Alkanes 12.1-12.5

Alkanes are saturated hydrocarbons.

Saturated--> a substance with only single C-C bonds 

Hydrocarbons--> containing hydrogen and carbon only 

Properties of alkanes 

Polarity 

• Almost non-polar beacuse the electronegativites of carbon (2.5) and hydrogen (2.1) are so similar 
• only intermolecular forces between their molecules are weak van der waals 
• larger the molecule the stronger the van der waals 

Boiling point 

• Increasing intermolecular forces is why the boiling points of alkanes increase as the chain length inceases 
• the shorter chains are gases at room temperatures 
• Pentane is a liquid with a low boiling point of 309k 
• Chain length of about 18 carbons the alkanes become soild at room temperature 
• Alkanes with branches have lower melting points than straight chain ones with the same number of C atoms because they can't pack together as closely so van der waals are not so effective.

Solubility 

• Insoluble in water because water molecules are held together by hydrogen bonds which are much stronger than the van der waals 
• Alkanes do mix with other relatively non-polar liquids 

Reactivity 

• relatively unreactive 
• have strong carbon-carbon and carbon-hydrogen bonds 
• do not react with acids, bases and oxidising agents and reducing agents 
• They do burn and will react with haogens under suitable conditions 
• They burn in a plentiful suply of oxygen to for CO₂ and water 

Crude oil 

Flammability --> As the carbon chains get longer they become less flammable as they are more difficult to light 

Viscosity--> As the carbon chains get longer they become more viscous as the chain gets longer theu become thicker 

Boiling point--> As the carbon chain gets longer the boiling point increases.

The boiling points of hydrocarbons change as the carbon chains get longer because there are more intermolecular forces in the chain so the boiling point increases.

Combustion 

--> reactions with oxygen 

• When hydrocarbons burn they produce carbon dioxide and water
• Alkanes burn with a clean flame due to low carbon: hydrogen ratio

                Alkane + Oxygen --> Carbon Dioxide + Water 

                C_nH_2n+2 + XO₂--> _nCO₂ + n+XH₂O

Incomplete Combustion 

• Occurs when not enough oxygen is available 
• more common with long chain hydrocarbons as need more oxygen to burn completely 
• instead carbon monoxide is formed. CO binds to haemoglobin in blood preventing transport of oxygen causing asphyxiation 
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