C5 Chemicals Of The Natural Environment

C5 Chemicals of the natural environment GCSE SCIENCE Stephen Repacholi

Part 1: Chemicals in four spheres

Learning Objectives

What are naturally occurring elements and compounds?

How abundant are elements in different spheres?

How do elements cycle between the spheres?

Lithosphere

- The rigid outer layer of the Earth

- Made up of the crust and the part of the mantle just below it.

~80 km deep

Elements in the Lithosphere

Oxygen comprises 47% of the earth’s crust (by mass) that is made up by minerals that are oxygenated (silicates, carbonates, etc.)

Therefore it is the most abundant element.

Silicon is the next most abundant element – rocks of the lithosphere are mainly made out of silicates.

Other abundant elements are aluminium, iron, calcium, sodium, magnesium, potassium and titanium.

Hydrosphere

All the water on Earth.

Includes oceans, lakes, rivers, underground reservoirs and rainwater.

Covers 2/3 of the globe.

The Earth’s ocean is 89% oxygen by element and contains dissolved oxygen.

The ocean is also vital for life as it contributes more than 50% of global photosynthetic oxygen production.

Look at the next diagram – what are the three most abundant elements in living tissue?

Biosphere

All the living things on Earth.

Includes all the plants, animals, and microorganisms.

The Three most abundant elements in the biosphere are…

Hydrogen

Oxygen

Carbon

And then nitrogen, calcium, potassium, silicon, magnesium, phosphorus, sulphur, aluminium, sodium, iron and chlorine

Atmosphere

The layer of gases that surround the Earth.

What elements are flowing or cycling between the spheres?

Flowing between the spheres

Water – water cycle

Carbon dioxide – carbon cycle

Nitrogen – nitrogen cycle

Oxygen – oxygen cycle

Carbon Cycle

Nitrogen fixing bacteria in root hairs

Part 2: Chemicals of the atmosphere

Learning objectives

What are the gases in the air?

What are the weak attractions between molecules?

What is strong covalent bonding?

These gases are held within the atmosphere by the force of gravity from the Earth. These gases have low melting points and boiling points. The atmosphere, plus a suitable temperature (water is a liquid usually on Earth) means that the Earth can sustain a variety of life. Note: CH 4 is methane

Molecules

Most non-metal elements and most compounds between non-metal elements are molecular.

In the atmosphere, there are diatomic molecules , such as N 2 and O 2 , as well as other small molecules such as CO 2 , and H 2 O.

Argon (Ar) is one of the exceptions – the noble gases exist as a single atom.

Atoms and molecules in the air

All molecules have a slight tendency to stick together.

These attractive forces between molecules are weak. E.g. O 2 ---- O 2

This explains the low melting points and boiling points of gases in the atmosphere.

Strong bonds in molecules

The forces inside molecules are many times stronger than the weak attractions between molecules.

Small atoms such as H 2 and O 2 do not split up, except at very, very high temperatures.

Model of an oxygen molecule

Hydrogen atoms share one electron each to make a stable outer shell

Oxygen atoms share two electrons each to make a stable outer shell

Water molecule

These strong bonds from shared electrons are called COVALENT BONDS Hydrogen has one covalent bond (H-H) and oxygen has two (O=O). Water has single covalent bonds. Note the diagram is not balanced, it should be… 2H 2 + O 2  2H 2 0

Carbon dioxide has what sort of bonds? With molecular models, by convention, hydrogen is white, oxygen is red and carbon is black.

COVALENT BONDS! How many on the outside of each atom? Hint: C is in Gp 4 O is in Gp 6

Carbon has 4 electrons in the outer shell, Oxygen has 6. Carbon needs 4 (4+4=8), Oxygen needs 2 (6+2=8). So each line represents one shared pair or 2 electrons. Count them to get 8 electrons in each outer shell.

Nitrogen molecule What group is in in? Therefore, how many electrons in the outer shell? How many does it need to get a complete outer shell? So how many bonds?

Nitrogen Group 5 5 electrons in the OUTER SHELL It needs 3 electrons to get to 8. So each share 3 electrons = TRIPLE BOND! VERY STRONG INDEED

Whoops!

The electrostatic forces on the previous slide are from ions… - Some atoms give electrons e.g Na+

- Some atoms take electrons e.g. Cl-

Then ionic bonds form when opposites attract!

With covalent bonds…

The electrostatic attraction is between the (positive) nuclei and the shared pair of (negative) electrons.

Again, opposites attract!

So, covalent bonds…

-- CO – together

-- VALENT –(from ‘valentia’ = strength)

They hold atoms together within molecules

The number of bonds depends on the number of shared pairs of electrons.

Part 3 Chemicals in the Hydrosphere

Learning Objectives:

What are the unusual properties of water?

What are the sorts of bonding within a water molecule and between water molecules?

What happens to ions in solution?

Properties of Water

Liquid at room temperature

Smaller mass than O 2 , N 2 or CO 2 .(gases)

Melts at 0 ○ C and boils at 100 ○ C

Water molecules tend to stick together

Ice is less dense that water, so it floats – very unusual property!

Properties of Water

It is a good solvent for salts – most common solvents do not dissolve ions, but water does.

Pure water does not conduct electricity – it does not contain charged particles that are free to move around.  INSULATOR

Water molecules

In the covalent bonds between hydrogen and oxygen, the electrons are not evenly shared –

oxygen has more than its fair share, so it has a slight negative charge.

hydrogen has a slight positive charge.

Water molecules The greater share of the electrons by oxygen  angular shape of the molecule. Overall, the molecules are still electrically neutral. The small charges on opposite sides of the molecules cause slightly stronger attractive forces between the molecules. Hence, water is ‘sticky’ AND these small charges help water dissolve ionic compounds

The charges on water help attract ions out of their crystals. Ions separate and can move freely when dissolved in water. In sea water there is a mixture of positive and negative ions.

Weathering slowly breaks down rocks. This exposes the inside of the rocks to water. Soluble chemicals in the rocks dissolve in the water and it get washed away. WHY IS SEA WATER SALTY? One litre of sea water has ~40 g of dissolved chemicals

Some salts in sea water

Sodium chloride Na+ Cl-

Potassium chloride K+ Cl-

Potassium bromide K+ Br-

Potassium iodide K+ I-

Magnesium chloride Mg 2+ Cl-

Magnesium sulphate Mg 2+ SO 4 2-

Positively charged metals and negatively charged non-metals  salts e.g. MgSO 4

Part 4: Chemicals in the lithosphere

Learning Objectives

What chemicals are in the crust of the Earth?

What is ionic bonding?

What is the importance of silica and silicates?

What are giant covalent structures?

Rocks and minerals

‘ Lithos’ = stone or rock

The crust is the top of the lithosphere.

Rocks are made of one or more minerals

E.g. sandstone is mainly silicon dioxide (SiO 2 )

Limestone is mainly calcium carbonate

(CaCO 3 )

Minerals are naturally occurring chemicals – they may be elements (Ag, Au) or compounds (NaCl, SiO 2 ,Fe 2 O 3 )

Granite

Granite is a mixture of quartz, feldspar and mica – it is a mixture of minerals.

The quartz is glassy grains of silica (silicon dioxide)

Mica is black

Feldspar is either pink or white

Which minerals of granite are these?

Which minerals of granite are these? MICA WHITE FELDSPAR PINK FELDSPAR QUARTZ

Quartz Pure silicon dioxide is transparent and very hard.

Amethyst

A form of quartz with traces of manganese and iron oxide  violet colour

Haematite Fe 3 O 4 You may choose to believe this, but I am a sceptic!

Calcite CaCO 3 calcium carbonate

Iron pyrites FeS 2 FOOL’S GOLD!  This one is on some quartz

Galena PbS

Lead sulphide!

Quick Quiz

What is the most abundant metal in the lithosphere?

What is the most abundant non-metal in the lithosphere?

Quick Quiz

What is the most abundant metal in the lithosphere?

SILICON (28%)

What is the most abundant non-metal in the lithosphere?

OXYGEN (47%)

SEA SALT

Sea Salt

When sea water evaporates, ionic compounds crystallize

E.g. NaCl (rock salt), which is the mineral halite

Salt mine in Cheshire

Gypsum

Calcium sulphate CaSO 4

It is the main ingredient of plaster and plaster-board.

The structure of salt

Sodium chloride is an ionic compound that forms from ionic bonding.

Na gives an e-  Na+

Cl takes an e-  Cl-

The Na+ attracts other Cl- ions that are close to it.

In fact 6 Cl- ions can fit around one Na+ ion, and vice versa..

6 to 1, half a dozen to the other… 6 Cl- ions are attracted around each Na+ ion 4 are on the same plane, in a square like formation 1 is above and 1 is below each ion. The converse is true: 6 Na+ are attracted around each Cl- ion  Cubic crystals form with a giant ionic structure

Properties of salts

There are millions of ions held together with very strong attractive forces

It takes a lot of energy to break down the regular arrangement of ions

So it has a reasonably high melting point and boiling point

E.g. NaCl melts at 808 degrees Celcius

Silica and silicates

Quartz is the commonest crystalline form

Is found in sand and used to make glass

Important role in computers (Silicon Valley)

95% if rocks in the Earth’s crust are formed by silica and silicate minerals.

Silicon Dioxide (SiO 2 ) Each Si atom is covalently bonded to four O atoms. Each O atom is bonded to 2 Si atoms. Si-O covalent bond is very strong, so this giant covalent structure is very strong and rigid.

Silica glass used as an insulator in electrical devices No free electrons or ions to carry electricity Electrical insulator Sandstone used as building stone Resists weathering, ending up as sand in rivers, beaches & deserts Insoluble in water Used to make linings for furnaces and high –temperature laboratory glassware Strong, rigid structure; difficult to break down High melting (1610 ○C) and boiling ( 2230 ○C) points Used as abrasive in sandpaper and scouring powders Strong rigid structure; will scratch steel Very hard Uses Comments Property of SiO 2

Deccan traps in India – layer upon layer of basalt (made from silicate minerals)

Part 5 - Chemicals of the Biosphere

Learning objectives

What chemicals are found in living things?

What are the chemicals found in carbohydrates, proteins and DNA?

Biochemistry

The study of chemicals of life

Most biochemicals are based on three elements – carbon, hydrogen and oxygen

Sometimes these large molecules will have nitrogen, sulphur and/or phosphorus

Biochemicals start with a carbon skeleton…

Carbon

Carbon is the element on which all life is based.

Carbon atoms can form chains by joining them together (the carbon skeleton)

Carbon forms 4 strong covalent bonds, so other atoms can join onto the chains – very often these are hydrogen atoms.

A whole branch of chemistry is based on this property of carbon (organic chemistry)

Some organic molecules Acetone – the active ingredient in nail polish remover. Glyceraldehyde – one of the breakdown products of carbohydrate metabolism.

Glucose and Starch Sometimes we don’t write in the carbons on our diagrams, e.g. starch above, as you would get sore fingers! Glucose (left) is really ring-shaped and starch stores glucose…

Glucose Courtesy of the sun and the action of photosynthesis!

Carbohydrate

Glucose, starch and cellulose are all examples

‘ Carbo’ – carbon

‘ hydrate’ – water

Glucose – very soluble sugar

Starch – insoluble polymer – made of long chains of glucose units

Cellulose – polymer found in plant walls, that we cannot digest  dietary fibre

Proteins are built from 20 different amino acids Besides, carbon, oxygen and hydrogen, amino acids have nitrogen in them – hence it is something that is very important to get in the diet (thank goodness for nitrogen-fixing bacteria!