C5.1 Metallic character

Metallic character is exemplified by the properties that are typical of metals. Metals have loosely held outer (valence) electrons. The more reactive the metal, the more easily it can lose these outer electrons forming a positive ion.

Group 1 metals are very reactive precisely for this reason - their atoms are much more stable as an ion than as a metal.

Na → Na⁺ + 1e^-

The oxidation of sodium by removal of an electron

All metals conduct electricity, as the metal structure has a regular lattice of metal ions surrounded by a “sea” of delocalised electrons. This is the most important physical property of metals and applies to ALL metals.

Metals are usually:

• Hard
• Malleable
• Ductile
• Shiny
• Good conductors of heat
• High melting point solids

Definitions

• Delocalised – not having a specific location
• Lattice – regular arrangement of repeating particles
• Malleable – can be moulded into shape
• Ductile – can be drawn into wires

C5.2 - Displacement reactions

More reactive metals can transfer their valence electrons to the ions of less reactive metals. This is called a redox reaction, where one species loses electron(s) and another species gains the electron(s).

Ionic compounds dissociate 100% in solution. This is how/why water dissolves ionic compounds.

CuSO₄(s) + (aq) → Cu²⁺(aq) + SO₄^2-(aq)

In all ionic solutions, the individual ions are free to move independently. In other words, copper(II) sulfate solution contains water, copper ions and sulfate ions all mixed together and moving independently.

REDOX stands for reduction and oxidation. Reduction is gain of electrons and oxidation is loss of electrons. An easy way to remember this is to use the mnemonic OILRIG.

Oxidation Is Loss, Reduction Is Gain

• Definitions
• Oxidation is loss of electrons (by a species)
• Reduction is gain of electrons (by a species)
• Spectator ions: Ions present in a solution that do not take part in the reaction
• Mnemonic: a memory aid

C5.3 Corrosion

Corrosion – Literally “chemical erosion”, breaking something down by chemical attack. Corrosion could be described as wearing down by chemical action, if you like, chemical erosion. The waves break down the rocks on the shore by mechanical erosion, but a process that wears something down by chemical attack and reaction is corrosion.

Corrosion quietly destroys structures and costs huge amounts of money each year.

Corrosion is an electrochemical process: the metal is oxidised (loses electrons), often by dissolved oxygen in water. On iron in aerated water: anode and cathode spots form on the same surface or between dissimilar metals.

Starter: why do structures fail?

• Watch a short video about a historical bridge failure; discuss how tiny defects + corrosion can lead to catastrophic failure.
• Brainstorm: where do you see corrosion in daily life? (bikes, boats, cars, outdoor fixtures)

The mechanism of corrosion

Corrosion is degradation caused by the reaction of metals with something in the air, usually oxygen. The most important and widely used construction metal is iron. Iron corrodes in air, particularly in the presence of air and water.

Electrons are transferred from iron to oxygen molecules. Solutions containing ions, which can carry charge, help this process along.

Iron reacts with oxygen in the presence of water to form hydrated iron(III) oxide (rust). The overall equation is:

4Fe(s) + 2xH₂O(l) + 3O₂(g) → 2Fe₂O₃.xH₂O(s)

We can break this equation down into just the oxidation and reduction parts (half-equations)

Oxidation: Fe → Fe³⁺ + 3e^-

Reduction: O₂ + 4e^- → 2O^2-

The balanced ionic equation is obtained by equalising the electrons and then adding them together.

4Fe + 3O₂ → 4Fe³⁺ + 6O^2-

C5.4 - Reduction of metal oxides

In the previous section we have seen that metals corrode by reacting with the oxygen, water and carbon dioxide in the air. The products of corrosion do not have the required metallic characteristics and so are not welcome.

Chemists extract metals from their oxides, reversing the process of oxidation.

Metal oxides can be reduced to the pure metal by using a reducing AGENT, which is a substance that is better at releasing electrons then the metal itself.

Copper(II) oxide can be reduced by heating in a stream of hydrogen gas or methane gas.

CuO + H₂ → Cu + H₂O

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Iron and steel

Industrially iron, which is the most important construction metal in modern society, is extracted from its ore by reduction with carbon monoxide, which in turn is formed by heating coke (dried coal) in a reduced supply of air.

Iron is extracted from various iron oxides by heating with carbon monoxide, formed from either charcoal (small scale) or coke (dried coal) burning in limited air supply.

The Blast furnace extraction of iron

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The thermit reaction

On a small-scale, iron can be extracted rapidly by reducing iron(III) oxide using aluminium as the reducing agent. This is known as the thermit reaction.

The Thermit Reaction

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C5.5 - Voltaic Cell

Voltaic or Galvanic cells are constructed to generate an electrical potential from chemical energy. The chemical energy is converted into electrical energy that can be made to drive a current around an electrical circuit.

In this process we are taking advantage of the different reactivity of metals and metal ion solutions. The more reactive metal passes electrons to the ions of the less reactive metal using the external circuit.

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The difference in reactivity between the metals in each half-cell results in the chemical potential energy. This causes electrons to attempt to move from one substance to another (a redox reaction). If the electrons are allowed to flow around the external circuit a flow of electrical charge occurs. This is known as an electrical current.

Representing Electrochemical cells

We can use a circuit convention, i.e. an agreed way to represent the physical apparatus used to generate electricity from the electrochemical cell. By convention the half-cell in which oxidation takes place is written first, followed by the salt bridge and then the half-cell in which reduction takes place. For the zinc|copper electrochemical cell the oxidation takes place in the zinc|zinc sulfate half-cell.

Zn|Zn²⁺ ∥ Cu²⁺|Cu

Oxidation reaction || Reduction reaction

Explaining the chemistry

The anode is the electrode where oxidation takes place. In this case, electrons are removed from the more reactive metal.

Zn(s) → Zn²⁺(aq) + 2e

The cathode is the electrode where reduction takes place. In this case, electrons are given to the ions of the less reactive metal to produce the metal again.

Cu²⁺(aq) + 2e → Cu(s)

The cell reaction

Cu²⁺(aq) + Zn → Zn²⁺(aq) + Cu(s)

The cell convention

Cell notation: Zn(s) | Zn²⁺(aq) ∥ Cu²⁺(aq) | Cu(s)

Left is the anode (oxidation), right is the cathode (reduction).

PS9.3 - Incomplete combustion & PM

Most energy resources are based on fossil fuels and rely on combustion. The hydrocarbons present in the fossil fuels burn in air to produce carbon dioxide and water vapour.

For example, natural gas (methane)

CH₄ + 2O₂ → CO₂ + 2H₂O

However, if the supply of air is limited then the combustion is incomplete and produces carbon monoxide and carbon micro-particulates (PM)

These carbon microparticulates are so small that they remain suspended in the air and become a major health hazard, causing bronchial complaints, asthma and cancer.

Most developed countries monitor these micro-particulates in cities and try to minimise their effect.

They are measured in μg per m³ air and determined according to the size of the microparticulates in micrometres (μm, 10^-6 m).

Particulate matter with particle diameter up to 50 μm is known as PM₅₀

PS9.4 - Pollution

Definitions

The terms "pollution" refers to anything in the environment that shouldn't be there and which has a detrimental effect, either in terms of health or environmental degradation.

In general, these substances are introduced into the environment as a result of human activity (anthropogenic).

Pollution of the air is particularly damaging, as everyone needs to breathe continuously introducing the pollution into the body. This has negative health consequences.

Air pollution is one of the world's leading risk factors for death. Air pollution is responsible for millions of deaths each year

Our World in Data - Air pollution

Some pollutants also have natural sources. However, the balanced ecosystems of the Earth usually have mechanisms to reduce the impact of natural pollutants.

Significant air pollutants

• Sulfur dioxide, SO₂
• Nitrogen oxides, NO_x
• Microparticulates, PM_2.5

Sulfur dioxide, SO₂

Sulfur dioxide is an acidic gas with a suffocating metallic smell (and taste).

It reacts with water making sulfuric(IV) acid, a weak acid:

The reaction of sulfur dioxide with water

SO₂ + H₂O → H₂SO₃

Anthropogenic sources

Sulfur is an element that appears in the protein building blocks of life, the amino acids methionine and cysteine. As fossil fuels are derived from living organisms, they contain sulfur. When fossil fuels are burned the sulfur content turns to sulfur dioxide.

Any industry or activity that burns fossil fuels produces sulfur dioxide. Energy generation (oil or coal fired power stations) and traffic are the main activities.

Natural sources

Sulfur dioxide also has a natural source in volcanic activity.

The reaction of sulfur with oxygen

S + O₂ → SO₂

Health effects

Sulfur dioxide creates sulfuric acid in the lungs if inhaled. This can lead to all sorts of health issues.

The reaction of sulfur dioxide with water

SO₂ + H₂O → H₂SO₃

Aerial oxidation of sulfuric(IV) acid to sulfuric(VI) acid

2H₂SO₃ + O₂ → 2H₂SO₄

Nitrogen oxides

These are formed naturally by electrical discharges in the air (lightning), and are part of the natural nitrogen cycle, which is essential for plant life.

Nitrogen oxides are made by electrical discharges:

N₂(g) + O₂(g) → 2NO(g)

Nitrogen(II) oxide reacts with oxygen in the air:

2NO(g) + O₂(g) → 2NO₂(g)

The nitrogen(IV) oxide, NO₂, dissolves in rainwater and gets into the ground where it is converted into soluble nitrates in the soil. The plants extract these soluble nitrates through the roots and use the nitrogen for growth. When the plants die the nitrogen is returned to the atmosphere as ammonia, where it is oxidised to nitrogen oxides again.

Microparticulates

Also known as particulate matter (PM).

The mass of particulate matter with maximum diameter of particles is defined. These are fundamentally small particles of carbon (sometimes called black carbon) produced during incomplete combustion.

PM_2.5 means microparticulates with diameter up to 2.5μm = 2.5 x 10^-6 m

These microparticulates remain in the air for long periods of time due to their small size and get inhaled by the population. They are responsible for respiratory diseases and lung cancers.

PS9.5 - Gases

As a state of matter, gases present specific difficulties when it comes to their collection and containment.

Properties of gases - volume

A substance is gaseous if its particles have enough energy to overcome the forces of attraction between them. For this reason gases expand to fill all of the available volume.

Properties of gases - temperature

The volume and pressure of a gas is directly proportional to its absolute temperature. As the temperature increases, so does the volume (at constant pressure) or the pressure (at constant volume).

The dependence of pressure on temperature

P ∝ T (at constant volume)

The dependence of volume on temperature

V ∝ T (at constant pressure)

At constant temperature, the product of the pressure and the volume is constant.

The relationship between volume and pressure at constant temperature

PV = constant (at constant temperature)

Properties of gases - collection

The preparation and gas collection apparatus must be gas-tight or the gas must be prevented from leaving the collection apparatus in some way.

This would be suitable for hydrogen, oxygen and carbon dioxide gases.

Gases such as ammonia, hydrogen chloride and sulfur dioxide are too soluble in water and must be collected in another way.

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