batteries and cells

BATTERIES AND CELLS

Batteries• A battery is a group of cells,

connected together in a series (to form more energy)

ELECTRIC CELL• Continuously converts

chemical energy into electrical energy• Real life electrochemistry!

• Each cell is composed of 2 electrodes (solid electrical conductors – usually 2 metals or graphite and metal)

• Each cell also contains 1 electrolyte (aqueous electrical conductor)

• 1 Positive electrode = CATHODE• Reduction occurs at the cathode

(GERC)

• 1 Negative electrode = ANODE• Oxidation occurs at the anode

(LEOA)

Voltaic Cells• A voltaic cell is an

arrangement of 2 half cells separated by a porous boundary

Half Cells• A half cell consists of 1

electrode and 1 electrolyte

Half cell Notation

• A half cell can be represented through the following shorthand

Zn(s) ZnSO4(aq) CuSO4(aq) Cu(s)

Porous Boundary• A porous boundary

separates the 2 electrolytes, while still permitting ions to move between the 2 solutions (through tiny openings in a salt bridge)

External Circuit• The connection between the

anode and the cathode through which the electrons travel (metal wire) • Often hooked to an voltmeter

ELECTRICITY• Electricity is

the flow of electrons from the anode to the cathode!!

Voltmeter• A device that is used to measure

the energy difference between any 2 points in an electric circuit• Energy is measured in VOLTS (V)

Energy Potential Difference

• Fancy way of describing the voltage (difference in energy)

• Voltage depends on the chemical composition of the reactants within the cell

Cell Potential = Voltage

• The theoretical voltage can be calculated using the formula:

Ecell = SOA – SRAOr……

Ecell = Cathode - Anode

Where Did We Get These #s???

• The standard Hydrogen electrode is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials.

• It is used to form a basis for comparison with all other electrode reactions, therefore hydrogen’s standard electrode potential is declared to be zero at all temperatures

• Potentials of any other electrodes are compared with that of the standard hydrogen electrode at the same temperature.

Challenging Diploma Example

• If the Ni2+(aq) + 2e- Ni(s) half reaction is defignated as the reference half reaction with an electrode potential of 0.00V, then what is the electrical potential for the Fe3+(aq) + e- Fe2+(aq) half reaction?

Electric Current• A measure of the rate of flow

of charge past a point in an electrical circuit

•Measured in Amperes (A)

Example:

• Write the equations for the half-reactions and the overall reaction that occurs in the following cell:

C(s) Fe2+(aq), Fe3+(aq) Cr2O72-(aq), H+(aq) C(s)

• Step 1: Label the ALL oxidizing and reducing agents.

• Step 2: Find the STRONGEST OXIDIZING AGENT and the STRONGEST REDUCING AGENT

C(s) Fe2+(aq), Fe3+(aq) Cr2O72-(aq), H+(aq) C(s)

• Remember the SRA gets oxidized at the ANODE!

• Remember the SOA gets reduced at the CATHODE!

• Step 3: Write the ½ reactions (from chart or using acid method)

• cathodeCr2O72-(aq) + 14H+(aq) + 6e- 2Cr3+ (aq) + 7H2O(l)

• anode 6 [ Fe2+(aq) Fe3+(aq) + e- ]

• Step 4: Balance electrons and cross out products and reactants to combine reactions

Cr2O72-(aq) + 14H+(aq) + 6e- 2Cr3+ (aq) + 7H2O(l)

6 [ Fe2+(aq) Fe3+(aq) + e- ]

Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) 2Cr3+ (aq) + 7H2O(l) + Fe3+(aq)

• Step 5: draw the cell representation of what is going on, including electron movement

Example 2:

• A silver copper voltaic cell consists of a copper half cell with a Cu(s) electrode and a 1.0M Cu(NO3)2 electrolyte, as well as a silver half-cell with an Ag(s) electrode and a 1.0M AgNO3 electrolyte. The 2 half cells are connected by a salt bridge containing KNO3. Write the half reactions and the net reaction.

• SRA = Cu(s) gets oxidized at the ANODE

• SOA = Ag+(aq) gets reduced at the CATHODE

• cathode2 [ Ag+(aq) + e- Ag(s) ]

• anodeCu(s) Cu2+(aq) + 2e-

• Net reaction

Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s)

Is this a spontaneous reaction????