is the relationship between the equilibrium constant (Keq) and electromotive force (Ecell)?
- 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
What is the purpose of the salt bridge?
to exchange anions and cations to balance, or dissipate, newly generated changes
- contain inert electrolytes (ionic compounds are strong electrolytes)
What is Faraday’s law?
- 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
What is one Faraday (F) equivalent too?
the amount of charge contained in one mole of electrons (1F = 96,485 C)
