C5 Revision

Chemicals of the Natural Environment

Objective

•To revise the whole of C5

By the end of this lesson…..

Three parts of the Earth

•Core •Mantle•Crust

The four spheres…

• Lithosphere: crust and upper mantle

• Hydrosphere: Oceans and rivers

• Atmosphere: Layer of air around the Earth

• Biosphere: sphere of life

Chemicals of the atmosphere

• 78% N2

• 21% O2

• 1% Ar• 0.03% CO2

• Small amounts water vapour

• All chemicals are gases – low melting and boiling points

Molecules

• Most of the gases in the atmosphere consist of small molecules

• Argon exists as single atom• Attractive forces between

molecules weak – gases – moving too quickly

Bonds

• Small molecules such as O2 or H2 do not split up unless at extremely high temperatures

• Very strong bonds between atoms

Different bonds

Single bonds Double bonds

Covalent Bonding

• Non-metal atoms combine to form molecules – share electrons in their outer shells.

• Held by electrostatic attraction between two nuclei and shared pair electrons

Chemicals of the Hydrosphere

• Water has special properties:• Liquid at room temperature, even though has

a smaller mass than the gases in atmosphere.• Melts at 0oC and boils at 100oC• Water molecules have a greater tendency to

stick together.• Contracts until 0oC, then starts to expand –

important in nature.

Good Solvent

• Good solvent for salts• Most common salts do

not dissolve ions, but water does.

• Does not conduct electricity, therefore does not contain charged particles free to move

Water molecules

• Three atoms appear at an angle

• The hydrogen region has a slight positive charge

• The oxygen region contains a slight negative charge

Water Molecule

• The angle is due to the fact that the electrons are not evenly shared.

• Oxygen has a greater share which explains the charges on each side of the molecule.

• Overall they are still electrically neutral.

Small Charges

• The small charges on opposite sides of the molecule cause slightly stronger attractive forces between them.

• Also help water dissolve ionic compounds by attracting the ions out of their crystals.

• Ice is less dense than water because of the molecules angular shape. They tend to line up in a very open structure.

Why is the sea salty?

• Soluble chemicals carried from rocks to sea in water cycle.

• Main chemical in sea sodium chloride

• Built up over millions of years.

• Most of compounds are salts – positive metal ions and negative non-metal ions.

Lithosphere

• Top part of lithosphere is crust• Rocks include boulders, stones and pebbles

Rocks..

• Rocks are made of one or more minerals

• Minerals are naturally occurring chemicals.

• Can be elements such as gold and silver

• More commonly compounds, such as silicon dioxide, calcium carbonate etc

Sandstone

• Made of mainly one mineral

• Silicon dioxide

Limestone

• Made mainly of calcium carbonate

Granite

• Mixture of quartz, feldspar and mica

Lithosphere

• Two most common elements in lithosphere are oxygen and silicon.

• These are abundant so form the major types of minerals, such as silicon dioxide.

Evaporite minerals

• Sea water contains lots of dissolved chemicals.• When evaporates ionic compounds crystallise

Rock salt – Halite is one example.• Minerals formed this way are called evaporites

– laid down millions of years ago

Ionic Bonding

• Ions held together strongly by the opposite charges of their ions

• E.g. Cl- and Na+• The structure is called a

giant ionic structure.• Takes lots of energy to

break down

Silica

• Consists of silicon dioxide• Commonest form is quartz• One silicon to four oxygens• Form a 3D giant covalent structure Strong and rigid

Silicon dioxide

• Very Hard – used as abrasive• High mp and bp – linings for furnaces and high

temp glassware• Insoluble in water – building• Electrical Insulator – silica glass used as

insulator

Biosphere

• Biochemicals – Carbon, hydrogen, oxygen• Carbon is base of all living things:• Can form chains by joining to themselves• Forms four strong covalent bonds so other

atoms can join• Means can make a variety of compounds.• Most are polymers

Proteins

• Hair skin and muscle.• Enzymes• Polymers made from amino acids• 20 different amino acids

Proteins from amino acids

Carbohydrates

• Carbon• Oxygen• Hydrogen

Sugars such as glucosePhotosynthesis produces glucoseGlucose very soluble

Nucleic Acids

• DNA and RNA are nucleic acids• Molecules that carry the genetic code• Back bone of DNA is a polymer

Natural Cycles

• As living things grow die and decay, elements move between the spheres

• Carbon cycle• Nitrogen cycle• Humans have an effect

on these cycles

Human Effects

• Carbon Cycle: burning fossil fuels.

• Raised CO2 from 277ppm to 360ppm

• Nitrogen Cycle: Adding fertilisers to soil

Nitrogen Cycle

• Essential for biochemicals, especially proteins• N2 is a gas

• NO3- (nitrate) and NH4

+ (ammonium) are ions.• Attracted to water molecules so makes them

dissolve

Fixing Nitrogen

• Action of micro organisms (bacteria or algae)• Chemical reaction in air during lightning

strikes• Haber process

Metals from Lithosphere

• Rocks which contain useful minerals are ores.• Valuable minerals are often oxides or

sulphides of metals.• Ore examples: bauxite (aluminium), haematite

(iron), rock salt (sodium).• Gold occurs naturally so has been used for

thousands years. Couldn’t use other metals until learned how to extract.

Mineral Processing

• Valuable mineral mixed with dirt and rock• Separating this off is called ‘concentrating the

ore.’• Issues: Reducing the ore; supply of ore; energy

costs; impact on environment.

Extracting metals

• Reduction – removing oxygen (eg ZnO to Zn)• Reducing agent – something that removes

oxygen (eg C)• In reducing something, the reducing agent

itself becomes oxidised (CO)• Carbon is a good reducing agent as at high

temperatures – it has a strong tendency to react with oxygen.

Relative atomic masses

Relative Formula Mass

• If you know the formula of a compound, then you can work out its relative formula masses.

• Eg Fe2O3

• Fe – relative atomic mass = 56• O – relative atomic mass = 16• Therefore relative formula mass of Fe2O3 = • 56+56+16+16+16 = 160

Atomic Mass Units

The actual mass of a hydrogen atom is 1.7x10-24g (that’s 0.0000000000000000000000017g!)

Far too small a number to easily get your head around…

So – we use Relative Atomic Masses.

Relative Atomic Mass (Ar) tells us the mass of an atom compared to 1 atom of carbon.

Carbon is given a Ar of 12 (as it has 6 protons and 6 neutrons).

e.g. Ar of magnesium = Ar of fluorine = Ar of calcium =

We can use relative atomic masses to work out the relative formula mass (Mr) of a molecule or compound.

E.g. NaClAr of Na = 23Ar of Cl = 35.5

So Mr of NaCl = 23 + 35.5 = 58.5

E.g.2 H2OAr of H = 1Ar of O = 16

So Mr of H2O = (2 x 1) + 16 = 18

Calculate the relative formula mass of the following compounds (showing your working!):

(a) Carbon monoxide CO(b) Carbon dioxide(c) Sulphur dioxide SO2

(d) Calcium carbonate CaCO3

(e) Sodium hydroxide NaOH(f) Sulphuric acid H2SO4

(g) Hydrochloric acid HCl(h) Copper sulphate CuSO4

(i) Magnesium chloride MgCl2(j) Sodium carbonate Na2CO3

(k) Lead nitrate Pb(NO3)2

(l) Calcium hydroxide Ca(OH)2

Electrolysis

• Some reactive metals hold on to their oxygen too strongly so cannot use carbon as a reducing agent.

• Use electrolysis• Aluminium extracted

from bauxite.• Electrolysis turns ions

back into atoms.

Electrolysis – What happens?

• The electrolyte (liquid for electrolysis)

• Aluminium forms at negative electrode because is attracted because positive.

• Oxygen is negative so forms at the positive electrode.

Electrolysis-

• Electrolysis turns ions back into atoms by giving them electrons.

• Metal ions are positive so attracted to negative electrode

• The flow of electrons from the power supply into the electrode is what makes it negative

• In the electrolysis of molten aluminium oxide, the aluminium ions turn into aluminium atoms.

Electrolysis

• Al3+ + 3e- Al Aluminium electrons supplied Aluminium ion by the negative electrode

Electrolysis

• Non-metal ions are negatively charged so attracted to positive electrode. Electrode is positive because electrons flow out of it to power supply.

• Negative ions give up electrons to positive electrode and turn into atoms.

• O2- O + 2e-

ion atom electrons removed by positive electrode

Metal Properties

• Strong• Can be bent and pressed into shape• High melting points• Conduct electricity

Metallic Structures

• Scientists imagine that the model for metallic structure uses:

• Tiny spheres• Arranged in a regular

pattern• Packed close together in

a crystal as a giant structure.

Metallic bonding

• Metals have a special kind of bonding – not ionic, covalent.

• Metallic• Strong but flexible –

allow to move atoms to new positions

Metallic Bonding

• Metal ions tend to lose electrons in their outer shell easily.

• In solid metal the atoms lose these electrons and become positive ions.

• The electrons drift freely between the metal ions.

• The attraction between them holds the structure together.

Metallic Bonding

• A metal crystal is not charged overall.

• Total negative charge on electrons = total positive charge on ions.

• Electrons can move freely between the ions – explains why metals conduct electricity well.

Life Cycle of Metals

• Mining – Large volumes waste rock

• Large holes in ground• Explosives – noisy and

produces dust• Open cast – large

volumes red mud.

Life Cycle of Metals

• Processing Ores• Many ores high value

but low grade• Large percentage waste

rock

Life Cycle of Metals

• Metal Extraction• All stages of metal

extraction and fabrication need energy

• Use large volumes water

• Give off air pollutants

Life Cycle of Metals

• Metals in Use• Lighter cars, trucks and

trains mean less fuel consumption and emssions.

• Less wear and tear on roads and tracks

Life Cycles of Metals

• Recycling• Scrap metal from all

stages of production routinely recycled.

• Metal recycled at the end of the usefulness of metal products.

• Save ores and reducing agents. Save water.