Galvanic Cells ELECTROCHEMISTRY/CHEMICAL REACTIONS SCH4C/SCH3U

 Consider the redox reaction between zinc and copper (II) sulfate: Zn (s) + CuSO 4(aq)  Cu (s) + ZnSO 4(aq) Oxidation: Zn (s)  Zn 2+ (aq) + 2e - Reduction: Cu 2+ (aq) + 2e -  Zn (s)  In 1836, John Frederic Daniell separated these 2 half reactions in the hopes of creating a current  He realized that each ½ reaction needed its own components – a metal to conduct current, a site for the redox reactions and a conducting fluid to supply ions.

The Daniell Cell  The beaker with the Zn ½-reaction is the oxidation half-cell  The beaker with the Cu ½-reaction is the reduction half-cell  The electrons flow from the zinc half cell to the copper half cell through a conducting wire, creating a current  If we placed a load between the two half cells, it would function as a battery

Galvanic Cells  The Daniell cell was the first galvanic cell, which converts chemical energy from redox reactions with electrical energy  Batteries are made to contain galvanic cells to produce electricity.

Galvanic Cells  Reactions that occur in a galvanic cell are spontaneous reactions, which means they need no outside assistance/ener gy to occur

Galvanic Cells  The oxidation of zinc and the reduction of copper ions occur in separate vessels, called half- cells  Each half cell contains a conducting electrode  Electron transfers occur in each half-cell between atoms on the electrode and the ions in the solution

Galvanic Cells  The electrode where oxidation occurs is known as the anode  The electrode where reduction occurs is known as the cathode  Each electrode is immersed in an electrolyte solution that has ions of the same metal as the electrode  Zinc electrode in zinc nitrate solution  Copper electrode in copper (II) nitrate solution

Galvanic Cells  The half-cells are connected by a wire (for electron flow) and a salt bridge (for ion flow)

Cell Reactions  The reactions that occur in each half-cell can be represented by half-reactions  Anode half-reaction (oxidation): Zn(s)  Zn 2+ (aq)+ 2e -  Cathode half-reaction (reduction): Cu 2+ (aq)+ 2e -  Cu(s)  Atoms from the zinc electrode lose electrons and decrease in mass while the copper electrode gains electrons and increases in mass

Cell Reactions  If we add the two half-reactions, we get the overall cell reaction for the zinc/copper galvanic cell: Zn(s)  Zn 2+ (aq)+ 2e - Cu 2+ (aq)+ 2e -  Cu(s). Zn(s) + Cu 2+ (aq)  Zn 2+ (aq)+ Cu(s) Electrons will always flow from the anode to the cathode!

Example  Write the anode, cathode, and overall cell reactions that occur when each pair of half cells is combined to form a galvanic cell.  A) a copper strip in a copper (II) nitrate solution and a tin strip in a tin (II) chloride solution  Establish the elements oxidized and reduced:  Tin is higher than copper in the activity series, so tin will be oxidized, copper reduced

A) a copper strip in a copper (II) nitrate solution and a tin strip in a tin (II) chloride solution  Anode ½-reaction:Sn(s)  Sn 2+ (aq) + 2e -  Cathode ½-reaction:Cu 2+ (aq) + 2e -  Cu(s)  Overall Reaction: Sn(s) + Cu 2+ (aq)  Sn 2+ (aq) + Cu(s)

B) An aluminum strip in a solution of aluminum nitrate and a silver strip in a solution of silver nitrate  Establish elements oxidized and reduced:  Al is higher on activity series, so is oxidized, silver is reduced  Anode ½ reaction: Al(s)  Al 3+ (aq) + 3e-  Cathode ½ reaction:Ag + (aq) + 1e-  Ag (s)  Balance the charges:  Multiply cathode reaction by 3 to ensure electrons are balanced between the two reactions. 3Ag + (aq) + 3e-  3Ag (s) Al(s)  Al 3+ (aq) + 3e-  Overall Reaction: 3Ag + (aq) + Al(s)  Al 3+ (aq) + 3Ag(s)

Electrochemical Cell Potential  The cell potential (voltage) for a galvanic cell can be predicted from half-reactions  We can find the reduction potential (E˚ reduction ) for the reduction ½ reaction on our tables  Find the oxidation ½ reaction on the reduction potential table but reverse the sign  E˚ oxidation = - E˚ reduction  Find the overall cell potential by adding the potentials of the half cells  E˚ cell = E˚ oxidation + E˚ reduction When E˚ cell is positive, the reaction is spontaneous

Standard Reduction Potentials