lecture 21- electrochemical cells
DESCRIPTION
Chapter 21 Lecture for Honors & Prep ChemistryTRANSCRIPT
BELLWORK
Balance the following redox reaction using the “half-reaction” method
HNO2 + I- NO + I2
2e- + 2H+ + 2HNO2 2NO + 2H2O
2I- I2 + 2e-
2H+ + 2HNO2 + 2I- 2NO + 2H2O + I2
Electrochemical cells produce an electric current from redox
reactions
CAN ALSO BE CALLED A
Voltaic cell Galvanic cell Battery
A simple battery separates the reduction reaction from the
oxidation reaction so that the electrons must travel through a wire
Oxidation e- e- e- e- e- e- e- Reduction
Lose e- e- e- e- e- e- e- e- e- e- e- e- Gain e-
Redox reactionZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)
Separate into 2 half-reactions and balance
Zn(s) Zn2+ + 2e- oxidation
2e- + Cu2+(aq) Cu(s) reduction
How to make a battery or an electrochemical cell
e- e- e- e- e- e- e- e- e- e- e- e-
e- e- e- e- e- e- e- e- e-
Create each half reaction in a separate chamber called a half cell and connect them by a wire
Oxidation Reduction
half cell half cell
To make the Zinc half cell Add a strip of Zinc metal to a
solution containing Zn2+
1M ZnSO4
Zn(s)
Zinc metal
To make the copper half cell Add a strip of copper metal to
a solution containing Cu2+
1M CuSO4
Cu(s)
Copper metal
The metal strips are electrodesThe solutions are electrolytes
oxidation occurs at the anode
Zn(s) Zn2+ + 2e- metal is lost
reduction occurs at the cathode
2e- + Cu2+(aq) Cu(s) metal is plated
An Ox Red Cat
When a wire connects the half cells an electrical current is created.
This can’t occur for long because positive charge will build up at the anode and negative charge will build up at the cathode
A salt bridge is used to keep the two cell compartments neutral
The salt bridge contains a salt (positive and negative ions) that flows into each beaker as
needed to keep the charge neutral
The salt bridge completes the circuit and the current will continue to flow
SALT BRIDGE
Gotta have it!!!
How do you determine the voltage of a battery?
Use the table of reduction potentials.• All of the half-reactions are for reduction• Eº is the electrical potential of the ½ reaction in Volts• If you need the oxidation potential, flip the equation and change
the sign of Eº. All reduction potentials are reversible.• The top of the table = elements that are easily reduced, and not
easily oxidized. They take electrons easily.• The bottom of the table= elements that are easily oxidized and
not easily reduced. They give electrons easily.
To determine the voltage of a battery you add the voltages of the two half-reactions
Flip the reaction for the oxidized element and change the sign of its voltage.
Ex. What is the voltage of the Zn-Cu battery?Cu2+ + 2e- Cu(s) 0.34V Zn2+ + 2e- Zn(s) -0.76V oxidized because it is the
lowest on the table of reduction potentials
Flip itZn(s) Zn2+ + 2e- 0.76V
The element at the top of the table will be reduced. The lower one will be oxidized.
ADD THESE FOR VOLTAGE
Describe the voltaic cell represented as:
Al(s) | Al2(SO4)3(aq) || NiSO4(aq) | Ni(s)