CHARGE ON THE ELECTRON BY ELECTROLYSIS


PURPOSE:	A value for the charge on the electron will be determined by measuring the 
mass deposited from solution in a cathode and the quantity of electric charge transferred during 
an electrolytic deposition.

WHAT'S THE POINT?	First, to show the connection between the electrical and physical 
changes in a copper electrode.  Second, to measure the amount of charge on a single electron.

BACKGROUND READING: Cutnell and Johnson 20.1.  See Fig 20.5.

THEORY:	According to modern chemical theory, when certain molecules are formed there 
are electron transfers from one atom to another so that the constituent parts of the molecule are 
said to be ionic in character.  For example, NaCl consists of Na+ ions and Cl-   ions in the 
ordinary crystal form of the material.  When the solid is placed in water it undergoes a process 
known as dissociation; that is, the crystal forces are broken down and free ions are liberated in 
the water.  The passage of electricity in a solution of positive and negative ions is directly related 
to the ion motion and the phenomenon is called electrolysis.

In this experiment a battery is connected to two electrodes which are in a copper sulfate solution 
(CuSO4) containing doubly positive copper ions (Cu2+) and doubly negative sulfate ions (SO42-
).  When a current is caused to move in the circuit, the negative ions are attracted to the anode, 
(positive electrode) while the positive copper ions are attracted to the cathode (negative 
electrode).  As long as the current is maintained, copper ions will thus be deposited on the 
cathode where they acquire the electrons needed to become copper atoms.  The increase in mass 
of the cathode can be measured, as can the magnitude and duration of the current.  Since the 
deposition of one copper ion corresponds to the transfer of two electrons, it is possible to 
calculate the electronic charge from the data mentioned and values for Avogadro's number and 
the atomic mass of copper.  With the apparatus provided, the results can be expected to be within 
5% of the accepted value.


PROCEDURE:	The coulometer used consists of a pair of plates connected together to form the 
anode and a single central plate forms the cathode (see Figure 1.).  It is important that the central 
electrode be negatively charged.  If it is positively charged, it will be found that a mass loss 
rather than a gain occurs (Why?).

Before assembling the circuit, clean all the plates by a light sanding with dry emery cloth.  Rinse 
in water and use the cleanest plate for the central position.  Clamp all three plates in the 
coulometer cap and fill the coulometer glass with copper sulfate solution (be sure the cap clamps 
are not in the solution as this makes the coulometer inoperative).  Now wire the circuit according 
to the diagram with the knife switch open.  Do not turn on the power supply until your wiring is 
verified by the instructor.

As a final step in preparation of the central electrode, it is desirable to perform an "ion cleaning".  
This is accomplished by reversing the leads at the coulometer, closing the knife switch and 
adjusting the rheostat so that the current is 0.50 amperes.  When this step has occurred for at least 
two minutes open the knife switch and return the leads to the diagram polarities.  Again close the 
switch and operate for another one or two minutes.  (This will deposit a fresh layer of copper on 
the "ion cleaned" central plate so that it should readily accept any further copper deposit due to 
an extended passage of current.)

Open the knife switch, remove the central plate being careful to HOLD IT BY THE EDGES 
ONLY.  Dip it into methanol and dry it with a CAREFUL air blast.  Determine the initial mass 
(mi) of the plate on a Mettler balance.  Replace the plate in the coulometer and close the knife 
switch, noting AND recording the EXACT time at which the current is restored.  By adjusting 
the rheostat as necessary to maintain a current of 0.50 amperes, copper deposition should 
continue for a period of about 35 minutes.  At the end of this period open the switch, noting 
AND recording the EXACT time the current stops.  The time interval of deposition will be the 
difference in the two recorded times and should be known to an accuracy of one or two seconds.  
(35 minutes is NOT the same as 35 minutes and 00 seconds!)

Remove the central plate, HOLDING IT BY ITS EDGES ONLY, rinse in clean water followed 
by methanol and air dry.  Do NOT dry the plate with a towel.  Determine the new mass (mf) of 
the cathode and record it.  Return the solution to the bottle, rinse and dry the coulometer glass 
and return all apparatus to the places where you obtained it.


ANALYSIS:	During the electrolysis the total charge Q transferred (in Coulombs) is given by

			Q = I (?t?					Equation 1

where I is the current (in Amperes) and ?t is the deposition time (in seconds).  A second 
expression for the total charge comes from the mass of deposited copper (mf - mi) and the 
atomic mass of copper (MW).  Specifically,

 			Q = (mf - mi)  NA  2e				Equation 2
		                                MW

where NA is Avogadro's number and e is the charge on the electron.

Explain each term in Equation 2, including the factor of 2.

Draw a picture of the experiment, with microscopic closeup pictures of the action at each 
electrode.  Label ions and electrons, and show their direction of movement.

Using Equation 1, calculate the total charge transferred during the deposition.  Use this result in 
Equation 2 and solve for the electronic charge e.  The atomic mass of Cu is 63.54 g/mole.  
Calculate the percent difference between the value and the currently accepted value of 1.6 x 10- 
19C.





















	 


Figure 1.  Wiring Diagram for Coulometer