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