Electrolysis uses electrical energy to drive a chemical reaction that would not happen on its own. It is important in metal refining, electroplating, production of elements, and rechargeable battery chemistry. In an electrolytic cell, a DC power supply pushes electrons through an external circuit while ions move through the electrolyte.
Faraday's laws let us predict how much substance is produced from the current and time.
Key Facts
- Charge passed: Q = It, where Q is in coulombs, I is in amperes, and t is in seconds.
- Faraday constant: F = 96485 C mol^-1, the charge carried by 1 mole of electrons.
- Moles of electrons transferred: n(e-) = Q/F.
- For copper plating: Cu^2+ + 2e- -> Cu(s), so 2 moles of electrons deposit 1 mole of copper.
- Mass deposited: m = (ItM)/(zF), where M is molar mass and z is electrons per ion.
- In an electrolytic cell, oxidation occurs at the positive anode and reduction occurs at the negative cathode.
Vocabulary
- Electrolysis
- Electrolysis is the use of electrical energy to force a nonspontaneous chemical reaction to occur.
- Electrolyte
- An electrolyte is a solution or molten substance containing mobile ions that carry charge through the cell.
- Anode
- The anode is the electrode where oxidation occurs, and in an electrolytic cell it is connected to the positive terminal.
- Cathode
- The cathode is the electrode where reduction occurs, and in an electrolytic cell it is connected to the negative terminal.
- Faraday constant
- The Faraday constant is the charge of one mole of electrons, approximately 96485 coulombs per mole.
Common Mistakes to Avoid
- Using minutes directly in Q = It is wrong because time must be in seconds when current is in amperes.
- Forgetting the electron coefficient z gives the wrong mass because ions with different charges require different numbers of electrons per atom or ion deposited.
- Calling the cathode positive in electrolysis is wrong because in an electrolytic cell the cathode is connected to the negative terminal even though reduction still occurs there.
- Assuming all current always plates the desired metal can be wrong because side reactions, impurities, or gas formation can reduce current efficiency.
Practice Questions
- 1 A current of 2.00 A flows through a CuSO4(aq) electrolytic cell for 30.0 min. How many grams of Cu are plated at the cathode? Use M(Cu) = 63.55 g/mol and z = 2.
- 2 How long, in minutes, must a 5.00 A current run to deposit 1.50 g of Ag from Ag+ solution? Use M(Ag) = 107.87 g/mol and z = 1.
- 3 In a copper electroplating cell with a copper anode and a metal object as the cathode, explain why the copper anode loses mass while the cathode gains a copper coating.