C2 - Electrolysis Flashcards
Why is electrolysis more complicated in aqueous solutions?
Because ions in the water are present as well as the ions in the compound
What is brine?
Sodium chloride solution
What will form at the positive electrode if possible?
A halogen
What is electrolysis?
The breakdown of a substance by passing electricity through it
How do you remember the difference between oxidation and reduction?
OIL RIG
Oxidation is loss (of electrons)
Reduction is gain (of electrons)
What 2 chemical properties are needed to use electrolysis in breaking down of a substance?
Substance is ionic
Ions are free to move
What are the cathode and anode?
Anode is positive electrode
Cathode is negative electrode
What is a half equation?
A chemical equation that shows how an ion gains or loses electrons at an electrode
2H+ + 2e- –> H^2
2Cl- –> Cl^2 + 2e-
What does diatomic mean?
An element that naturally has 2 covalently bonded atoms such as H^2
Which substances made by electrolysis are diatomic?
Hydrogen at the cathode
Halides at the anode
How do OH- ions oxidise? (Chemical equation)
4OH- –> O^2 + 2H^2O + 4e-
What is Faraday’s law of electrolysis?
The higher the current used, the faster the substance is produced
What is an electrolyte?
An ionic substance that is either a liquid or dissolved in water
Why isn’t an ionic solid an electrolyte?
The ions are not free to move so the current cannot pass through them
What are the 4 uses of electrolysis?
Extract a metal from an ore
Purify copper
Create substances from salt
Improve surface of a metal
Why is electrolysis avoided if possible?
Expensive due to the cost of the electricity as a high current is needed
Why is electrolysis used for purifying copper?
Copper needs to be very pure for use as electrical wiring
Show how sodium chloride solution would break down in electrolysis
Hydrogen is less reactive so at the negative electrode, the H+ ions lose their charge
Because a halogen will form at the positive electrode if possible, the Cl- ions will lose their charge and become the halogen chlorine
When the electricity is turned off, the Na+ and OH- ions are left so they react to form sodium hydroxide
What are the rules about reactions at the negative electrode?
If metal ions and hydrogen ions are present, the metal ions will stay in solution and hydrogen will form if the metal is more reactive than hydrogen
This is because, the more reactive an element, the keener it is to stay as ions
What are the rules about reactions at the positive electrode?
If hydroxide and halide ions are present, halogens will be produced and the hydroxide will stay as ions
If no halide is present, oxygen will be formed
What are the 2 reasons that the electrolysis of sodium chloride solution is useful in industry?
Chlorine is useful in bleach and plastics
Sodium hydroxide is a very strong alkali and used widely in the chemical industry
Why is electrolysis of aluminium oxide so expensive and what is done to counter this?
It has to be heated up to 2000C for it to become molten
Cryolite (less common aluminium ore) is added and this brings the melting point down to 900C
Why does the positive electrode have to be replaced?
When oxygen forms at the electrode, it reacts with the carbon of the electrode and forms carbon dioxide
This reaction eats away the carbon of the electrode
Describe electroplating
You can coat the surface of one metal with another metal
The negative electrode is the metal to be plated and the positive electrode is the metal that will plate it. The electrolyte also contains the ions if the playing metal
The ions that plate the metal come from the solution and the positive electrode keeps the solution topped up
