iGCSE Chemistry - Chemistry in Society

Extraction and uses of metals

 

• Metals are found in ores containing minerals mixed with rock
• The mineral is a compound of the metal and needs to be purified
• Aluminium is extracted from the ore bauxite
• The aluminium oxide in the bauxite needs to be purified, and it is melted so that electrolysis can be carried out.
• The anode is made from carbon
• The cathode is the steel-lined case.
• Aluminium forms at the cathode
• Oxygen forms at the anode
• The oxygen formed reacts with the carbon anode to form CO₂, and this causes the anode to be worn down
• This process requires large amounts of electricity
• Copper is obtained by heating the ore malachite with carbon
• All metals below carbon in the reactivity series can be obtained this way
• Iron is produced on a large scale by this process in a blast furnace:
1. Iron ore (source of iron), coke (source of carbon) and limestone are added at the top of the blast furnace
2. Hot air is blasted from the bottom of the furnace to the top
3. Oxygen in the air reacts with coke to form CO₂, which in turn reacts with the coke again to from carbon monoxide
4. Carbon monoxide is a reducing agent and thus turns iron oxide into iron.
5. Molten iron runs to the bottom of the furnace and is run off
6. The limestone is broken down by heat into calcium oxide, which in turn reacts with impurities to form slag

·        Zinc can be extracted from zinc blende:

1.       Zinc sulphide is turned into zinc oxide by heating

2.     Zinc oxide is either reduced by carbon monoxide or

3.     it is dissolved in sulphuric acid and then electrolysed

• Uses of metals:

Metal	Properties	Uses
Aluminium	Very light	Aircraft construction
Iron	Strong + malleable	Car bodies, cutlery
Zinc	Low melting point	Galvanising
Chromium	Strong and non-corrosive	Stainless steel production
Copper	Good electrical conductor	Electrical wiring

 

 

Natural Gas and Oil

 

• Crude oil is a mixture of hydrocarbons
• Separate parts, or fractions, of crude oil can be separated using fractional distillation.
• The crude oil is heated in a furnace and passed into the bottom fo the fractionating column
• The mixture evaporates and the different fractions condense at different levels.
• The fractions are:

                   Gases

                   Gasoline

                   Kerosene

                   Gas oil

                   Fuel oil

                   Bitumen

• Fractional distillation produces more long-chain hydrocarbons than required, and these are turned into short-chain hydrocarbons by cracking
• When hydrocarbons are burnt (combusted) they release CO₂, and if they do not combust completely, they release CO (carbon monoxide)
• Carbon monoxide molecules bind with haemoglobin in the blood more strongly than oxygen, and thus are dangerous
• Hydrocarbons are damaging to the environment because when burnt they release CO₂ which is a greenhouse gas
• If crude oil is spilled, it can destroy habitats and kill animals.

 

 

Synthetic Polymers

 

• Polymers are large molecules made up of many small molecules called monomers
• Polymers can be made in tow ways: addition and condensation
• Addition polymerisation consists of alkenes reacting with each other to form long chains
• Condensation polymerisation consists of joining two different monomers, and when they react they expel a small molecule, often water
• Ethene is used to make polyethene through addition polymerisation
• Nylon is manufactured using condensation polymerisation, and its starting monomers are: hexanedioic acid and 1,6-diaminohexane

 

The Manufacture of some Important Chemicals

 

• Ammonia is produced in the Haber Process from nitrogen (from the air) and hydrogen (from natural gas)
• The Haber Process involves an iron catalyst, a temperature of 450°C and 200 atmospheres
• Any unused hydrogen and nitrogen is recycled.
• Ammonia is used for the manufacture of nitric acid and fertilisers
• Sulphuric acid is manufactured in the contact process
• The raw materials needed are sulphur and air
• The conditions are as follows: 450°C, 2 atmospheres and a vanadium oxide catalyst.
• Four steps in process:
1. Sulphur + Oxygen -> Sulphur Dioxide
2. Sulphur Dioxide + Oxygen -> Sulphur trioxide
3. Sulphur Trioxide + Sulphuric Acid -> Oleum
4. Oleum + Water -> Sulphuric Acid

·        Sulphuric acid is used in the manufacture of chemicals

·        Sulphur dioxide and nitrogen oxides are pollutant gases which, when mixed with water, form acid rain

·        Acid rain kills plants and animals and depletes the soil of minerals

·        Sodium hydroxide is manufactured through the electrolysis of brine (sodium chloride solution)

·        At the cathode, hydrogen ions are turned into atoms, and at the anode, chloride ions are turned into atoms.

·        What is left is sodium hydroxide solution.

·        Sodium hydroxide is used for the manufacture of soap.

·        Chlorine is used in the manufacture of bleach.

iGCSE Chemistry - Physical Chemistry

States of Matter

 

• There are three states of matter: gas, liquid, and solid
• In gases, particles are far apart from each other and expand to fill a space.
• In liquids, particles are held together but have energy to move around, and thus have no definite shape
• In solids, particles are held in a fixed position and thus solids have a definite shape.
• In mixtures, individual substances can be separated through physical means. The chemical properties are the same as the components’.
• In compounds, the components can only be separated by chemical means. The chemical properties different to the components’.
• Distillation is used to separate mixtures of two or more liquids by relying on the fact that they have different boiling points.
• Fractional distillation is used to refine oil and involves a fractionating column which separates the mixture into “fractions”.
• Filtration separates a liquid from a solid using filter paper.
• Crystallisation involves evaporating the liquid in a solution to leave crystals.
• Paper chromatography separates mixtures of several soluble solids. The different solids have different solubilities and thus move at different speeds across the paper.

 

 

Acidity, Alkalinity and Neutralisation

 

Colour of litmus	Type of solution
Red	Acidic
Purple	Neutral
Blue	Alkaline

 

• Phenolphthalein indicator is colourless in acid and pink in alkaline.
• Methyl orange indicator is pink in acid and yellow in alkaline.
• Universal indicator: Brick = Acid, Green = Neutral, Blue = Alkaline
• pH scale is 1-14, 1 being acid and 14 being alkali. 7 is neutral.
• Acids lose protons in reactions

 

Salt equations

Acid + Alkali -> Salt + water

Acid + Base -> Salt + water

Acid + Carbonate -> Salt + water + CO2

Acid + Metal -> Salt + hydrogen

 

 

Energetics

 

• Reactions in which the temperature is increased are exothermic
• Reactions in which the temperature is decreased are endothermic
• Energy changes occur when bonds are made or broken
• Enthalpy is the heat energy in chemical equations
• Enthalpy change: Mass x heat capacity x rise in temperature
• No energy can be created or destroyed, and thus the enthalpy change comes from the chemical reaction
• In exothermic reactions, enthalpy change is negative, as energy is given out to the surroundings.
• In endothermic reactions, enthalpy change is positive, as energy is taken in from the surroundings.
• Breaking bonds is endothermic and creating bonds is exothermic.

 

Rates of Reaction

 

• Increased surface area increases the rate of reaction.
• Increased concentration increases the rate of reaction.
• Increased temperature increases the rate of reaction.
• Use of a catalyst increases the rate of reaction.

 

Equilibria

 

• Equilibrium can only occur in reversible reactions.
• Equilibrium is affected by concentration, temperature, and pressure.
• Le Chatalier’s principle states that when a variable such as temperature is changed, the equilibrium will shift to counteract that change.

iGCSE Chemistry - Organic Chemistry

Alkanes

 

• Alkanes are saturated hydrocarbons, meaning they carbons share only 1 covalent bond.

 

Alkane	Molecular Formula
Methane	CH4
Ethane	C2H6
Propane	C3H8
Butane	C4H10
Pentane	C5H12

 

• A family of hydrocarbons, such as the alkanes, are called a homologous series.

 

Properties of alkanes
General formula	CnH2n+2
Description	Saturated
Combustion	Burn in oxygen to form CO2 and H2O
Reactivity	Low
Test	None
Uses	Fuels

 

• Alkanes are tetrahedral in shape.
• Isomers of an alkane are two or more alkanes with the same molecular formula but a different structure.
• Halogenation is the replacement of one or more hydrogens in an organic compound with halogen atoms.

 

Alkenes

 

• Alkenes are unsaturated hydrocarbons, meaning they contain a double covalent bond (C=O)
• Alkenes are formed through cracking

 

 

 

 

Properties of alkenes
General formula	CnH2n
Description	unsaturated
Combustion	Burn in oxygen to form CO2 and H2O
Reactivity	High
Test	Turn bromine water colourless
Uses	Making polymers

 

• The bonds on each carbon are directed to the corners of an equilateral triangle
• Alkenes undergo addition reactions, in which the double covalent bond splits open and halogens attach themselves.

 

Ethanol

 

• Alcohols are molecules containing –OH.
• Alcohols have the general formula: C_nH_2n+1OH
• Ethanol can be made in two ways:

1. Fermentation
• Yeast are added to sugar in an anaerobic environment
• Pure alcohol is extracted using fractional distillation
2. Industrial scale

·        Ethene and steam are passed over a phosphoric acid catalyst

·        Ethanol can be oxidised to make ethanoic acid

·        Ethene can be made from ethanol by dehydrating ethanol together with sulphuric acid (as a catalyst)

·        Carboxylic acids are acids which contain –COOH (i.e. have been oxidised)

·        When ethanol is reacted with carboxylic acids, esters are formed

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Hydrogen

H₂

Oxygen

O₂

Nitrogen

N₂

Chlorine

Cl₂

Bromine

Br₂

Iodine

I₂

 

• Balancing numbers can only be put in front of formulae.
• States and their symbols:

 

State	Symbol
Solid	s
Liquid	l
Gas	g
Solution	aq

 

• Ions and their charges:

 

Positive Ions (cations)		Negative ions (anions)
Ammonium	NH4+	Bromide	Br-
Hydrogen	H+	Chloride	Cl-
Copper	Cu+	Iodide	I-
Potassium	K+	Hydroxide	OH-
Sodium	Na+	Nitrate	NO3-
Silver	Ag+	Carbonate	CO32-
Calcium	Ca2+	Oxide	O2-
Magnesium	Mg2+	Sulphate	SO42-
Copper (II)	Cu2+	Sulphite	SO32-
Iron (II)	Fe2+	Sulphide	S2-
Zinc	Zn2+	Phosphate PO43-
Aluminium	Al3+
Iron (III)	Fe3+

 

• Ionic equations show reactions involving ions.
• In many ionic reactions some of the ions play no part in the reaction. These ions are called spectator ions.
• A solution is made when a solute dissolves in a solvent.
• Moles present in solution = volume (cm³) x mol cm³.

 

Ionic Compounds

 

• Metals lose electrons from their outer shell to form positive ions.
• Non-metals gain electrons in their outer shell to form negative ions.
• Ionic bonding can be represented in dot and cross diagrams.

 

Properties of Ionic compounds

High melting + boiling points

Form giant lattices

Strong forces between ions

Electrolytes – conduct electricity when molten or dissolved in water

 

Covalent Substances

 

• Covalent bonds involve electron sharing.
• Covalent bonds only occur between non-metals.
• The displayed formula of H₂ is written as H—H.
• Covalent bonds can be single, double, or triple. This means they each contribute one, two, or three (respectively) electrons to the molecule formed.
• Shapes of molecules:

          Carbon dioxide is a linear molecule.

          Water molecules are v-shaped.

          Methane molecules are tetrahedral.

          Molecules of ammonia are pyramidal.

• Covalent compounds can form simple molecular crystals.

 

Properties of covalent compounds

Low melting + boiling points

Very strong intramolecular forces

Weak forces between ions

Do not conduct electricity

 

• Not all covalently bonded compounds exist as simple molecular structures: diamond exists as a giant structure.
• Different forms of the same element are called allotropes.
• Diamond ore is called kimberlite.

 

Electrolysis

 

• Substances that can conduct electricity are called electrolytes.
• For electrolysis to occur, the substance must be molten or dissolved in water, and it must contain ions.
• Electrode connected to positive terminal is the anode.
• Electrode connected to negative terminal is called cathode.
• The negative ions are attracted to the anode and lose electrons (oxidised).
• The positive ions are attracted to the cathode and gain electrons (reduced).
• A Faraday is one mole of electrons.
• Reactions at electrodes can be shown as ionic half equations.
• e^- stands for electron.
• Electroplating is the process by which electrolysis is used to coat an object with a thin film of metal. The object is used as the cathode.
• To purify metals, make the anode a large impure block and the cathode a thin pure block

 

Metallic Crystals

 

• Metals are giant structures with high melting and boiling points.
• Metal atoms give up electrons to form cations.
• These electrons given off form a swarm around the metal ion, and thus are free to move (delocalised) meaning metals conduct electricity.

 

Properties of Metals

High melting + boiling points

Malleable (can be hammered into a sheet)

Ductile (can be drawn into a wire)

Conduct heat and electricity