Faraday electrochemistry

Michael Faraday was one of the greatest experimental scientists of the 19th century. He is best known for contributions made to the study of electricity in both its physical and chemical aspects. These exercises are based on an experiment from which Faraday obtained stoichiometric information by electrochemical means. He noted that the same quantity of electricity that decomposed a tin chloride into chlorine and 3.2 grains of tin also is capable of decomposing 0.49742 grains of water into hydrogen and oxygen.

1a) How many moles of water are in 0.49742 grains? (1 grain = 0.0648 grams.)
b) If one mole of tin is formed for every mole of water decomposed, then the 3.2 grains of tin represents the same number of moles as you found in the previous question. If so, what is the mass of one mole of tin?

2) Now let us analyze the problem using our knowledge of electrons and valences.
a) Consider the decomposition of water:

2 H2O -- > 2 H2 + O2
Which element is oxidized in the course of the reaction? Which is reduced? How many electrons must be transferred in the course of decomposing two molecules of water?

b) Consider the decomposition of tin(II) chloride:

SnCl2 -- > Sn + Cl2
Which element is oxidized in the course of the reaction? Which is reduced? How many electrons must be transferred in the course of decomposing one molecule of SnCl2?

c) How many moles of Sn are electrochemically "equivalent" to one mole of water? (Hint: Write the chemical equations above so that they refer to the same number of electrons transferred and then compare the stoichiometric numbers for water and tin.)

3) The Faraday constant, 96485 coulombs per mole, was named after Michael Faraday; it is the magnitude of the electrical charge of one mole of electrons. What quantity of electrical charge passed through Faraday's electrochemical apparatus to decompose 0.49742 grains of water?

Reference

Michael Faraday, "Experimental Researches in Electricity. Seventh Series," Philosophical Transactions of the Royal Society 124, 77-122 (1834)
Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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