Electroplating is an important industrial process to prevent corrosion and upvalue jewelry and cutlery.
Electroplating is an important industrial process to prevent corrosion and upvalue jewelry and cutlery (0:30). Electroplating takes place in electrolytic cells, which need an external power source (1:25). Our episode qualitatively describes electroplating using the example of a zinc spoon and silver (1:52). Quantitatively, we discuss Faraday’s law and the relationship between electric charge, current, time and Faraday’s constant first theoretically (3:41) and then with an example problem asking how much silver is being produced when applying 5.0A for 31 minutes (5:07).
Question: Electrolysis can also be used to produce or refine metals in electrometallurgy. What is the name of the electrolytic process to produce aluminum?
Thank you for listening to The APsolute RecAP: Chemistry Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2021 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Electrolysis.
Lets Zoom Out:
Unit 9 - Applications of Thermodynamics
Topic 9.10 - Electrolysis and Faraday’s Law.
Big idea - Energy
Introduction:
Today we are discussing a chemical process that saves lives - as chemistry always does - or at least makes our life a lot prettier, at low costs. Wonder what I am talking about? Electroplating! We can use electrolysis to deposit a thin layer of one metal on another metal. How does that make your life prettier or even safe lives? Electroplating is used in the automobile industry to prevent corrosion by coating steel, which easily rusts, with a thin layer of a zinc-nickel alloy. It is also used frequently to make the cars look nicer, when parts are coated with chromium. You can also buy jewelry that is electroplated with thin layers of silver or gold. Same look, but for a cheaper price!
Let’s zoom in:
Electrolysis reactions and the process of electroplating happen in electrolytic cells. Quick recap of episode 41: Electrolytic Cells need a power source, because electrolytic cells contain reactions that are thermodynamically unfavored. Therefore the chemical reaction needs electrical energy that can be converted to chemical energy.
Let’s start right away with an example and make our cutlery a bit fancier by electroplating a zinc spoon with a thin layer of silver. As discussed in episode 41, the zinc spoon and the silver metal will be in a beaker together, in our case filled with silver cyanide and connected with wires to a power source. The zinc spoon is our cathode, where electrons are gained, the silver metal is our anode, where electrons are lost. When silver metal loses electrons, it forms silver(I) cations that go into solution. At the same time, at the cathode, the silver(I) ions in solution will gain an electron and deposit as elemental silver on our spoon - tada!
Now there are a few things to consider during electrolysis and electroplating: How thick do I want my layer of coating to be? What current do I need? And how long do I have to let the reaction run? These questions can be answered with Faraday’s law and stoichiometry!
Faraday’s law tells us that the amount of substance that is being oxidized or reduced during electrolysis is proportional to the amount of electricity passed through and the time: electric charge (q) = current (I) in Ampere times time (t) in seconds. Your unit of electric charge therefore is Ampere second, which is more commonly referred to as coulomb. If we define it the other way around, coulomb is the quantity of charge passing a point in a circuit in 1 s when the current is 1 Ampere. Now how does that help with calculating a mass? The connection is Faraday’s constant, which states that the charge of 1 mol of electrons is 96,485 C. This constant is also given on the equation sheet - Yippie! But I am still not at mass? True, but now I can use my half reaction to relate the number of moles of electrons to the moles of elemental silver and then use the molar mass to determine the mass of silver.
This sounds way more complicated than it actually is, so let’s go back to our example: We apply 5.0 A for 31 minutes. How much silver is produced?
First step: Determine the electric charge in coulombs. As mentioned before, electric charge in coulomb is Ampere times time in seconds - in our case we would convert 31 minutes to seconds - 1,860 seconds - times 5.0 ampere, which is 9,300 Coulomb. Second step: Using Faraday’s constant, we can now convert our 9,300 Coulomb to moles of electrons: 9,300 Coulomb x 1 mole of electrons over 96,458 Coulomb is 0.096 moles of electrons. Third step: Looking at the half reaction, we know that we need 1 mole of electrons for 1 mole of elemental silver. Therefore, our 0.096 moles of electrons equal 0.096 moles of silver. And now, our fourth and last step, convert our moles of silver to grams of silver, using the molar mass of 107.8 g/mol and have... 10.4g of silver!
As you’ve seen in this example, the key is stoichiometry - and really dimensional analysis! As always with dimensional analysis, USE YOUR UNITS and make sure they cancel out! If they don’t you have to go back! With this, we can go in any direction: If we know the time and the mass we want to plate, we can calculate the current needed; if we know the current and mass we want to plate, we can calculate the time needed.
To recap:
Electrolytic Cells need a power source, because electrolytic cells contain reactions that are thermodynamically unfavored. Electroplating can be used to deposit a thin layer of metal on an object, like with corrosion protection. Using Faraday’s law, Faraday’s constant and dimensional analysis we can calculate the amount of metal deposited, the time needed or the current needed for electroplating.
Coming up next on the APsolute RecAP Chemistry Edition: its our Listener’s Choice episode! If you have a topic you’d like us to cover, please contact us through social media or our website.
Today’s Question of the day is about Electrolysis
Question: Electrolysis can also be used to produce or refine metals in electrometallurgy. What is the name of the electrolytic process to produce aluminum?