11th2nd: APCHEM Electrochemistry

How does a salt bridge help prolong a battery?

semi-permeable passageway between the cathode and anode that contains an aqueous ionic salt containing a common ion with both solutions; negative ions flow to anode where there is a buildup of positive charges, positive ions flow to cathode where there is a buildup of negative charge

Electromotive Force (EMF)

the reduction potential difference between the cell’s anode and cathode

EMF other names (2)

E(cell), cell potential

Reduction potential (E) tends to increase with an atom’s _; why?

electronegativity, because greater ability to attract e-

Base (0) reduction potential reaction and voltage value

2H+ + 2e- → H2(g) ; 0V

What do you have to note about the formatting of reduction potentials?

Even though the oxidation reaction has the non charged species as the reactant, its correct reduction potential is still given by the equation that is the oxidation reaction’s reverse

E˚ conditions

1 atm or 1M solution

At equilibrium, ∆G is _; why?

0; at equilibrium, the forward and reverse rates are the same, meaning that there is no net gain/loss of free energy

At equilibrium, EMF is _; why?

0V; the forward and reverse rates are the same, meaning the net current of electrons is the same

Equation that relates delta G naught and K

∆G˚ = -RTlnK

Equation that relates delta G naught and Q (hint, requires another variable!)

∆G = ∆G˚ + RTlnQ

Equation that relates delta G( ˚ ) and E( ˚ ) of cell

∆G(˚) = -nFE(˚)

Equation that relates E cell naught and K

E˚ = (RT/nF)lnK

Equation that relates E cell naught and K (assuming that temperature is 25C)

E˚ = (.0592/n)logK

Which equation relates Q and E˚? (hint: there is another variable!) 2 ways!

1. E = E˚ - (RT/nF)lnQ

2. E = E˚ - (.0592/n)logQ

Why is E = E˚ at standard conditions?

Using the equation, E = E˚ - (RT/nF)lnQ, ln Q evalulates to zero because the concentrations/pressures are all 1, meaning the whole term is zeroed

Faraday’s Constant (F) value

96485 Coulombs / mol e- transferred

n =

mol of electrons transferred / mol of reaction

E units

V

∆G units

J/mol of reaction

E vs E˚ in relation to Le Chatelier?

Remember that E˚ doesn’t change, but the E will because you’d be changing pressures and concentrations permanently (equilibrium will eventually be reached because the proportions will even out again, but the proportions don’t consist of the same amounts)

How does E relate to Le Chatelier? (discuss both positive and negative E˚)

+E˚: E becomes higher if a change causes a forward shift, E lowers if a change causes a reverse shift

-E˚: E becomes more negative if a change causes a reverse shift, E becomes less negative if forward shift

Balancing Redox tips: If a solution containing the redox reaction is basic and the combined half reactions contain H+, what do you do?

Solution is basic, so doesn’t make sense to have hydrogen ions, meaning add enough OH- to the H+ to make H2O, and add that same number of OH- to the other side as well

In voltaic/galvanic cells, you (CAN/CANNOT) assume that the E˚ value indicates whether the half reaction is reduction or oxidation, but with electrolytic cells, you (CAN/CANNOT) assume that

CAN; CANNOT

Departing from standard conditions (1M and 1atm), how will E compare to E˚ if water is added to the anode solution?

E larger; the concentration of the anode’s ions, which are one of the products, decreases, causing Q to go from 1 to a fraction and causing lnQ to evaluate as negative, meaning the whole term will actually be added onto E˚

Oxidation agents are atoms that are …. and ….

smaller; more electronegative

Which is the stronger oxidizing agent? Cl2 or HClO?

HClO; Cl2 oxidation number is 0, and considering that HClO functions based on its central atom, Cl+ has an oxidation number of +1 and the oxygen pulls electrons further away, creating greater electronegativity

When you’re given the reduction potential of a species, how do you find its oxidation potential?

negate the reduction potential

Electrolytic Cell definition; E˚ value

nonspontaneous process that require external electricity to run; negative

For electrolytic cells that only contain positive and negative ions at the respective ends, how do you determine which are involved in the half reactions?

The ion with the lower charge will most likely be the one to be oxidized

I =

current, or speed of electron flow, in A (Amps)

Express Amps in Coulombs/second

1 Amp = 1C/s

1 mole of electrons is … Coulombs

96485 C

In the Q or K expression, what can go in together?

pressure and molarity