Section 10.3—Batteries & Redox Reactions

How do we harness the electricity to form a battery?

Voltaic Cells & Electricity

Voltaic Cell (also called Galvanic Cell) – Turns chemical energy into electrical energy.

They separate the reduction reaction from the oxidation reaction and harness the electricity as electrons flow from one side to the other

Electricity – Flow of electrons over a wire.

What Makes up a Voltaic Cell?

If the both the reduction and oxidation reaction happened in one container together, there would be no way to harness the electricity

What Makes up a Voltaic Cell?

If the both the reduction and oxidation reaction happened in one container together, there would be no way to harness the electricity

Electrons flow from the oxidation compartment to the reduction compartment

Metal and a wire are needed for the electrons to flow from one compartment to another.

What Makes up a Voltaic Cell?

If the redox reactions include solid metals, then those are used to conduct the electrons

If not, a non-reactive metal (such as platinum) is used.

CathodeReduction occurs

AnodeOxidation

occurs

Over time, there will be a build up of negative charge in the reduction compartment.

A salt bridge is added. As negative electrons flow to the reduction compartment, ions in the salt bridge flow to balance the charge.

What Makes up a Voltaic Cell?Voltaic cells are made of several components.

CathodeReduction occurs

AnodeOxidation

occurs

----

--

This would cause the cell to stop when enough charge builds up.

Salt Bridge

----

---

-

----

-+

++

++

+

+

Line Notation

Line Notation – A short-hand method of describing the components of a voltaic cell

The oxidation reaction (anode) is always written first.The reactant is written first for each half-reaction.The anode and cathode are separated by a “║”Different states of matter are separated by a “│”Species of the same state of matter are separated by a “,”

Line Notation

ExampleWrite the line notation for:

Mg (s) + Al+3 (aq) Mg+2 (aq) + Al (s)

The oxidation reaction (anode) is always written first.The reactant is written first for each half-reaction.The anode and cathode are separated by a “║”Different states of matter are separated by a “│”Species of the same state of matter are separated by a “,”

Mg0 Mg+2 is oxidation reaction (anode)Al+3 Al0 is the reduction reaction (cathode)

Mg (s)│Mg+2 (aq) ║ Al+3 (aq) │Al (s)

Line Notation

ExampleWrite the line notation for:

Mg (s) + Al+3 (aq) Mg+2 (aq) + Al (s)

How is Electricity Measured?

Electrons have potential energy based on their position

As electrons change places in a redox reaction, they have a different potential energy

The difference in potential energy as the electron moves is how electricity is measured.

The potential difference (or Electromotive Force, EMF or E) is measured in Volts (V)

Standard Reduction Potential

Standard Reduction Potential – Electromotive Force (EMF) produced when a reduction reaction occurs with hydrogen as the reference.

The hydrogen reaction has been defined as “0” and all others are compared to it.

Standard Reduction Potential is an intensive property…it doesn’t matter how many atoms undergo the change, the Standard Reduction Potential is the same!

The table lists standard reduction potential

An oxidation reaction is the opposite process from reduction

Therefore the oxidation potential is “- reduction potential”

reactionoxidationpotentialreduction

reactionreductionpotentialreductionEMFcell

anodecathodeEMF

Calculating Cell Potential

Remember—the number of atoms or moles doesn’t matter…don’t multiple reduction potentials by balanced equation coefficients!

Cell Potential & Spontaneity

A spontaneous reaction is one that occurs on its own

A voltaic cell will operate spontaneously if the EMF is positive

ExampleDetermine if a cell will react as written

spontaneously: Fe+3 + Cu Cu+2 + Fe+2

Example #2

ExampleDetermine if a cell will react as written

spontaneously: Fe+3 + Cu Cu+2 + Fe+2

Look up standard reduction potentials:Fe+3 + e-1 Fe+2 0.77VCu+2 + 2 e-1 Cu 0.34 V

Fe+3 Fe+2 reduction (cathode)Cu Cu+2 oxidation (anode)

anodecathodeEMF

VVEMF 34.077.0

EMF = 0.40 VIt will proceed spontaneously

Example #2

Electrolysis & Electrolytic Cell

Electrolysis – Putting in electrical energy to force a redox reaction in the non-spontaneous direction.

Electrolytic Cell – Cell that converts electricity to chemical energy.

Electrolytic Cell Example

To force a cell in the non-spontaneous direction, you must put in at least the voltage that is produced from the spontaneous process.

Fe+3 + Cu Cu+2 + Fe+2

Produces 0.44 V Spontaneously

Requires at least 0.44 V to push in non-spontaneous direction

Batteries as Electrolytic Cells

When a battery is being re-charged, it’s acting as an electrolytic cell!

What did you learn about batteries?

Batteries

Voltaic Cells

Voltaic Cells

are

Oxidation-Reduction Reaction

Oxidation-Reduction Reaction

Which produce electricity

through

ElectronsElectrons

Electrolytic cells

Electrolytic cells

OxidationOxidation

ReductionReduction

Single replacement

reactions

Single replacement

reactions

Activity Series

Activity Series

Is transfer of

When lost

When gained

One type

Possibility determined

by

When being recharged are

Force a non-spontaneous