SCH4U                             Lab #16 -  Lemon Voltage

Introduction:  We know that a flow of electrons occurs because electrons will move a needle on a voltmeter.  We will try to make a needle move on a voltmeter using two different metal strips and a lemon as the electrolyte source.  The metals must be freshly cleaned with steel wool.  Voltage is a measure of how readily electrons flow from one electrode to the other.  The lemon is a good wet cell.  The pulp acts as a barrier and a salt bridge at the same time.   The acid in the lemon juice reacts with some of the metal ions of the metal that are in the solution surrounding the metal electrode.  The lemon juice also supplies hydrogen ions, which are more spontaneously reduced than some of the metals.  Alessandro Volta, an Italian scientist, devised the first battery in the 1790's by sandwiching salty wet cardboard between alternating pieces of zinc and copper.  Perhaps he could have used a lemon or two just as easily.

Materials:
1.    Lemon
2.    Metal pieces such as nickel, tin, lead, iron, zinc, magnesium etc.
3.    Wire leads with connectors (or alligator clips)
4.    Voltmeter

Procedure:
1.   Make two deep slits in the lemon several centimeters apart.

2.    Select two different metals and insert them in the lemon.

3.    Using the wire leads, connect the metal strips to the voltmeter.  If no reading occurs , reverse the poles.

4.    Test as many combinations of metals a you can and record the voltage reading in a chart of metal strip combinations.

5.    Wash you hands thoroughly when finished the laboratory work and discard the lemon in the proper waste container.

Reactions:    When two different metals are connected by wires and ionic solutions that will conduct electricity, electrons flow from one metal to the other through the wire.  If occurs only if the solutions are connected.  Within the lemon, the cells containing the lemon juice are separated by cell membranes which are semi permeable.  These are ideal for the conduction of ions across the membrane.   Within the juice of the lemon one metal (Mo) is oxidized and the other dissolved metal will be  reduced (M+1):

                                Mo  --->   M+1  + e-1       oxidized
                                M+1  + e-1  --->   Mo      reduced

    The dissolved metal is formed when the lemon juice reacts with the metal electrode.  In other cases, the metal is oxidized, as hydrogen ions at the other electrode are reduced: .

                                Mo  --->   M+1  + e-1              oxidized
                                2 H+1  + 2 e-1  --->   H2(g)      reduced

Oxidation means the lose of electrons.  Reduction means the gain of electrons.  The electrons flow through the wire and move the needle on the voltmeter .  You can tell which way the electrons are flowing by observing the voltmeter needle.

Questions:
1.   Which two metals produce the greatest voltage when used together?

2.    Use these two metals in combination with the others to rank the metals from "hardest to lose electrons"  at the head of the list to "easiest to lose electrons" at the end.  You need a starting point.  Silver is the common metal that is the most stable. i.e. "hardest to lose electrons"