Topic: Electrochemical Cells

Topic: Electrochemical CellsDo Now: 5 color pencils

Electrochemistry – the study of the relationship between chemical

potential energy and electrical energy

All this occurs Via flow of electrons

• Chemical rxns can produce electricityOR

• Electrical energy can be used carry out chemical reactions

A Chemical reactions can generate electrical energy if it occurs spontaneously

How does a spontaneous reaction generate electricity?

Electrochemical Cells AKA voltaic cell or galvanic cell

• Used when the chemical rxn is spontaneous • It produces electricity (energy) via flow of

electrons through a wire therefore is exothermic• Oxidation and Reduction Reaction have to be

separated

Remember the lab SR lab

AgNO3 + Cu _______ +_______

Was this spontaneous?Was this a redox?

So why didn’t we produce electricity?

Ag CuNO3

yesYes, Ag was Reduced & Cu was Oxidized

-2+5+1 0 -2+50 +1

Half reactions weren’t separated

Electrochemical Cell Requirements• 2 half-cells containing

aqueous solution & an electrode

• Need to be connected by a wire for the electrons to flow through.

• Need to be connected by a salt bridge to maintain electrical neutrality. (if electrons are moving ions need to move also = + ions will flow in same direction as electrons)

Zn+2 aq

SO4-2 aq

Cu+2 aq

SO4-2 aq

Voltmeter = measures electrical current

When the cells reach equilibriumvoltage = 0 =no more electrical

current

Electrode

Surface at which oxidation or reduction half-reaction occurs.

Anode – OxidationThe anode = location for the oxidation half-reaction.

Reduction – CathodeThe cathode = location for the reduction half-reaction.

MOVIE CLIP

• MEMORIZE–Fat Red Cat ate An Ox –Anode oxidation–Fat (getting bigger) Reduction

Cathodeyummy

Crap

• Since Cu is the cathode – a fat red cat = gain mass– Where does the mass come from?– Reduction of Cu+2 is gaining 2 e- to make Cu– So the concentration of Cu+2 ions decreases

• Since Zn is the anode – An ox was eaten = lost e- = getting smaller– Oxidation = Lose electrons, who is going to lose Zn or Zn+2

– Zn Zn+2 + 2e-

– So the concentration of Zn+2 increases

Is this reaction at equilibrium? How can you tell?What is happening to the concentration of Zn+2 ions?What is happening to the concentration of Cu+2 ions?

No – volts not a 0increasing

decreasing

How do you know which electrode is which?

• Use Table J to predict which electrode is the anode / cathode.– higher in Table J. gets oxidized– Anode = Oxidation = Electron Donor

– lower in Table J. gets reduced– Cathode = Reduction = Electron Acceptor

Label anode and cathode

anode cathode

• Electrons flow from Anode to Cathode

• Cathode is (+) Anode is (-)• Positive Ions flow in same direction as electrons

via Salt bridge (too offset the negative electrons)• Which direction would the negative ions flow?

anode cathode

e- e- e- e- e- e-

+ + + +

Anode = site of oxidation =getting smaller

cathode = site of reduction =getting larger

What’s wrong with this picture?

No salt bridge = needed to complete the circuit so ions can flow

Now you tryCreate an galvanic (electrochemical) cell with Al and Pb and

AlNO3 and Pb(NO3)2

Label anode and cathode(Use Table J), direction of electron flow in wire, direction of positive ion flow in salt bridge,

positive electrode, negative electrode.

Pb Al

Pb Al

Create an galvanic (electrochemical) cell with Al and Pb and AlNO3 and Pb(NO3)2

Label anode and cathode(Use Table J), direction of electron flow in wire, direction of positive ion flow in salt bridge,

positive electrode, negative electrode.

Alanodecathode

e- e- e- e- e- e-

Pb+2 & NO3-1 Al+3 & NO3

-1

What half-reactions occured?REMEMBER Al got smaller and Pb got bigger

Al Al+3 + 3e-

Pb+2 + 2e- Pb

Al was the anode, it was oxidized, it lost e-

Pb got bigger. How? By gaining electrons. The Pb+2 ions gained 2 e- to make Pb.

Overall Spontaneous Rxn

2(Al Al+3 + 3e-)

3(Pb+2 + 2e- Pb)+ _______________________

2Al + 3Pb+2 2Al+3 + 3Pb

2Al + 3Pb+2 2Al+3 + 3Pb

• Which electrode is losing mass?

• Which electrode is gaining mass?

• What’s happening to the [Al+3]?

• What’s happening to the [Pb+2]?

Al because now Al+3 ions in solution

Pb, Pb+2 gains 2e- an makes solid Pb

Increasing

Decreasing

Notation for Cells

Application: Batteries

Dry Cell

Mercury battery