Alkenes and Alkanes

Combustion

Fuels are substances that react with oxygen to release useful energy as heat and light. 

21% of the air is oxygen, when fuel burns in plenty of air, it receives enough oxygen complete combustion

Complete Combustion

• Complete combustion needs a good supply of air so the elements in the fuel react fully with oxygen

• Natural gas and petrol contain hydrocarbons which are compounds of hydrogen and carbon only

• When hydrocarbons burn completely:

• The carbon oxidises to carbon dioxide

• The hydrogen oxidises to water

• Complete combustion equation → hydrocarbon + oxygen → carbon dioxide + water

• Complete combustion of propane used in bottled gas → propane + oxygen → carbon dioxide + water

• C3H8 + 5O2 → 3CO2 + 4H2O

Incomplete Combustion

• Occurs when supply of air is poor

• Water still produced but carbon monoxide and carbon instead of carbon dioxide

• Incomplete combustion → hydrocarbon + oxygen → carbon monoxide + carbon + water

• Carbon is released as soot

• Carbon monoxide is a poisonous gas

• Gas fires and boilers must be serviced regularly to ensure they don’t produce carbon monoxide

• Incomplete combustion of propane

• Propane + oxygen → carbon + water

• C3H8 + 2O2 → 3C + 4H2O

Alkanes

• Homologous series of hydrocarbons - similar chemical properties and have trends in physical properties

• Alkane formula → CnH2n+2
• Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the covalent bondsbetween them by a straight line.
• Alkanes are saturated hydrocarbons - joined together by single bonds
• Relatively unreactive apart from their reaction with oxygen in the air which we call burning or combustion 
• Alkanes show isomerism 
• Their atoms can be arranged differently to make slightly different compounds with different properties. For example, an isomer of butane is methylpropane.

In substitution reactions one atom is swapped with another

Useful reactions in chemical industry - allow chemists to change one compound into something more useful building up designer molecules like drugs

Alkanes undergo a substitution reaction with halogens in the prescence of light

In ultraviolet light methane reacts with halogen molecules such as chlorine and bromine

E.g. Methane + bromine --> methylbromine + hydrogen + bromide - CH4 + Br2 --> CH3Br+HBr

Substitution reaction because one if hydrogen atoms from methane is replaced by a bromine atom

Homologous series of hydrocarbons with a double carbon - carbon bond

General formula - CnH2n

Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more reactive than the alkanes.

Testing for alkenes

c-c double bond allows alkenes to react like alkanes cannot

To tell them apart we use bromine water (orange solution of bromine) 

Becomes colourless when shaken with alkenes

Alkenes can decolourize bromine wawter but alkanes cannot

Alkanes and Alkenes