Alkanes (T3)

Question 1

What are Alkanes?

Answer 1

• one of the most common forms of molecule found in crude oil - alkanes are a saturated hydrocarbon, meaning there are no free electrons within the molecule and all of the bonds are single covalent bonds with one electron being shared by the atoms - each carbon, which has four free electrons in its outer shell, bonds with four atoms, and each hydrogen bonds with one

Question 2

What are the three smallest / first straight chain alkanes and their chemical

Answer 2

• methane: CH4 - ethane: C2 H6 - propane: C3 H8

Question 3

Briefly explain, using alkanes as an example, how the general formula for alkanes works…

Answer 3

• members of a homologous series have a general formula - in the case of alkanes, if you know the number (n) of carbon atoms, there are 2n + 2 hydrogens - you multiply the number by 2 then add 2 - the general formula for alkanes is C n H 2n+2 - so, if there are three carbons: (2x3) + 2 = 8 hydrogens - this makes up the formula for propane: C3 H8 - if you wanted the formula for an alkanes with 15 carbons, you could easily work out its C15 H32 etc.

Question 4

Describe the physical state of the first eight alkanes…

Answer 4

The first four are gases at room temperature and the next four are liquid

Question 5

At what formula do solids start to appear in the alkanes?

Answer 5

C17 H36

Question 6

Describe the combustion of alkanes…

Answer 6

• all alkanes burn in air or oxygen - if enough oxygen, they burn completely to give carbon dioxide and water Hydrocarbons + oxygen > carbon dioxide + water + ENERGY - if there is not enough oxygen, there is ‘incomplete combustion’ of the hydrocarbon and you get carbon monoxide or carbon (soot) produced instead of carbon dioxide Hydrocarbons + oxygen > carbon monoxide + water + ENERGY

Question 7

Explain the reaction of alkanes and bromine…

Answer 7

• alkanes react with bromine in the presence of ultra-violet light, eg: sunlight - a hydrogen atom in the alkanes is replaced by a bromine atom - this is known as a substitution reaction, because on atom has been substituted by a different one

Question 8

What is a ‘substitution reaction’? Give an example..

Answer 8

When two substances react and one (or more) atoms in one substance is replaced by an atom from the other Eg: when alkanes react with bromine in the presence of UV light, a hydrogen atom is replaced by a bromine atom

Question 9

What happens if a mixture of methane and bromine is exposed to sunlight?

Answer 9

• they can be carefully boiled to allow fractional distillation - the largest molecule chains have a considerably higher boiling point than the shorter chains - the viscosity of those longer chains is also higher with the shorter chains being thinner and able to run more easily

Question 10

What are the names of the first five straight chain alkanes?

Answer 10

• methane - ethane - propane - butane - pentane

Question 11

What is the formula and room temperature state of methane

Answer 11

• CH4 - gas

Question 12

What is the formula and room temperature state of ethane?

Answer 12

• C2 H6 - gas

Question 13

What is the formula and room temperature state of propane?

Answer 13

• C3 H8 - gas

Question 14

What is the formula and room temperature state of butane?

Answer 14

• C4 H10 - gas

Question 15

What is the formula and room temperature state of pentane?

Answer 15

• C5 H12 - liquid

Question 16

What is the general formula for alkanes?

Answer 16

Cn H2n+2

Question 17

How are the physical properties of alkenes similar to alkanes?

Answer 17

• the small alkenes with up to four carbon atoms are gases - they are gases up to C4 H8

Question 18

How are the chemical reactions of alkenes similar to alkanes?

Answer 18

They also burn in air or oxygen to give carbon dioxide and water