Topic: Electrochemical Cells Do 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 electricity OR 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 AgNO 3 + Cu  _______ +_______ Was this spontaneous? Was this a redox? So why didn’t we produce electricity? Ag CuNO 3 yes Yes, Ag was Reduced & Cu was Oxidized Half reactions weren’t separated

Electrochemical Cell Requirements 2 half-cells aqueous solution electrode 2 half-cells containing aqueous solution & an electrode wire Need to be connected by a wire for the electrons to flow through. salt bridge 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 SO 4 -2 aq Cu +2 aq SO 4 -2 aq

Voltmeter = measures electrical current When the cells reach equilibrium voltage = 0 =no more electrical current

Electrode Surface at which oxidation or reduction half-reaction occurs. Anode – Oxidation The anode = location for the oxidation half-reaction. Reduction – Cathode The cathode = location for the reduction half-reaction.

MOVIE CLIP MEMORIZE MEMORIZE – FatRed Cat ate An Ox – Fat Red Cat ate An Ox – Anode oxidation – Fat (getting bigger) Reduction Cathode yummy 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 e - – 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 0 increasing decreasing

How do you know which electrode is which? Use Table J 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 try Create an galvanic (electrochemical) cell with Al and Pb and AlNO 3 and Pb(NO 3 ) 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 AlNO 3 and Pb(NO 3 ) 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. Al anode cathode e -  e -  e -  e -  e -  e - Pb +2 & NO 3 -1 Al +3 & NO 3 -1

What half-reactions occured? REMEMBER Al got smaller and Pb got bigger Al  Al e - Pb e -  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 e - ) 3(Pb e -  Pb) + _______________________ 2Al + 3Pb +2  2Al Pb

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