Chapter 20- Electrochemistry

Elements oxidation number

0

Monatomic ion oxidation number

charge

F oxidation number

-1

oxygen oxidation number

-2

oxygen oxidation number as peroxide

-1

hydrogen oxidation number

+1

hydrogen in metal hydride

-1

Sum of the oxidation numbers =

overall charge (0 in compound)

Oxidized

substance that loses electrons

Reduced

susbtance that gains electrons

Oxidizing Agent (oxidant)

thing that is reduced; causing oxidation

Reducing Agent (Reductant)

thing that is being oxidized; causes reduction

Half-Reactions

separate oxidation and reduction reactions

Oxidation Half-Reactions

electrons are products

Reduction Half-Reactions

electrons are reactants

How do you balance Half-Reactions in acidic solution?

balance elements (not H or O); balance O by adding water; balance H by adding H+; finishing balancing charge (add electrons); multiply each half-reaciton to make electrons equal; add and symplify

How do you balance Half-Reactions in basic solutions?

balance elements (not H or O); balance O by adding water; balance H by adding H+; add OH- to each side to neutralize H+; combine H+ and OH- to get water; combine water molecules; finishing balancing charge (add electrons); multiply each half-reaciton to make electrons equal; add and symplify

What is a redox reaction?

a reaction with oxidation AND reduction

Good Oxidizing Agents

electron acceptors, high reduction potential, electronegative, oxyanions; ex: fluorine, chlorine, oxygen, ozone, hydrogen peroxide, halogens, nitric acid (HNO₃)

Good reducing Agents

readily donate electrons; active metals (low ionization energies and electronegativities); Na, Mg, Al, Zn, Li; Metal Hydrides (contain H-)

Voltaic (galvanic) cells

devices in which electron transfer occurs via an external circuit rather than directly between reactants; spontaneous

What process occurs at the anode?

oxidation half-reaction

What process occurs at the cathode?

reduction half-reaction

Salt Bridge

used to complete electrical circuit; needs to have ions

Where do cations move in voltaic cell?

anode → cathode

Where do anions move in voltaic cell?

cathode → anode

Electrodes

two solid metals; cathode and anode

Flow of electrons in voltaic cell

from anode to cathodes; through switch, not salt bridge

Anode is (sign)

negative

Cathode is (sign)

positive

Why do electrons flow from anode to cathodes?

cathode has lower electrical potential energy

Electromotive force (emf)

force required to push electrons through the external circuit

Cell potential, E_cell

emf of a cell; = E°_red (cathode) - E°_red (anode)

Standard Cell Potential, E°_cell

emf for 1 M solutions at 25°C (standard conditions)

Standard Reduction potentials, E°_red

measured relative to standard hydrogen electroe (SHE)

SHE voltaic cell

SHE is cathode; Pt electrode in 1 M H+ solution

Large + reduction potential

nonmetals that are easily reduced; stronger the oxidizing agent

Large - reduction potential

metals easily oxidized; stronger reducing agent

Does changing the stoichiometric coefficient affect E°_red?

no

Reaction with E°_red > 0 are

spontaneous reductions

Reactions with E°_red < 0 are

spontaneous oxidations

+ E°_cell

spontaneous process (galvanic)

- E°_cell

nonspontaneous process

ΔG = -nFE

ΔG- change in free-energy; n- number of moles of electrons transferred; F- faraday’s constant (96,500 C/mol or J/V-mol); E- emf of cell

+ E°_cell and - ΔG

reaction is spontaneous

E°_cell = 0

equilibrium has been reached; non-functional voltaic cell

Nernst Equation

E = E° - (RT/nF)lnQ; relates emf to concentration

Concentration Cells

cell with emf based solely on difference in concentration; tends to equalize the concentrations in each compartment

Is the concentrated solution the cathode or anode?

cathode bc it has to reduce amount of concentrated ions

Q increase, E and ΔG do what?

E decreases (becomes more negative) and ΔG becomes more positive (less favorable)

E = 0, ΔG = 0, Q = K_eq

equilibrium is reached

Electrolysis reactions

nonspontaneous reactions that require external current in order to force the reaction to proceed

Where does reduction occur in electrolytic cells?

cathode

Where does oxidation occur in electrolytic cells?

anode

How do electrons flow in electrolytic cells?

anode to cathodes (forced)

In electrolytic cells, anode is (sign) and cathode is (sign)

positive; negative; opposite of galvinic cells

Active Electrodes

electrodes that take part in electrolysis; ex: electrolytic plating

Faraday’s Law of Electrolysis

amount of substance that undergoes oxidation or reduction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the cell; Q = current * time

Q = current * time

amperes/seconds to get quantitiy of charge in coulombs

Free-energy

measure of the maximum amount of useful work that can be obtained from a system

If work is negative, then work is _____ by the system and E is (sign)

performed; positive; work is watts