Transcript Further Electrochemistry

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Rusting
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What happens when iron rusts?
When iron rusts it reacts with oxygen from the air to form
iron (III) oxide.
iron + oxygen
4Fe +
3O2
iron (III) oxide
2Fe2O3
This is a redox reaction.
Remember OIL RIG:
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
What is oxidized?
iron
What is reduced?
oxygen
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Redox equations for rust
When iron rusts, iron (Fe) atoms form Fe3+ ions.
Fe
Fe3+
+ 3e–
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.
Oxygen molecules are reduced: they gain electrons to form O2–
.
O2 + 4e–
2O2–
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Preventing corrosion
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Stainless steel
Stainless steel is used to make cutlery, saucepans, surgical
instruments and food transporters.
It is very resistant to corrosion and staining.
Stainless steel is an alloy of iron which contains around
12% chromium and smaller amounts of nickel and carbon.
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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.
He called this new alloy ‘rustless steel’.
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Why is stainless steel rust-proof?
Stainless steel
Mainly iron and
chromium (12%)
with small
amounts of
nickel and
carbon.
air
protective
Cr2O3 layer
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.
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Rust summary
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Electrolysis
anode
An ionic compound is formed of
charged particles. When molten or in
solution, the ions are able to move
freely and conduct electricity.
cathode
Electrolysis is the decomposition of
a liquid by passing an electric
current through it.
Electrolysis involves redox reactions: ions lose electrons at
the positive electrode (anode) and gain electrons at the
negative electrode (cathode).
In the electrolysis of molten potassium chloride:
anode: 2Cl–
cathode: K+ + e–
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Cl2 + 2e– oxidation
K
reduction
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Balancing redox half equations
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Electrolysis of molten substances
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Electrolysis of solutions
When electrolysing aqueous ionic compounds,
the hydrogen and hydroxide ions in water may
become involved in the reaction.
If this happens, hydrogen or oxygen may form
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.
Electrolysis of sodium chloride (aq) produces hydrogen at the
cathode, as sodium is more reactive than hydrogen.
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Electrolysis products
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Producing hydrogen and oxygen
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KNO3 and K2SO4 solutions: redox
What happens during the electrolysis of potassium nitrate and
potassium sulfate solutions?
At the negative electrode: reduction
2H+ + 2e–
H2
At the positive electrode: oxidation
4OH–
2H2O + O2 + 4e–
What is the overall equation for this electrolysis?
2H2O (l)
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2H2 (g) + O2 (g)
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Electrolysis of KNO3 and K2SO4 solutions
The solution contains potassium (K+) and hydrogen (H+) ions.
Hydrogen ions are reduced at the negative electrode to form
hydrogen gas, as potassium is more reactive than hydrogen.
The negative ions in the solutions are:
 nitrate (NO3–) in potassium nitrate
 sulfate (SO42–) in potassium sulfate
 hydroxide (OH–) from the water
It is the hydroxide ions react, producing
oxygen at the positive electrode. This is because
OH– ions are less reactive than both SO42– and NO3– ions.
The other ions (K+ and NO3– or SO42–) stay in the solution.
What happens if the current is left flowing for a long time?
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Electrolysis summary
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Electrolysis of copper sulfate solution
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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.
This can be increased in two ways:
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.
2. Increasing the time current flows through the circuit.
This increases the total amount of charge flowing, increasing
the total amount of metal deposited.
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The Faraday constant
Charge is calculated using the equation:
time
Q
= current ×
I
×
=
(Coulombs)
(Amps)
(seconds)
charge
t
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.
A charge of 96,500 coulombs is equivalent to one mole of
electrons flowing through the circuit.
This amount of charge is called the Faraday constant, which
can be used to calculate the mass of an electrolysis product.
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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.
Question: A charge of 4550 coulombs passes through a
circuit. How many moles of electrons is this?
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.
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Using Q = I × t
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Find mass using Q = I × t
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Glossary
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Anagrams
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Multiple-choice quiz
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