3.10 Alkanes - Properties & Combustion

Question 1

Describe how the properties of hydrocarbons change depending on their length (4)

Answer 1

• The shorter the carbon chains, the more runny they will be - that is, the less viscous (gloopy) they are.
• Molecules with shorter carbon chains are also more volatile, i.e. they have lower boiling points.
• The shorter the carbon chains, the more flammable the compounds will be.
• The properties of these compounds affect how they’re used for fuels. E.g. compounds with short carbon chains and lower boiling points are used as “bottled gases” - stored under pressure as liquids in bottles.

Question 2

What is the word equation for the complete combustion of any carbon-based compound?

Answer 2

carbon-based compound + oxygen -> carbon dioxide + water

Question 3

What is the main use of hydrocarbons and why?

Answer 3

• Fuel: as they release a lot of energy when they combust completely. As long as there is a lot of oxygen available, we get complete combustion.

Question 4

As the chain length (number of carbon atoms) of alkanes increases, they become: (3)

Answer 4

• More viscous
• Less volatile
• Less flammable

Question 5

Is combustion an exothermic or endothermic reaction?

Answer 5

exothermic

Question 6

Describe how carbon and hydrogen are oxidised during a combustion reaction

Answer 6

Carbon (C) becomes CO2, whilst hydrogen (H2) becomes H2O. Both have gained oxygen, so have been oxidised.