Chapter 20: Electrochemistry

Redox reaction

There is a transfer of electrons from one reactant to another.

Nonredox reaction

No transfer of electrons from one reactant to another.

Oxidation

Reactant in chemical reaction loses one or more electrons.

Reduction

Reactant in chemical reaction gains one or more electrons.

Oxidation number (ON)

Represents charge that atom appears to have when electrons in each bond are assigned to more electronegative of two atoms involved in bond.

Rules for determining ON

1) ON of element in elemental state is 0. 2) ON of monatomic ion = to charge on ion. 3) ON of hydrogen is +1 in most H-containing compounds. 4) ON of Oxygen is -2. 5) In binary molecular compounds, more electronegative element is assigned negative ON equal to its charge in binary ionic compounds. 6) For compound or ion, sum of individual ON = 0; for polyatomic ion, sum = charge.

Identifying Redox Reaction

1) Determine ON for elements in reactants and products. 2) Compare ON for element on reactant and product side. Redox occurs if ON changes.

Oxidizing agent

Reactant in redox reaction that causes oxidation of another reactant by accepting its electrons (undergoes reduction).

Reducing agent

Reactant in redox that causes reduction of another reactant by providing electrons for other reactant to accept (undergoes oxidation).

Half reactions

Reactions involving just oxidation or reduction.

Oxidation half-reaction

Electrons are products.

Reduction half-reaction

Electrons are reactants.

electric current

the amount of charge passing a point in a conductor every second (ampere)

potential difference

difference in potential energy between reactants and products (V)

Electromotive Force (EMF)

amount of force pushing the electrons through the wire

cell potential (Ecell)

difference in potential energy between cathode and anode

Electrical Circuits

electrons flow through wire and ions flow through solution

electrolyte

ion exchange between 2 half cells of system

anode

where oxidation occurs

cathode

where reduction occurs

salt bridge

contains strong electrolyte and connects two half cells

substances in different phases are separated by what?

separated by "I"

multiple species in same phase are separated by what?

a comma

what indicates a salt bridge in an equation? Where the oxidation half cell is on the left and the reduction is on right

separated by "II"

what is a standard hydrogen electrode (SHE)?

reference tool to measure standard electrode potentials as half cell electrode potentials can't be measured on its own.

measure it relative to another half rxn

what are standard reduction potentials?

compare tendency for particular reduction half rxn to occur relative to reduction of H to H2

half rxn with stronger tendency toward reduction than SHE are what?

E°red = (+)

half rxn with stronger tendency toward oxidation than SHE are what?

E°red = (-)

Spontaneity of Redox Reactions

(+) E°cell = ΔG° < 0 = spontaneous; (-) Ecell = ΔG° > 0 = nonspontaneous.

Electrolytic cells

Nonspontaneous reaction driven by external electrical current.

electrons draw away from anode (connected to (+) terminal) and are forced to cathode (connected to (-) terminal)

Electrolysis

Process of using electrical current to drive nonspontaneous reaction (in electrolytic cells).

Overvoltage

When electrolysis reaction requires more voltage than E° predicts.

Electrolysis of pure compounds

Requires compound to be in molten/liquid state.

Voltaic cells

Spontaneous reaction generates electricity; anode is source of electrons, has negative charge; cathode draws electrons, has positive charge.