Chem271 Ch. 20

Front 1

Steps for balancing redox rxns in acidic solns:

Back 1

1. Assign oxidation states

2. Write as 2 half rxns

3. Balance the rxns: 1) elements other than H and O, 2) balance O with H2O, 3) balance H with H+, 4) balance charge by adding e-

4. Multiply half rxns by integers so that # of e- lost = # of e- gained

5. Add half rxns and simplify

6. Check to make sure that # of atoms and total charge match up on both sides

Front 2

Steps for balancing redox rxns in basic solns:

Back 2

Follow steps just like in acidic solns.

Add the same # of OH- to both sides of half rxns as # of H+.

This will replace H+ with H2O, and OH- will replace H+.

Front 3

E released in a spontaneous redox rxn can be used to perform electrical work.

Accomplished in _______.

Back 3

voltaic or galvanic cells, where the transfer of e- takes place through external pathway rather than directly b/w reactants.

Front 4

Describe setup of voltaic (galvanic) cells.

Electrodes.

Back 4

Oxidation and reduction occur in separate compartments called half-cells, and e- must flow through external circuit (connecting wire).

One half-cell contains oxidation half rxn, other half-cell reduction half rxn.
Electrodes-metals connected by external circuit. Can be made of materials in reaction, or of conducting material which serves as a surface for e- transfer

Anode--electrode at which oxidation occurs

Cathode--electrode at which reduction occurs

Front 5

For a voltaic cell to work, the solns in the two half-cells must remain electrically _______.

Solution to this?

Back 5

neutral.

Ex. As Zn is oxidized at anode, Zn2+ enter soln. There must be a way for the cations to migrate out of anode compartment, or for anions to migrate in.

A porous glass disc separating the two compartments, or a salt bridge--to allow migration of ions to maintain neutrality.

A salt bridge consists of a U-shaped tube containing an electrolyte soln, whose ions will not react with ions in cell. Electrolyte is often in paste or gel. Ions from salt bridge migrate to neutralize charge in compartments.

Anions always migrate toward anode and cations toward cathode, for neutralization.

Front 6

In any voltaic cell the e- flow from ______ through external circuit to the _____.

Which is labeled with a (+), and which is labeled with a (-)?

Back 6

e- flow from the anode to the cathode.

anode (-)
cathode (+)

Front 7

e- flow from anode to cathode b/c of a difference in _________.

How is this measured? (units)

Back 7

potential E.

The difference in PE per electrical charge (the potential difference) b/w electrodes is measured in units of volts.

One volt is the potential difference required to impart 1 Joule of E to charge of 1 coulomb.

1 V = 1 J/C

one e- has charge of 1.6 * 10^-19 C

Front 8

The potential difference between electrodes provides driving force that pushes e- through external circuit.

This potential difference is called the __________.

Back 8

electromotive force (emf).

emf of a cell (E_cell) is called cell potential, or cell voltage.

Front 9

For any cell rxn that is spontaneous, the cell potential (or cell voltage, or emf of cell) will be _________.

Back 9

positive.

Front 10

What does the emf of a voltaic cell depend on? (3)

Back 10

the specific rxns occurrring in half cells

the conc. of reactants and products

temperature (usually assumed to be 25 C)

Front 11

Standard conditions for standard emf or standard cell potential:

Back 11

Eo_cell

1 M conc. for reactants and products in soln

1 atm pressure for gases

25 degrees C

Front 12

How do you determine the emf of a voltaic cell?

Back 12

By tabulating standard reduction potentials for all half-cell rxns, and using them to determine Eo_cell.

Eo_cell = Eo_red (cathode) - Eo_red (anode)

Front 13

How do you measure the standard reduction potential of half rxns, since all voltaic cells must contain two half cells?

Back 13

All standard reduction potentials are assigned according to one reference half rxn.

The reference half rxn is reduction of H+ to H2(g) under standard conditions

2H+ + 2e- --> H2
Eo_red = 0V

The rxn being measured is always written as a reduction, even if it is being oxidized.

Front 14

What is the electrode desiged to produce the reference half rxn of H+ to H2?

Back 14

SHE (standard hydrogen electrode), or NHE (normal hydrogen electrode)

SHE or NHE consists of a platinum wire connected to a piece of platinum foil covered w/ finely divide platinum to serve as inert surface for rxn.

Front 15

B/c electrical potential measures PE per electrical charge, standard red potentials are _________ properties.

Back 15

intensive properties.

In other words, if the amt of substances were increased, the E and charges involved would increase, but the RATIO of E to electrical charge would remain constant. V = J/C

Changing the stoichiometric coefficient in a half rxn does not affect the value of the standard red potential.

Eo_red for reduction of 50 Zn2+ = reduction of 1 Zn2+

Front 16

The more positive the value of Eo_red, the ________ the driving force for the rxn.

Or, the greater the tendency for the reactant of the half rxn to be ______, and to _______ another species.

Back 16

greater.

be reduced, and to oxidize.

In a voltaic cell, the red rxn at cathode must have a more positive value than anode rxn--b/c its greater driving force forces the anode rxn to occur "in reverse" as an oxidation.

The cathode Eo_red is always more positive or less negative than the anode Eo_red!

Front 17

Most frequently used oxidizing agents are:

Back 17

halogens, O2, and oxyanions like MnO-4, Cr2O7-2, and NO3-, whose central atoms have high pos. oxidation states.

F2 is the strongest oxidizing agent.

Front 18

The half rxn with the smallest red potential is most easily reversed as an _______.

Back 18

oxidation.

Li+ + e- --> Li
Eo_red = -3.05 V
Most difficult to reduce, so it is the strongest reducing agent.

Front 19

Most frequently used reducing agents are:

Back 19

H2, active metals (alkali and alkaline earth metals), cations with neg E0_red like Zn and Fe

Front 20

Solns of reducing agents are difficult to store for extended periods b/c of the ubiquitous presence of ______.

Back 20

O2, a good oxidizing agent.

Front 21

To remember relps b/w strengths of oxidizing and reducing agents:

exothermic rxn b/w sodium metal and chlorine gas to form sodium chloride

Back 21

Cl2(g) + Na(s) --> NaCl(aq)

Cl2 is reduced--stg oxidant

Na is oxidized--stg reductant

Cl- is very weak reductant

Na+ is very weak oxidant

Front 22

eq of emf for general redox rxns, not necessarily in voltaic cells:

A pos value of E indicates a _____ process, and a neg value indicates a ______ process.

Back 22

Eo = Eo_red (reduction) - Eo_red (oxidation)

spontaneous, nonspontaneous.

Front 23

Change in Gibbs free E (delta G) is a measure of _________.

What is the relp b/w emf and free E change?

Back 23

spontaneity of a process at a constant temp and pressure.

delta G = -nFE

n = # of e- transferred
F = Faraday's constant, the quantity of electrical charge on one MOLE of e-
1 F = 96485 C/mol = 96485 J/V-mol

units of delta G are J/mol (per mol of rxn as written in eq)

Front 24

A _____ value of E and a _____ value of delta G indicate that a rxn is spontaneous.

Back 24

positive, negative.

Front 25

Equations for relating standard emf Eo, to standard free E change delta G, to equilibrium constant K:

Back 25

-nFEo = delta Go = -RTlnK

Front 26

As a voltaic cell is discharged, the reactants are consumed so that conc. change. The emf ____, until E = ___, at which point the cell is dead. At this point the conc of reactants and products are in ________.

Back 26

The emf drops, until E = 0.
in equilibrium.

Front 27

The dependence of the cell emf on conc. can be obtained from the dependence of delta G on conc.

Derive the equation that can be used to calculate the emf generated under nonstandard conditions.

Back 27

delta G = delta Go + RTlnQ

substitute with delta G = -nFE:
-nFE = -nFEo + RTlnQ

solve for E for Nernst eq:
E = Eo - RT/nF * lnQ
or
E = Eo - 2.303RT/nF *logQ

Front 28

Increasing conc. of the reactant and decreasing conc. of the product relative to standard conditions _______ the emf of the cell relative to standard conditions.

Back 28

increases.

It pushes the conc. further away from equilibrium, so the emf is larger. As the rxn proceeds, Q increases and emf decreases. When E = 0, delta G = 0 and no net rxn occurs.

Front 29

Define concentration cell.

Back 29

A cell based on the emf generated b/c of a difference in conc. only, not b/w different reactive species.

B/c cell emf depends on conc., a voltaic cell can constructed using the same species in both the anode and cathode compartments.

Front 30

Example of a concentration cell.

half rxns and overall rxn.

Back 30

Anode: Ni(s) in 1M soln of Ni+2(aq)

Cathode: Ni(s) in 0.001M soln of Ni+2(aq)

half-cell rxns are reverse of each other, and though Eo_cell = 0, the cell is under NONSTANDARD conditions b/c conc. of Ni+2 are not equal.

Cell will operate until conc. of Ni+2 in both compartments are equal.

Anode: Ni --> Ni+2 (dilute) + 2e-
Cathode: Ni+2 (conc) + 2e- --> Ni
Overall: Ni+2 (conc) --> Ni+2 (dilute)

Front 31

How do you calculate the emf of a conc. cell?

Back 31

Using the Nernst eq.

Ex. Q = [Ni+2 dilute]/[Ni+2 conc]

E = Eo - 2.303RT/nF * logQ
E = Eo - 0.0592/n * logQ

E = 0 - 0.0592/2 * log[0.001M]/[1M]
E = +0.0888V

difference in conc. provides driving force. When conc. become equal, Q = 1 and E = 0.

Front 32

What is a battery?

Back 32

A battery is a portable, self-contained electrochemical power source that consists of one or more voltaic cells.

Front 33

1.5-V batteries are single voltaic cells. Greater voltages can be achieved by using multiple voltaic cells in a series.

When batteries are connected in series, the total emf is the _____.

Back 33

sum of the individual emfs.

Front 34

primary vs. secondary cells

Back 34

primary cell must be discarded or recycles after emf drops to 0.

secondary cell can be recharged from an external power source.

Front 35

Common types of batteries: (5)

Back 35

-lead-acid
-alkaline
-nickel-cadmium, nickel-metal-hydride, lithium-ion
-hydrogen fuel cells
-direct methanol fuel cells

Front 36

Lead-acid batteries

12-V lead-acid automotive battery consists of ______ in series, each producing ______.

cathode:
anode:

B/c reactants are _____, there is no need to separate the cell into compartments. To keep the electrodes from touching each other, ____ or _____ spacers are placed between.

Back 36

6 voltaic cells in series, each producing 2 V.

cathode: PbO2(s) in HSO4-
anode: Pb(s) in HSO4-

solids, wood or glass-fiber spacers.

Front 37

More lead-acid

Using rxn where reactants and products are solids has another benefit. B/c solids are ______ from rxn quotient Q, amts of Pb, PbO2, and PbSO4 have ________ on emf. This helps battery maintain a relatively ________ during discharge. But conc. of H2SO4 ______, b/c it is ______.

One advantage of lead-acid battery is that it can be ________.

Recharging in the automotive battery is provided by the _______. It is possible b/c PbSO4 formed ______ to the electrodes, and is converted to ____ at one electrode and _____ at the other.

Back 37

excluded
no effect
constant
varies, consumed

recharged.
An external source is used to reverse the direction of the overall cell rxn to regenerate Pb and PbO2.

alternator, driven by the engine
adheres, Pb, PbO2

Front 38

Alkaline Battery is the most common primary battery.

anode:
cathode:

Battery is sealed in a _____ to reduce risk of leakage of the _______.

The emf is _____ at room temp.

Back 38

anode: powdered zinc metal immobilized in a gel, in conc. soln KOH (this is why it's named alkaline)

cathode: mixture of MnO2(s) and graphite, separated from anode by porous fabric

steel can, conc. KOH

1.55 V

Front 39

Nickel-cadmium (nicad) is one of the most common rechargeable batteries for cell phones, notebooks, and video recorders.

cathode:
anode:

Like the ____ battery, the solid products adhere to the electrodes, which permits the electrode rxns to be _____.

A single nicad voltaic cell has emf of _____. Battery packs usually contain _____ or more cells in a series.

Drawbacks: Cd(s) is a ____ and ____ metal.

Back 39

cathode: nickel oxyhydroxide [NiO(OH)]
anode: cadmium metal Cd(s)

lead-acid, reversed during charging.

1.30V
three

Cd(s) is toxic and heavy.

Front 40

Development of __________ batteries to replace nicad.

cathode:
anode:

During oxidation at anode, hydrogen atoms lose electrons, and the H+ ions react with _____ to form _____, which is reversed during charging.

_______ automobiles, powered by both a _______ engine and an ______ motor, use NiMH batteries to store _____. Batteries are recharged by the electric motor while ____, so the batteries can last up to _____.

Back 40

nickel-metal-hydride (NiMH)

cathode: same, nickel oxyhydroxide [NiO(OH)]
anode: metal alloy such as ZrNi2, that has ability to absorb hydrogen atoms

OH- ions to form H2O

Hybrid gas-electric automobiles
gasoline, electric
electrical power
braking
8 years

Front 41

Newest rechargeable battery to receive large use in consumer electronic devices is the ______ battery, found in ____ and ____.

b/c lithium is a very _____ element, Li-ion batteries achieve a greater ______ _____, amt of energy stored per unit mass, than ______-based batteries.

Technology of Li-ion batteries is very different, based on ability of Li+ ions to be ______ in and out of ______.

For example, the cathode is ______, the anode is _____. When charging, the cathode is oxidized, the Li ions migrate to the anode. During discharge...

Li-ion produces max voltage of _____, considerably higher than typical 1.5-V _____ batteries.

Back 41

lithium-ion (Li-ion)
cell phones, laptops

light, energy density, nickel-based

inserted and removed out of layered solids
layered graphite, cobalt oxide (LiCoO2)

During discharge, Li ions migrate from anode to cathode, enabling e- to flow through external circuit.

3.7V, alkaline

Front 42

The ________ energy released by the __________ of fuels convert water to _______, which drives a turbine which drives a generator. Typically a max of ____ of the energy from combustion is converted to electricity; the rest is lost as ______.

Direct production of electricity from fuels by a ______ could, in principle, yield a ______ rate of conversion. Voltaic cells that perform this conversion using conventional fuels, such as ___ and ____, are called _________.

Back 42

thermal, combustion, steam
40%, heat
voltaic cell, higher
H2, CH4, fuel cells

Front 43

The most promising fuel-cell system involves rxn of ___ and ___ to form ____ as the only product, generating electricity twice as efficiently.

The standard emf of an H2-O2 cell is ____, reflecting the large driving force for the rxn.

This fuel cell, aka _____ for "___-exchange ____," the anode and cathode are separated by a thin polymer membrane that is permeable to _____, which acts like a ______. The electrodes are typically made of _____. It operates at 80 degrees Celsuis, catalyzed by a thin layer of _____ on electrodes.

Efforts are directed towards developing fuel cells that use conventional fuels like _____ and _____, which are not as difficult to handle and distribute as H2.

Back 43

H2, O2, H20
1.23V
PEM fuel cell, proton-exchange membrane
protons, salt bridge
graphite
platinum
hydrocarbons
alcohols

Front 44

The direct methanol fuel cell is similar to the _____ fuel cell. It uses methanol, CH3OH.

These cells operate around 120 degrees Celsius. The shortcoming is that the methanol cell needs a greater quantity of _______ than PEM cells. Also, H20 and _____ are produced, the latter of which is not as environmentally friendly. But liquid methanol is far easier to store and transport than H2.

Back 44

PEM fuel cell
platinum catalyst
CO2

Front 45

Corrosion of metals are spontaneous ______ rxns in which a metal is attacked by some substance in environment and converted to an unwanted cpd.

For nearly all metals, ________ is a thermodynamically favorable process in air at room temp. But it can form an insulating protective _____ layer that prevents further rxn of underlying metal.

Ex. ____ metal forms a thin protective coat of _____, a hydrated form of ____.
Other Ex. ____ metal, ____ steel, semiconductor ____

Back 45

redox, oxide
aluminum (emf of Al3+ is favorable), oxide, Al2O3
magnesium, stainless steel, silicon (forms SiO2 coating)

Front 46

Common corrosion process: ____ of ____.

It requires ___ and _____.

Reduction of O2 requires ____, so ______ the pH makes the reduction less favorable.

___ formed at the anode is further oxidized to ____, which forms the hydrated _____ known as rust.

Enhanced corrosion cause by the presence of ____ is evident on autos during the winter, b/c it provides the ______ necessary to complete the electrical circuit.

Back 46

rusting of iron
O2, water
H+, increasing
Fe2+, Fe3+, hydrated iron(III) oxide
salts, electrolytes

Front 47

Iron is often covered with a coat of _____ or another metal such as ___ or ____ to protect its surface from corrosion.

________ iron, which is iron coated with a thin layer of ____. Even if coating is broken, __ is ____ to oxidize than Fe, so it serves as the ____ and Fe as the ____, at which ___ is oxidized.

Protecting a metal by making it the cathode is called ___________. The metal that is oxidized while protecting the cathode is called the __________.

Back 47

paint, zinc, tin
Galvanized, zinc
Zn is easier to oxidize
anode, cathode at which O2 is oxidized
cathodic protection, sacrificial anodeT

Front 48

Nonspontaneous redox rxns that are driven by an outside source of electrical energy are called _________, and they take place in _________.

It consists of two electrodes in ______ or a soln. The source of current acts like an ________ pump.

Ex. NaCl --> Na(l) and Cl2(g)
Net movement of Na+ to _____ and Cl- to _____. Electrons move from anode to cathode, and the anode is connected to the ____ terminal and the cathode to the _____ terminal of the source.

The _______ of ionic substances requires _____ temp for electrolysis.

Back 48

electrolysis rxns, electrolytic cells
molten salt

cathode, anode
positive, negative
high melting pts, high temp

Front 49

Electrodes that are ___ do not undergo rxn, but serve as the surface for redox rxns.
Active electrodes are electrodes that ________ in the electrolysis process.

_________ uses electrolysis to deposit a __________ to improve beauty or resistance to corrosion.

Ex. electroplating nickel onto piece of steel
cathode:
anode:
overall rxn looks as if nothing has been accomplished, but Ni has been ________.
Standard emf for rxn is ____.

Back 49

inert, participate

Electrolysis, thin layer of metal on another metal

cathode: steel strip, where Ni +2 is reduced
anode: nickel strip, where Ni is oxidized
transferred from Ni anode to steel cathode

0, and only small emf is needed to push the atoms from one electrode to another.

Front 50

For any half rxn, the amt of a substance that is reduced or oxidized in an electrolytic cell is _______ to the # of e- passed into cell.

Quantity of charge passing through the electrical circuit is measured in ______. The charge on one mole of electrons is ______.
A _____ is the quantity of charge passing a point in a circuit in ___ when the current is ___.
Eq?

Back 50

directly proportional
coulombs
96485 C (1 faraday)
coulomb, 1s, 1 ampere
coulombs = amperes * seconds

Front 51

How do you calculate the max useful electrical work from a voltaic cell?

Work done by a system on its surroundings will be negative or positive?

To force a nonspontaneous rxn, E_cell is negative, and an external potential E_ext must be applied. Magnitude of E_ext must be _____. Then the amt of work performed by surroundings on system is now ____.

Back 51

b/c delta G = -nFE, w_max = -nFE
negative, b/c E is positive
E_ext > -E_cell
w = nFE_ext (positive b/c surr are doing work on sys)

Front 52

Electrical work can be expressing in watts times time. The watt (W) is a unit of electrical power that equals ____.
A kilo-watt-hour (kWh) equals ____. It is the amt of energy needed to _____.

Back 52

1 W = J/s
1 kWh = 3.6 * 10^6 J
provide 1000 W for one hour.