GCSE Chemistry- Module C5

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Module C5

Chemicals in the atmosphere

Dry air is a mixture of gases

1) The earth's atmosphere contains many gases

2) Some of the gases are elements and they contain only one type of atom

3) Other gases are compounds which contain more than one type of atom bonded together

4) There are different amounts of these gases in the atmosphere-

78% nitrogen

21% oxygen

1% argon

0.04% carbon dioxide

Molecular substances have low melting and boiling points

1) molecular substances usually exist as small molecules like CO2 and H2O.

2) The atoms within the molecules are held together by very strong covalent bonds

3) But the forces of attraction between molecules are very weak

4) You only need a bit of energy to overcome the weak forces between the molecules- so molecular substances have low melting and boiling points.

5) This means that they're usually gases and liquids at room temperature

6) Pure molecular substances don't conduct electricity because the molecules aren't charged- no free electrons or ions

7) Most non metals and most compounds formed from non metals are molecular.

Covalent bonding

Covalent bonds- sharing electrons

1) sometimes atoms make covalent bonds by sharing electrons with other atoms

2) This way both atoms feel that they have a full outer shell

3) Each covalent bond provides one extra shared electron for each atom

4) Each atom involved has to make enough covalent bonds to fill up its outer shell

5) The atoms bond due to the electrostatic attraction between the positive nuclei and the negative electrons shared between them.

Hydrogen

Hydrogen needs just one extra electron to fill its outer shell. So two hydrogen atoms share their outer electron so they each have a full shell and a covalent bond is formed.

Carbon dioxide

Carbon needs four more electrons to fill it up

Oxygen needs two

So two double covalent bonds are formed. A double covalent bond has two shared pairs of electrons.

Module C5

Chemicals in the Hydrosphere

The Earth's Hydrosphere is the oceans

1) the earth's hydrosphere consists of all the water in the oceans, seas, lakes, rivers, etc

2) it also contains any compounds that are dissolved in the water

3) Many of these compounds are ionic compounds called salts

4) Examples of salts are sodium chloride, magnesium chloride and potassium bromide.

Solid ionic compounds form crystals

1) Ionic compounds are made of charged particles called ions

2) Ions with opposite charges are strongly attracted to one another. You get a massive lattice of ions built up.

3) There are very strong chemical bonds called ionic bonds between all the ions

4) A single crystal of salt is one giant ionic lattice, which is why salt crystals tend to be cuboid shaped.

Ionic compounds have high melting and boiling points

1) The forces of attraction between the ions are very strong

2) It takes a lot of energy to overcome these forces and melt the compound and even more energy to boil it

3) So ionic compounds have high melting and boiling points which makes them solids at room temperature.

They conduct electricity when dissolved or molten

1) When an ionic compound dissolves the ions separate and are all free to move in the solution

2) This means that they're able to carry an electric current

3) When an ionic compound melts the ions are again free to move, so they carry an electric current

4) When an ionic compound is a solid the ions aren't free to move so no electric current can be passed.

Identifying positive ions

Flame tests- spot the colour

Sodium- orange/yellow

Potassium- lilac

Calcium-brick red

Copper- blue/green

Add sodium hydroxide and look for a coloured precipitate

Many metal hydroxides are insoluble and precipitate out of solution when alkalis are added.

Some of the precipitates have a characteristic colour.

So in this test you add a few drops of sodium hydroxide solution to a solution of your mystery compound in the hope of forming an insoluble hydroxide.

Metal Colour of

precipitate

calcium Ca2+ white

copper II Cu2+ blue

iron II Fe2+ sludgy green

iron III Fe3+ Reddish brown

Zinc Zn2+ white then no colour

Identifying negative ions

Hydroxide acid can help detect carbonates

With dilute hydrochloric acid, carbonates will fizz because they give off carbon dioxide. You can test for CO2 using limewater as it turns cloudy.

Test for sulphates with HCl and Barium chloride

To identify a sulphate ion, add dilute HCl, followed by barium chloride solution. A white precipitate of barium sulphate means the original compound was a sulphate.

Test for halids with nitric acid and silver nitrate

A chloride gives a white precipitate of silver chloride

A bromide gives a cream precipitate of silver bromide

An iodide gives a yellow precipitate of silver iodide.

Module C5

Chemicals in the lithosphere

The earth's lithosphere is made up of a mixture of minerals

1) The lithosphere is the earth's rigid outer layer, the crust and part of the mantle below it

2) Its made up of a mixture of minerals often containing silicon, oxygen and aluminium.

3) Different types of rock contain different minerals and different elements

Carbon forms giant covalent structures

Carbon forms giant covalent

Carbon can form two types of giant covalent structure, diamond and graphite. Both diamond and graphite are minerals which are found in the Earth's crust.

Diamond

1) The carbon atoms in diamond each form four covalent bonds in a very rigid giant covalent structure

2) This structure makes diamond the hardest natural substance

3) All those strong covalent bonds give diamond a very high melting point

4) It doesn't conduct electricity because it has no free electrons

5) Its insoluble in water

Graphite

1) Graphite is also made from carbon but it has a different giant covalent structure.

2) Each carbon atom only forms three covalent bonds, creating sheets of carbon which are free to slide over eachother. This makes it slippery so its used as a lubricant.

3) The layers are held together so loosely that they can be rubbed off onto paper

4) Graphite has a high melting point- the covalent bonds need lots of energy to break

5) Graphite has lots of free spare electrons, so it can conduct electricity.

Silicon dioxide is also a Giant Covalent Structure

1) Most of the silicon and oxygen in the Earth's crust exists as the compound silicon dioxide.

2) Silicon dioxide is what sand is made of

3) Each grain of sand is one giant structure of silicon and oxygen

4) Silicon dioxide has a similar structure to diamond so has similar properties too, e.g. high melting point & no electricity conduction.

Metals from minerals

Ores contain enough metal to make extraction worthwhile

1) Rocks are made of minerals. Minerals are just solid elements and compounds

2) Metal ores are rocks that contain varying amounts of minerals from which metals can be extracted.

3) In many cases the ore is an oxide of the metal.

4) For some metals large amounts of ore need to be mined just to obtain small percentages of valuable minerals. A good example of this is copper mining- copper ores typically contain about 1% copper.

More reactive metals are harder to get

1) A few unreactive metals like gold are found in the Earth as metal itself rather than as a compound

2) But most metals need to be extracted from their ores using a chemical reaction

3) More reactive metals like sodium are harder to extract so thats why it took longer to discover them

Some metals can be extracted by reduction with carbon

1) A common way of extracting a metal from its ore is chemical reduction using carbon or carbon monoxide

2) When an ore is reduced, oxygen is removed from it.

3) When an oxide loses its oxygen it is reduced. The carbon gains the oxygen and is oxidised.

4) How reactive the metal is compared to carbon determines whether it can be extracted by reduction with carbon or carbon monoxide.

- Metals that are less reactive than carbon can be extracted by reduction by heating with carbon

- This is because carbon can take the oxygen away from metals which are less reactive than itself.

- Metals that are more reactive than carbon can't be extracted by reduction- they have to be extracted by electrolysis.

Module C5

Electrolysis

Electrolysis means 'splitting up with electricity'

1) Electrolysis is the decomposition of a substance using electricity.

2) It needs a liquid to conduct the electricity, called the electrolyte. Electrolytes are usually free ions dissolved in water or molten ionic compounds

3) Its the free ions that conduct the electricity

4) For an electrical circuit to be complete there has to be a flow of electrons. In electrolysis, electrons are taken away from ions at the positive electrode and given to other ions at the negative electrode. As ions gain or lose electrons they become atoms or molecules.

Electrolysis removes aluminium from its ore

1) The main ore of aluminium is bauxite which contains aluminium oxide

2) Molten aluminium oxide contains free ions so it will conduct electricity

3) The positive Al3+ ions are attracted to the negative electrode where they each pick up three electrons and turn into neutral aluminium atoms. These then sink to the bottom

4) The negative O2- ions are attracted to the positive electrode where they each lose two electrons. The neutral oxygen atoms will then combine to become O2 molecules.

Metals form positive ions so they're attracted to the negative electrode.

Non metals form negative ions so they're attracted to the positive electrode

at the negative electrode

Al3+ + 3e- -> Al

at the positive electrode

2O2- -> O2 + 4e-

Metals

Ores are finite resources

1) This means that there's a limited amount of them

2) People have to balance the social, economic and environmental effects of mining the ores

3) So mining ores is good because useful products can be made. It provides people with jobs and brings money into the area. This means services such as transport and health can be improved.

4) But mining ores is bad for the environment as it uses loads of energy, scars the landscape and destroys habitats. Mine shafts are also dangerous.

Recycling metals

1) Mining and extracting metals takes lots of energy most of which comes from burning fossil fuels.

2) Fossil fuels are running out so it's important to conserve them. Not only this, but burning them contributes to acid rain, global warming and climate change.

3) Recycling metals only uses a fraction of the energy needed to mine new metal

4) Recycling saves money and conserves resources and doesn't contribute to landfil

molecular substances & covalent bon

IONIC COMPOUNDS&Identifying ions

lithosphere, COVAlent structures&minerals

electrolysis