Alkanes

What can alkanes be used for?

Used as fuels and lubricants, and as starting materials for a range of compounds. So they are very important in industry.

What is an expression to deduce the Mr of an alkane?

14N +2

General formula of ring alkanes?

Cn H2n

Why are alkanes non polar?

The electronegativities of carbon and hydrogen are very similar

Why does alkane boiling point increase as chain length increases?

Van der Waals forces increase so it takes more heat energy to break them.

What are the states of chain lengths with less than 4 carbons, 5-18 carbons, and above 18 carbons?

Less than 4 = gas, 5-18 = liquid, above 18 = solid

Why do branched alkanes have lower melting points than straight chain alkanes?

They cannot pack as closely together as straight alkanes so van der Waals forces aren’t as effective

Are alkanes insoluble, and why/why not?

They are insoluble because water molecules are held by hydrogen bonds which are much stronger than van der Waals forces. Alkanes don’t mix with other non polar liquids

How do alkanes react?

They are quite unreactive as they have strong C-H and C-C bonds, so don’t react with acids, bases etc. They do burn in a plentiful supply of oxygen to form carbon dioxide and water.

What is a fraction?

A group of hydrocarbons with a similar chain length and therefore similar boiling points.

What is crude oil?

A fossil fuel formed millions of years ago by breakdown of organism remains at high pressure and temperature

How does fractional distillation work?

1. Crude is heated in a furnace to vaporise it

2. Passes into the fractionating tower that is cooler at the top and hotter at the bottom

3. Vapours rise through the trays and cool until they condense to liquid, then get piped off.

4. High boiling point molecules at the bottom - they dont rise far up before reaching their boiling point, shorter chain molecules at the top as they have lower boiling points so rise further up.

What are features of fractional distillation that allows it to separate the crude oil?

Fractions have different boiling points, boiling point depends on chain length, cooler at the top and hotter at the bottom.

Why is cracking important?

Shorter chain hydrocarbons are in huge demand from the petrochemical industry, but the supply is low, however longer chain alkanes have a low demand but higher supply. Shorter chain products are more economically valuable

What does cracking produce?

Shorter, more useful and valuable products, and some alkenes which are more reactive so have many uses

Why are harsh conditions required for cracking?

Carbon carbon bonds are strong

What does thermal cracking involve?

Heating alkanes to a high temperature (700,1200 K) and high pressure (up to 7000kPa). The alkanes are only kept in these conditions for a very short time to avoid too much decomposition of the longer chains

How is a free radical produced in thermal cracking?

The bonds break in a way so that each shorter chain initially ends in a carbon with an unpaired electron (known as a free radical, a chemical species with an unpaired electron)

How are alkenes then produced from thermal cracking?

As there are not enough hydrogen atoms to produce two alkanes, one of the new chains must have a double bond, os a high proportion of alkenes are produced from cracking

What does catalytic cracking involve?

Takes place at a lower temperature (720K - still classed as a high temp though on the exam) and at a slight pressure. Uses a zeolite catalyst consisting of silicon dioxide and aluminium oxide. Zeolites have a honeycomb structure with a huge surface area

What does catalytic cracking produce?

Produces aromantic compounds with carbon rings