Further Electrochemistry (1)

What happens when iron rusts?

When iron rusts it reacts with oxygen from the air to form

iron(III)oxide.

This is a redox reaction.

What is oxidized?

What is reduced? oxygen iron

iron 4Fe

+

+

Remember OIL RIG:

Redox equations for rust

When iron rusts, iron (Fe) atoms form Fe3+ ions.

This is oxidation, because the iron atoms have lost electrons.

We say that the oxidation state of the iron is 3, because the ions have a charge of 3+.

This is why rust is called iron(III)oxide.

Fe Fe3+

+

Oxygen molecules are reduced: they gain electrons to form O2–_.

Stainless steel

Stainless steel is used to make cutlery, saucepans, surgical instruments and food transporters.

The story of stainless steel

In the 1910s, before the First World War, people were concerned about how easily gun barrels made of steel would corrode.

A scientist called Henry Brearly found that adding about 10% chromium to the steel made an alloy which was very

resistant to corrosion at high temperatures.

He also tested his new alloy with food acids like lemon juice and vinegar and found that it was very resistant to chemical attack as well, making it perfect for cutlery.

Why is stainless steel rust-proof?

Chromium is more reactive than iron. It reacts quickly with oxygen from the air to form a very thin layer of chromium oxide on the surface of the steel.

This protects the iron atoms from reacting with the oxygen in the air and prevents rust forming.

If the steel is scratched or cut, more chromium atoms quickly form a new protective layer.

protective

Cr₂O₃ layer

air Stainless steel

Mainly iron and chromium (12%) with small

Electrolysis

Electrolysis involves redox reactions: ions lose electrons at the positive electrode (anode) and gain electrons at the

negative electrode (cathode). An ionic compound is formed of

charged particles. When molten or in solution, the ions are able to move freely and conduct electricity.

Electrolysis is the decomposition of a liquid by passing an electric

current through it.

In the electrolysis of molten potassium chloride:

anode: 2Cl– Cl

2 + 2e– cathode: K+ + e– K

oxidation

reduction

Electrolysis of solutions

When electrolysing aqueous ionic compounds, the hydrogen and hydroxide ions in water may become involved in the reaction.

If an ionic compound contains a metal that is more reactive than hydrogen, electrolysis of a solution of the compound produces hydrogen at the negative electrode.

Electrolysis of copper sulfate(aq) produces pure copper at the cathode, as copper is less reactive than hydrogen.

If this happens, hydrogen or oxygen may form

KNO

and K

SO

solutions: redox

What happens during the electrolysis of potassium nitrate and potassium sulfate solutions?

What is the overall equation for this electrolysis? At the negative electrode:

At the positive electrode:

reduction

2H+

+

2

4OH– 2H

2O

+

+

oxidation

Electrolysis of KNO

and K

SO

solutions

The solution contains potassium (K+) and hydrogen (H+) ions.

The negative ions in the solutions are:

 nitrate (NO3–) in potassium nitrate

 sulfate (SO₄2–) in potassium sulfate

 hydroxide (OH–) from the water

Hydrogen ions are reduced at the negative electrode to form hydrogen gas, as potassium is more reactive than hydrogen.

It is the hydroxide ions react, producing

oxygen at the positive electrode. This is because OH– ions are less reactive than both SO

42– and NO3– ions.

The other ions (K+ and NO

3– or SO42–) stay in the solution.

What determines the amount of metal?

The amount of metal which is deposited at the negative electrode depends on the number of electrons available to reduce the metal ions.

1. Increasing the current which flows through the circuit.

This increases the amount of charge flowing through the circuit every second, meaning more metal ions are reduced at the cathode.

This increases the total amount of charge flowing, increasing the total amount of metal deposited.

This can be increased in two ways:

The Faraday constant

A charge of 96,500 coulombs is equivalent to one mole of electrons flowing through the circuit.

Faraday’s First Law of Electrolysis states that the mass of a substance produced at an electrode is directly proportional to the charge at that electrode.

This amount of charge is called the Faraday constant, which can be used to calculate the mass of an electrolysis product. Charge is calculated using the equation:

(Coulombs) (Amps) (seconds)

charge

=

×

Electrolysis calculations

Faraday’s law and constant allow us to gain quantitative information about an electrolysis.

Faraday’s constant allows us to find the number of moles of electrons that have passed through an electrolysis.

1 mole of electrons has a charge of 96,500 coulombs

Therefore: number of moles of electrons = 4550 ÷ 96,500

= 0.047 moles e–

Once you have the number of moles of electrons, it is

possible to calculate the mass of a product in an electrolysis.