C12 Electrolysis

Question 1

What is an electrode?

Answer 1

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

Question 2

What is the anode?

Answer 2

Anode: the positive electrode of an electrolysis cell

Question 3

What is the cathode?

Answer 3

Cathode: the negative electrode of an electrolysis cell

Question 4

What is an electrolyte?

Answer 4

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

Question 5

Why is direct current used in electrolysis rather than alternating current?

Answer 5

Direction of Current: Electrolysis requires a unidirectional flow of current to drive the chemical reactions at the electrodes. In electrolysis, ions move toward the electrodes, and the chemical reactions at the anode and cathode depend on the consistent polarity of the electrodes. AC reverses direction periodically, meaning that the electrodes would alternate between being positive and negative. This would prevent the continuous chemical reactions necessary for electrolysis.

Electrode Reactions: In electrolysis, specific reactions occur at the anode (oxidation) and the cathode (reduction). With AC, the same electrode would not consistently serve as the anode or cathode, leading to inconsistent and inefficient reactions. This could also result in unwanted side reactions or the dissolution of the electrodes.

Question 6

What happens at the cathode?

Answer 6

The cations (positively charged ions)
receive electrons (reduction) to change into atoms or molecules

Question 7

What happens at the anode?

Answer 7

The anions (negatively charged ions) lose electrons to form atoms or molecules (oxidation)

Question 8

Predicting the electrolysis of Molten Compounds

Answer 8

A molten binary compound consists of just two elements joined together by ionic bonding and is molten

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

For example,

Zinc chloride will produce zinc at the cathode and chlorine at the anode

Question 9

For an aqueous solution, what happens at the anode?

Answer 9

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

Question 10

For an aqueous solution, what is produced at the cathode?

Answer 10

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

Question 11

Why does the anode need to be replaced?

Answer 11

The carbon in the graphite anodes reacts with the oxygen produced to produce CO2

C (s) + O2 (g) → CO2 (g)

As a result the anode wears away and has to be replaced regularly

A lot of electricity is required for this process of extraction, this is a major expense

Question 11

Explain how electrolysis is used to electroplate objects.

Answer 11

The negative electrode should be the object to be electroplated.

The positive electrode should be the metal that you want to coat
the object with.

The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate.

Question 11

What is the reactivity series?

Answer 11

Potassium (K) - Most reactive
Sodium (Na)
Lithium (Li)
Calcium (Ca)
Magnesium (Mg)
Aluminium (Al)
Zinc (Zn)
Iron (Fe)
Lead (Pb)
Hydrogen (H) - Used as a reference
Copper (Cu)
Silver (Ag)
Gold (Au)
Platinum (Pt) - Least reactive

Question 12

What is the setup for electrolysis?

Answer 12

When an electric current is passed through a molten ionic compound the compound decomposes or breaks down

The process also occurs for aqueous solutions of ionic compounds

Liquids and solutions that are able to conduct electricity are called electrolytes

Covalent compounds cannot conduct electricity hence they do not undergo electrolysis

An electrolytic cell is 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

Question 13

The general electrolysis process

Answer 13

Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge

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

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, electrons flow from the positive terminal (anode) to the negative terminal (cathode) through the external circuit, i.e., from the anode to the cathode

Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode

Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode

Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode

Question 14

Where do the electrons flow during electrolysis?

Answer 14

Away from the positive electrode towards the negative electrode