AP Chem - Chapter 20: Electrochemistry

What are the most common type of reactions?

Oxidation-Reduction Reactions (Redox) among the most common and important

What processes are redox reactions involved in?

rusting of iron, manufacture and action of bleaches, respiration of animals

Oxidation

loss of electrons (electrons are products)

Reduction

gain of electrons (electrons are reactants)

When do oxidation-reduction reactions occur?

when electrons are transferred from the atom that is oxidized to the atom that is reduced

Describe the thermodynamics of redox reactions

produce energy in the form of heat or electricity- thermodynamically "downhill" (spontaneous)

Are redox reactions spontaneous?

yes

What is electricity used for?

to make nonspontaneous processes occur

Electrochemistry

study of the relationships between electricity and chemical reactions

20.1 Oxidation-Reduction Reactions

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How is it determined whether a given chemical reaction is an oxidation-reduction reaction?

done by keeping track of oxidation numbers of all the elements involved in the reaction

Oxidation Number

the charge (in general)

What must occur in any redox reaction?

both oxidation AND reduction

Oxidizing Agent/Oxidant

substance that makes it possible for another substance to be oxidized - removes electrons from another substance by acquiring them itself (therefore is itself reduced)

Reducing Agent/Reductant

substance that gives up electrons causing another substance to be reduced (this substance is oxidized in the process)

20.2 Balancing Oxidation-Reduction Equations

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Important to note when balancing a chemical equation

law of conservation of mass (amount of each element must be the same on both sides of the equation)

What is special about balancing redox reactions?

the gains and losses of electrons must be balanced

Half-reactions

equations that show either oxidation or reduction alone - provides a general method for balancing redox reactions

How is an acidic solution balanced?

1. Divide the equation into two incomplete half-reactions
2. Balance each half-reaction
a. first balance the elements other than H and O
b. next, balance the O atoms by adding H20
c. then balance the H atoms by adding H+
d. finally balance the charge by a

20.3 Voltaic Cells

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Voltaic/Galvonic Cell

device in which the transfer of electrons takes place through an external pathway rather than directly between reactants (energy released from spontaneous redox reaction can then be used to perform electrical work)

Electrodes

two solid metals that are connected by the external circuit

Anode

electrode at which oxidation occurs (more +, lose e)

Cathode

electrode at which reduction occurs (more -, gain e)

Half-cell

name of each of the two compartments of the voltaic cell

Which way do electrons flow in a voltaic cell?

towards the cathode

What purpose does the salt bridge serve?

keeps both half-cells electrically neutral (contains electrolyte solution whose ions will not react with other ions in the cell)

Migration patterns

anions migrate to the anode, cations migrate to the cathode

20.4 Cell EMF

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Why do electrons transfer spontaneously in a voltaic cell?

electrons flow from the anode of a voltaic cell to the cathode because of a difference in potential energy (like a waterfall) - difference in potential energy per electrical charge is measured in volts

Electromotive Force (emf)/Cell Potential

potential difference between two electrodes of a voltaic cell - provides the driving force that pushed electrons through the external circuit

Cell Voltage

cell potential or emf because it is measured in volts

Cell potential will be positive for...

any reaction that proceeds spontaneously

What does the mf depend on?

specific reactions that occur at the cathode and anode, the concentrations of reactants and products, and the temperature

Standard Conditions

1 M concentration, 1 atm pressure, 25 degrees Celsius

Standard emf/cell potential

E^o cell - emf under standard conditions (superscript ^o indicated standard-state conditions)

Calculating Standard Reduction Reaction Cell Potential

Ered(cathode) - Ered(anode)

How is a half-cell potential written?

as a reduction - E^o red (intensive property and therefore changing coefficient makes no difference)

Standard Reduction Potentials Chart

higher means cathode and reduction and higher reducing agent

What information is presented with the strength of the driving force?

more positive Ered, greater driving force of reduction

Ered cathode vs anode

cathode is more positive

What information is presented with the sign of E?

more positive Ered = greater tendency for the reactant to be reduce or oxidize another species

Most common oxidizing agents

halogens, O2, and oxyanions (MnO4-...)

Most common reducing agents

H2 and active metals

20.5 Spontaneity of Redox Reactions

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Voltaic cells use redox reactions that proceed...

spontaneously

Using E to determine spontaneity

Positive E = spontaneous

Free energy charge

Delta G = -nFE where n is the number of electrons transferred, F is Faraday's constant (1 F = 96,500 C/mol) - note: both n and F are positive numbers

Spontaneity determined by delta G

negative delta G = spontaneous

20.6 Effect of Concentration on Cell EMF

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Nerst equation

calculate emf at nonstandard conditions - helps to understand why the emf of a voltaic cell drops as it discharges

Relationship between concentrations and emf

[reactants] up = emf up, [products] up = emf down

Concentration cell

contains the same substances on either side with different concentrations

20.7 Batteries

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Battery

portable, self-contained electrochemical power source that consists of one or more voltaic cells

Labels on battery parts

cathode labeled +, anode labeled -

Primary Batteries

cannot be recharged, one time use - must be discarded after emf drops to 0

Secondary Batteries/cell

can be recharged from an external power source after its emf has dropped

Lead-Acid Battery

6 voltaic cells each producing 2 V - cathode of lead dioxide (PbO2) and anode of Pb - both immersed in sulfuric acid

What happens when a battery is recharged?

direction of the reaction is reversed - done in a car by a generator

What is the most common primary battery?

alkaline battery - zinc anode with MnO2 cathode

20.8 Corrosion

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Corrosion Reaction

spontaneous redox reactions in which a metal is attacked by some substance in its environment and converted to an unwanted compound

Galvanized iron

iron coated with a thin layer of zinc

Cathodic protection

protecting a metal from corrosion by making it the cathode in a electrochemical cell

Sacrificial anode

the metal that is oxidized while protecting the cathode

20.9 Electrolysis

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What are electrolysis reactions?

process in which electricity is used to cause nonspontaneous redox reactions to occur

Where does electrolysis take place?

electrolytic cells - consists of two electrodes in a molten salt or solution

What does electrolysis of molten salts require?

high temperatures due to the high melting points of ionic substances

Inert electrodes

did not undergo reaction but merely served as the surface where oxidation and reduction occurred

Active electrodes

participate in the electrolysis process

Electroplating

uses electrolysis to deposit a thin layer of one metal on another in order to improve beauty or resistance

Units for charge passing through electrical circuit

coulombs - 1 mol of electrons has a charge of 96,500 C or 1 faraday (F) therefore 1 F = 96,500 C/mol e

Calculating coulombs

coulombs = (amperes)(seconds)