SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Oxidation-Reduction and Electrochemistry
QUANTITATIVE ASPECTS OF ELECTROLYSIS
Relationship Between Quantity of Electricity and Amount of Products
The amounts of products liberated at the electrodes of an electrolytic cell are related to the quantity of electricity passed through the cell and the electrode reactions.
In electrolysis, 1 mole of electrons is called 1 faraday of electric charge. In the electrolysis of molten NaCl, 1 faraday will liberate 1 mole of sodium atoms.
Cathode reaction: Na+(aq) + e− → Na(s)
At the same time, 1 mole of Cl− ions at the anode will form 1 mole of chlorine atoms and thus 0.5 mole of chlorine molecules.
If the metallic ion had been Ca2+, 1 faraday would release only 0.5 mole of calcium atoms since each calcium ion requires 2 electrons, as shown here:
Ca2+(aq)+ 2e− → Ca(s)
SAMPLE PROBLEM: How many moles of electrons are required to reduce 2.93 grams of nickel ions from melted NiCl2?
Use the equation to determine the quantity of electrons needed.
The reaction is
Ni2+(aq)+ 2e− → Ni(s)
The 2.93 g represents
Since 2 mol of electrons are needed to produce 1 mol of Ni(s),
0.1 mol of electrons