Electrolysis

What happens when an electric current passes through a molten ionic compound?

The compound decomposes or breaks down

What type of solutions does this process also occur for?

aqueous solutions of ionic compounds

What are liquids and solutions that are able to conduct electricity called?

electrolytes

why do covalent compounds not undergo electrolysis?

Because they cannot conduct electricity

What is an electrolytic cell?

The name given to the set-up used in electrolysis and which consists of the following:

• Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte

• Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity

• Anode: the positive electrode of an electrolysis cell

• Anion: negatively charged ion which is attracted to the anode

• Cathode: the negative electrode of an electrolysis cell

• Cation: positively charged ion which is attracted to the cathode

Why can’t ionic compounds in the solid state conduct electricity?

They have no free ions that can move and carry charge

What states can ionic compounds conduct electricty?

They must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous

What happens to the ions when the cell is turned on and an electric current is passed through an electrolyte?

The ions in the solution start to move towards the electrodes

During electrolysis, what direction do the electrons move?

… ions within the electrolyte migrate towards the negatively charged electrode which is the …

… ions within the electrolyte migrate towards the positively charged electrode which is the …

• From the power supply towards the cathode

• positive, cathode

• negative, anode

When a … conducts it is the electrons that are moving through the metal. When a salt solution conducts it is the … in the solution that move towards the electrodes while carrying the electrons

electrons, ions

When predicting the products of any binary ionic compounds that are in their molten state and so they can conduct electricity as their ions can move freely and carry the charge, and do when they undergo electrolysis produce their corresponding elements, what is the first step?

Identify the ions present:

The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode, therefore the cathode product will always be metal and the product formed at the anode will always be non-metal

What is it important for electrodes to be so they don’t participate in a side reaction with the electrolyte?

Inert, such as graphite or platinum

What will aqueous solutions always have?

Water

In the electrolysis of aqueous solutions, what do the water molecules do?

Therefore, what do we now have?

• Dissociate producing H⁺ and OH^– ions: H₂O ⇌ H⁺ + OH^–

• an electrolyte that contains ions from their compound plus ions from the water

In aqueous solutions, what do which ions get discharged and at which electrode depend on?

Depend on the relative reactivity of the elements involved

In electrolysis of aqueous solutions, … and … solutions of the same compound give different products

Concerning anions, what ion tends to get discharged?

• concentrated and dilute

• the more concentrated ion will tend to get discharged over a more dilute ion

What’s attracted to the positive electrode (anode)?

What happens if halide ions ((Cl^-, Br^-, I^-) and OH^- are present?

What happens if no halide ions are present?

What happens in both cases?

• Negatively charged OH^– ions and non-metal ions

• then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)

• then OH^- is discharged at the anode, loses electrons and forms oxygen

• the other negative ions remain in the solution

What is attracted to the negative electrode (cathode) and how many will gain electrons?

What could be produced?

How do we know which and why?

• Positively charged H⁺ and metal ions are attracted to the negative electrode but only one will gain electrons

• Either hydrogen gas or the metal will be produced

• If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode

• This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged

• Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced

why are inert electrodes such as graphite usually used in electrolysis?

as they don’t take part in the process, just provide a surface for the reactions to happen on

What is the experimental process for the electrolysis of an aqueous solution use inert electrodes?

- Set up the apparatus as shown in the diagram

• Add the aqueous solution to the beaker

• Add two graphite rods as the electrodes and connect this to a power pack or battery

• Turn on the power pack or battery and allow electrolysis to take place

• Record the results in a suitable table (see below) and repeat for another solution, checking the electrodes in between runs to see if any metal has been deposited

• The gases produced can be collected in the test tubes to be tested later

Diagram showing electrolysis using inert electrodes 

Are non-electrodes ever used and for what?

Sometimes, for electroplating and purifying copper

Why is purifying copper necessary?

Because the copper obtained from its ore is not pure enough for the use in, for example, electrical wires

What is electroplating?

Give 3 examples

• A process used to coat metals in a thin layer of a different metal

  • coating copper or nickel jewellery with silver

  • coating taps with chromium

  • coating steel cutlery with silver

What is the practical setup for purifying copper?

Similar to that of inert electrodes except both electrodes are made from copper

• The anode is impure copper ore

• The cathode is pure copper 

• The electrolyte is copper (II) sulfate solution 

• Once the power supply is turned up

  • At the anode Copper atoms lose electrons to become ions in the electrolyte 

Cu  ⟶  Cu²⁺  +  2e^-

• As a result the anode decreases in mass and impurities collect underneath

  • At the cathode the copper ions from the electrolyte gain electrons to form copper atoms 

Cu²⁺ + 2e^-  ⟶  Cu

• Copper collects on the cathode causing the mass to increase 

Describe electroplating

• The cathode is the object you want to coat

• The anode is the metal you want to coat it with

• The electrolyte is a solution containing ions of the coating metal 

• During the process:

  • At the anode metal atoms lose electrons to form ions in the electrolyte

  • At the cathode metal ions from the electrolyte gain electrons and are discharged on the surface of the metal that needs coating 

What’s the key difference between inert and non-inert electrodes?

non-inert electrodes take part in the process, inert ones don’t

What is oxidation?

When a substance loses electrons

What is reduction?

When a substance gains electrons

What happens when the ions come into contact with the electrode?

Electrons are either lost or gained and they form neutral substances

What then happens to these neutral substances?

They are then discharged as products at the electrodes

What happens at the anode?

What happens at the cathode?

• Negatively charged ions lose electrons and are thus oxidised

• the positively charged ions gain electrons and are thus reduced

What can half-equations be used for?

What is important when writing them?

• To demonstrate the oxidations and reduction reactions that occur at each electrode

• that the number of atoms/ions on each side of the equation is balanced, as well as the charges

Describe the half equations in the electrolysis of molten lead(II) bromide

• negative electrode (cathode) : Pb²⁺ + 2e^– ⟶ Pb

• positive electrode (anode): 2Br^- – 2e^– ⟶ Br₂ OR 2Br^- ⟶ Br₂ + 2e^–

Describe the half equations in the electrolysis of molten aluminium oxide

• cathode: Al³⁺ + 3e^– ⟶ Al

anode: 2O^2- – 4e^– ⟶ O₂ OR 2O^2- ⟶ O₂  + 4e^–