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, branched alkanes, and cycloalkanes. Mainly used to produce motor fuels

How are the products of catalytic cracking separated?

Fractional distillation

What are the products of alkane combustion in a plentiful oxygen supply

Carbon dioxide and water

What are some examples of alkane fuels?

Methane, propane, butane, petrol, paraffin

What are fuels?

Substances that release a large amount of energy per gram.

What are the products of combustion in a limited oxygen supply?

Carbon monoxide or carbon (in even less oxygen) and water. Often happens with longer chain alkanes.

What polluting products can hydrocarbons produce?

CO - poisonous gas

Nitrogen oxides (NOx) - these oxides react with water vapour and O2 to form nitric acid - acid rain contributor.

Sulfur dioxide - acid rain contributor.

Carbon particulates exacerbate asthma and cause cancer.

Unburnt hydrocarbons are major greenhouse gases

CO2 is a greenhouse gas.

What is the equation where SO2 combines with O2 to form sulfuric acid?

SO2 (g) + ½ O2 (g) + H2O (l) → H2SO4 (l)

What are the gases called given out by power stations, and why does SO2 need to be removed from them?

Flue gases, SO2 is a gas that causes acid rain as it reacts to form sulfuric acid.

What is the first method of flue gas desulfurisation that involves calcium oxide?

A slurry of it and lime is sprayed into the flue gas which reacts to form calcium sulfite, further oxidised to calcium sulfate (called gypsum).

What is the overall reaction of the formation of gypsum?

CaO (s) + 2H2O (l) + SO2 (g) + ½ O2 (g) → CaSO4. 2H2O (s)

What is the equation of the alternative process of flue gas desulfurisation that involves calcium carbonate?

CaCO3 (s) + ½ O2 (g) + SO2 (g) → CaSO4 (s) + CO2 (g)

What are petrol cars fitted with, how do they work and what do they filter out?

Catalytic converters in their exhaust, which reduces the output of CO, NOx, and unburnt hydrocarbons. The converter is a honeycomb (large s.a.) made of ceramic material coated with platinum and rbodium metals. They act as as catalysts.

What is the general equation of reacting hydrocarbons with nitrogen oxide?

hydrocarbons + nitrogen oxide → nitrogen + carbon dioxide + water

How does the enhanced greenhouse effect work?

CO2 traps infrared radiation so Earth’s atmosphere heats up, making Earth warm enough to inhabit. However since the industrial revolution, more fossil fuels have been used so CO2 has been rising. This has caused a rise in temperature.

What is a free radical?

A species that has an unpaired outer electron, highly reactive.

How are free radicals made?

Via breaking single bonds, and one electron from the single bond goes to each atom to form 2 radicals, called homolytic fission. Represented with a dot.

What is the essential condition in free radical substitution?

UV light is needed to initiate it so homolytic fission happens.

What is the general equation for the initiation reaction?

molecule → radical + radical - breaks up a halogen

What is the general equation for propagation?

radical + molecule → radical + molecule. Usually first reaction involves the free radical from initiation and the other molecule from the main reaction, and the second reaction involves the free radical made from reaction 1 and the halogen molecule.

What is the general equation of termination?

radical + radical → molecule, which decreases the free radicals to use them up and form a stable product.

What is the propagation reaction known as?

A chain reaction as it results in multiple substitutions, and multiple steps that are repeated indefinitely until the chain is terminated.

What is homolytic bond fission?

When two atoms are the same (eg Cl2) resulting in the bond breaking homolytically so each Cl atoms gets one electron from the bond

What may occur in free radical substitution if a large excess of a halogen is used?

More than one halogen atom may be substituted onto the alkane forming a different organic product, and eventually all the hydrogen atoms may be substituted to form tetrachloromethane.

What is a CFC, and what is it doing to the ozone layer?

Chlorofluorocarbons where all hydrogens are replaced by halogens, and they are destroying the ozone layer

What is ozone, and what is its impact at ground level?

Molecule made from 3 oxygen atoms, and can cause lung irritation and degradation of plants.

Why is the ozone layer important in the atmosphere?

Protects the Earth from the exposure to UV light, as too much can cause plankton damage/ skin cancer

How are chlorine free radicals formed from CFCs?

The chlorine-carbon bond breaks homolytically

What is the equation for chlorine free radicals attacking ozone?

• Cl + O3 → ClO • + O2

What is the second propagation reaction involving ClO• and ozone?

ClO• + O3 → 2O2 + Cl•

What is the overall equation for the decomposition of ozone?

2O3 → 3O2