The spontaneous chemical reaction between zinc metal and aqueous copper (II) sulfate is conducted so as to obtain the available chemical energy released in the form of electrical energy.
In principle, the energy from any chemical reaction may be harnessed in the form of electrical energy. Placing a strip of zinc metal into a solution of aqueous copper (II) sulfate causes a chemical reaction to occur. The surface of the zinc becomes coated with copper metal. Energy is released in the form of heat.
The model for this chemical reaction is one where electrons leave the zinc and react with copper (II) ions. It is often written in terms of substeps, or half-reactions:
Zn(c) + Cu2+(aq) Zn2+(aq) + Cu(c)
Zn(c) Zn2+(aq) + 2e-
Cu2+(aq) + 2e- Cu(c)
In designing a galvanic cell, the chemical reaction is engineered so that electrons flow from the zinc to the copper (II) solution through an external wire. In order to complete the circuit, an electrolytic connection is needed. There are several ways in which this connection can be accomplished. A 'salt bridge' is often used.
A paper wick moistened with electrolyte can be used to replace the glass bridge.
Wear gloves. Use steel wool or sandpaper to polish 1.5- x 15-cm strips of copper and zinc sheet metal.
Place 60 mL 1 M ZnSO4 solution in a 150-mL beaker. Place a polished zinc metal strip in the cup.
Fill a porous cup three-fourths full with 1 M CuSO4 solution. Copper (II) sulfate solution will seep through the cup giving it a wet appearance. Place a polished copper metal strip in the cup. Place the porous cup inside the beaker. This arrangement of the two pieces of metal, the beaker, and the porous cup is called a cell.
Connect alligator clip probes to a DC voltmeter. Connect the clips to the metal strips. Note and record the meter reading.
Prepare a second cell using the procedure just described. Select a wire with alligator clips at each end. Connect the zinc strip in one cell with the copper strip of the other cell.
Connect the alligator clips of the meter to the unconnected metal strips in the cells. Note and record the meter reading.
Use the two-cell battery to drive low power devices. For example, a bank of light emitting diodes can be powered by the battery.
Demonstrate alternatives for electrolytic connection of the half-cells. A salt bridge filled with an inert electrolyte (such as 1 M Na2SO4) may be used. A salt bridge consists of a U-tube filled with the electrolyte and stoppered with cotton or glasswool plugs.
A salt bridge also may be constructed from paper. Form a U-shape from several layers of 1.5- x 15-cm filter paper strips. Insert one leg of the U-shape into the beaker with the zinc metal and zinc solution. Insert the other leg into the beaker with the copper metal and copper solution. Connect the meter to the metal strips. Moisten the paper U-shape with 1 M sodium sulfate solution.
Why is the current low when the paper bridge is dry?
To illustrate a galvanic cell.
200 mL 1 M aqueous copper (II) sulfate (dissolve 50 g CuSO45H2O in enough distilled water to make 200 mL of solution.)
200 mL 1 M aqueous zinc sulfate (dissolve 58 g ZnSO47H2O in enough distilled water to make 200 mL of solution.)
2 strip zinc metal (1.5- x 15 cm)
2 strip copper metal (1.5- x 15 cm)
2 150-mL beaker
2 porous cup (5-cm diameter x 8-cm length)
DC meter (1-5 volts; modern digital meters are rugged and serve the purpose well. Use alligator clips on the meter probes.)
15-cm length copper wire with alligator clips at each end
leather or canvas gloves
5- x 5- x 1-cm glass U-tube, cotton plugs
100 mL 1 M Na2SO4 (dissolve 32 g Na2SO410H2O in enough distilled water to make 100 mL of solution.)
several circles filter paper cut into 2- x 15-cm strips
thin stem plastic transfer pipet
1 qt household ammonia solution
Use an autoranging digital DC meter if possible. A variety of inexpensive meters may be used.
Unglazed flower pots make suitable porous cups. Scale the size of the vessels to fit the porous cups used.
Teacher preparation: 20 minutes
Presentation: 10-15 minutes
The chemicals are toxic. Cuts from the metals strips and splinters from the steel wool are possible.
Do not ingest the chemicals. Wear leather or canvas gloves when polishing the metal strips.
Save the metal strips. Treat the copper sulfate solution with dilute sodium hydroxide; filter; place the recovered solid in a plastic bag and dispose of with ordinary solid trash. Discard excess zinc (II) sulfate using large amounts of water at the sink.
Soak the porous cups in dilute ammonia for 15 minutes at a hood. Change the soaking solution twice. Discard the used soaking solution at the sink. Soak for 15 minutes with distilled water three times. Permit the porous cups to dry. Repeat the procedure if the dried cups appear blue or show a crust of dried salt.
The salt bridge makes an electrical connection which allows electrons to flow from one container to the next.
Dry paper does not conduct electricity well. Ions in the solution conduct electricity through the paper bridge.
This static was created at 12:31:56 PM on Monday, April 20, 2015