Electrolysis

Why do covalent compounds not conduct electricity?

Electrical conductivity is the flow of charged particles.

In this case, charged particles means either delocalised electrons or ions.

These particles need to be free to move in a substance for that substance to be conductive.

Covalent compounds do not conduct electricity because there are no charged particles that are free to move.

Why do ionic compounds conduct electricity only when molten or in aqueous solution?

Conductivity of ionic compounds

• Ionic compounds can conduct electricity in the molten state or in solution

• This is because they have ions that can move and carry charge

• They cannot conduct electricity in the solid state as the ions are in fixed positions within the lattice and are unable to move

  

Molten or aqueous particles move and conduct electricity but cannot in solid form

What are anions and cations and where do they go in electrolysis?

Cations and anions

• Anions are negatively charged ions 

  • E.g. Cl^-, O^2-, SO₄^2-

• Cations are positively charged ions

  • E.g. K⁺, Mg²⁺, H⁺

• During electrolysis the electrons move from the anode towards the cathode

• Cations within the electrolyte migrate towards the negatively charged electrode which is the cathode

• Anions within the electrolyte migrate towards the positively charged electrode which is the anode

Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl

Cations are attracted to the cathode and anions to the anode due to their opposite charges

How do you write ionic half-equations representing the reactions at the electrodes during electrolysis and why are these reactions classified as oxidation or reduction?

Electrolysis half equations

• In electrochemistry we are mostly concerned with the transfer of electrons, hence the definitions of oxidation and reduction are applied in terms of electron loss or gain rather than the addition or removal of oxygen

• Oxidation is when a substance loses electrons

• Reduction is when a substance gains electrons

• As the ions come into contact with the electrode, electrons are either lost or gained and they form neutral substances

• These are then discharged as products at the electrodes

• At the anode, negatively charged ions lose electrons and are thus oxidised

• At the cathode, the positively charged ions gain electrons and are thus reduced

• This can be illustrated using half equations which describe the movement of electrons at each electrode

Electrolysis of molten lead(II) bromide

• In the electrolysis of molten lead(II) bromide the half equation at the negative electrode (cathode) is:

Pb²⁺ + 2e^– ⟶ Pb         Reduction

• At the positive electrode (anode) bromine gas is produced by the discharge of bromide ions:

2Br^– – 2e^– ⟶ Br₂          Oxidation

OR

2Br^– ⟶ Br₂  + 2e^–

Electrolysis of aqueous sodium chloride

• In the electrolysis of aqueous sodium chloride the half equation at the negative electrode (cathode) is:

2H⁺ + 2e^– ⟶ H₂           Reduction

• At the positive electrode (anode) chlorine gas is produced by the discharge of chloride ions:

2Cl^– – 2e^– ⟶ Cl₂          Oxidation

OR

2Cl^– ⟶ Cl₂  + 2e^–

Electrolysis of dilute sulfuric acid

• In the electrolysis of dilute sulfuric acid the half equation at the negative electrode (cathode) is:

2H⁺ + 2e^– ⟶ H₂                     Reduction

• At the positive electrode (anode) oxygen gas is produced by the discharge of water molecules:

2H₂O – 4e^– ⟶ O₂  +  4H⁺    Oxidation

OR

2H₂O  ⟶ O₂  +  4H⁺ + 4e^– 

Electrolysis of aqueous copper(II) sulfate

• In the electrolysis of aqueous copper(II) sulfate the half equation at the negative electrode (cathode) is:

Cu²⁺ + 2e^– ⟶ Cu                Reduction

• At the positive electrode (anode) oxygen gas is produced by the discharge of water molecules:

2H₂O – 4e^– ⟶ O₂  +  4H⁺    Oxidation

OR

2H₂O  ⟶ O₂  +  4H⁺ + 4e^–

How do you investigate electrolysis, using inert electrodes, of molten compounds and predict the products?

Electrolysis of molten compounds

• Binary ionic compound are compounds consisting of just two elements joined together by ionic bonding

  • E.g. lead(II) bromide

• When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge

• These compounds undergo electrolysis and always produce their corresponding element

• To predict the products of any binary molten compound first 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 the metal and the product formed at the anode will always be the non-metal

The electrolysis of molten lead(II) bromide Method

1. Add lead(II) bromide into a crucible and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge

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

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

Diagram showing the electrolysis of lead(II) bromide

Lead ions are attracted to the cathode, and bromide ions to the anode

What happens at the anode?

• Negative bromide ions move to the positive electrode (anode)

• At the anode, they lose two electrons to form bromine molecules

• There is bubbling at the anode as brown bromine gas is given off

What happens at the cathode?

• Positive lead ions move to the negative electrode (cathode)

• At the cathode they gain electrons to form grey lead metal 

• The lead deposits on the bottom of the electrode

How do you investigate electrolysis, using inert electrodes, of aqueous solutions and predict the products?

Electrolysis of aqueous solutions

• Aqueous solutions will always contain water molecules (H₂O)

• In the electrolysis of aqueous solutions, the water molecules dissociate producing H⁺ and OH^– ions:

H₂O ⇌ H⁺ + OH^–

• These ions are also involved in the electrolysis process and their chemistry must be considered

• We now have an electrolyte that contains ions from the compound plus ions from the water

• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

What is produced at the anode?

• Negatively charged OH^– ions and non-metal ions are attracted to the positive electrode

• If halide ions (Cl^-, Br^-, I^-) and OH^- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)

• If no halide ions are present, then OH^- is discharged at the anode, loses electrons and forms oxygen

• In both cases the other negative ion remains in solution

What is produced at the cathode?

• 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

The electrolysis of aqueous solutions

• The apparatus can be modified for the collection of gases by using inverted test tubes over the electrodes

• The electrodes are made from graphite which is inert and does not interfere with the electrolysis reactions

How do you investigate the electrolysis of aqueous solutions?

Practical: Investigate the electrolysis of aqueous solutions

Aim:

To electrolyse aqueous solutions of sodium chloride, sulfuric acid and copper(II)sulfate, and to collect and identify the products at each electrode

Diagram:

Electrolysis cell for collecting gaseous products from aqueous solutions

Method:

1. Add the aqueous solution to a beaker and cover the electrodes with the solution

2. Invert two small test tubes to collect any gaseous products

3. Connect the electrodes to a power pack or battery

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

5. Observations at each electrode are made

6. Gases collected in the test tube can be tested and identified

Testing the products

• If the gas produced at the cathode burns with a ‘pop’ when a sample is lit with a lighted splint, the gas is hydrogen

• If the gas produced at the anode relights a glowing splint dipped into a sample of the gas, the gas is oxygen

• If the anode gas bleaches of a piece of litmus paper, chlorine has been produced

• If a solid forms around the electrode, the metal have been formed

  • The colour can indicate the metal formed

Results:

Solution	Cathode observation	Anode observation
Sodium chloride	Colourless gas evolved which goes 'pop' with a lighted splint	Gas evolved which bleaches litmus paper
Dilute sulfuric acid	Colourless gas evolved which goes 'pop' with a lighted splint	Colourless gas evolved which relights a glowing splint
Copper(II) sulfate	Pink-brown deposit seen on the electrode	Colourless gas evolved which relights a glowing splint

Conclusions:

1. Sodium chloride solutions produces hydrogen at the cathode and chlorine at the anode

2. Dilute sulfuric acid produces hydrogen at the cathode and oxygen at the anode

3. Copper(II)sulfate solution produces copper at the cathode an oxygen at the anode