Chem-7

Question 1

what is a hydrocarbon

Answer 1

Hydrocarbons are compounds that contain hydrogen and carbon atoms only.

Question 2

what is a crude oil

Answer 2

Crude oil is a finite resource that is found in the Earth’s crust. It is the remains of organisms that lived and died millions of years ago - mainly plankton which was buried in mud.

Crude oil is a complex mixture of hydrocarbons. The carbon atoms in these molecules are joined together in chains and rings. In the ball and stick models below, carbon atoms are black and hydrogen atoms are white.

Question 3

What is crude oil used for

Answer 3

fuels such as petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases
feedstock for the petrochemical industry

Question 4

What is a feedstock

Answer 4

A feedstock is a raw material used to provide reactants for an industrial reaction. A petrochemical is a substance made from crude oil using chemical reactions. For example, ethene is produced from crude oil. It is used as feedstock to make poly(ethene), a polymer.

Question 5

What are some useful substances made from compounds found in crude oil

Answer 5

solvents
lubricants
detergents

Question 6

what is a homologous series

Answer 6

A ‘family’ of organic compounds that have the same functional group and similar chemical properties.

Question 7

What are some factors of alkanes

Answer 7

have the same general formula
differ by CH2 in molecular formulae from neighbouring compounds
show a gradual variation in physical properties, such as their boiling points
have similar chemical properties

Question 8

what is the general formula for alkanes

Answer 8

The general formula for the alkanes is CnH2n+2, where n is the number of carbon atoms in the molecule.

Question 8

What does saturated mean

Answer 8

A saturated hydrocarbon contains no carbon-to-carbon double bonds, only single bonds.

Question 9

Why are alkanes saturated hydrocarbons

Answer 9

hydrocarbons, because they are compounds containing hydrogen and carbon only
saturated, because their carbon atoms are joined by C-C single bonds

Question 9

What is fractional distillation

Answer 9

Fractional distillation is used to separate crude oil into simpler, more useful mixtures. This method can be used because different hydrocarbons have different boiling points.

Question 10

What is the process of fractional distillation

Answer 10

During the fractional distillation of crude oil:

heated crude oil enters a tall fractionating column, which is hot at the bottom and gets cooler towards the top
vapours from the oil rise through the column
vapours condense when they become cool enough
liquids are led out of the column at different heights
Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points. They do not condense, but leave the column as gases. Long hydrocarbon molecules have stronger intermolecular forces, so they have high boiling points. They leave the column as hot liquid bitumen.

Question 11

Properties of hydrocarbons in each fraction

Answer 11

Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. They have similar (but not identical):

numbers of hydrogen and carbon atoms in their molecules
boiling points
ease of ignition

Question 12

what properties do the gases fraction contain

Answer 12

the gases fraction contains hydrocarbons with one to four carbon atoms. These:

have boiling points below room temperature
are very flammable
have a low viscosity

Question 13

Do all fractions of a fractioning tower have the same properties

Answer 13

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These:

have boiling points well above room temperature
are very difficult to ignite
have a high viscosity

Question 14

What can a hydrocarbon fuel undergo depending on the amount of oxygen available

Answer 14

combustion or incomplete combustion

Question 15

what is complete combustion

Answer 15

Complete combustion of a hydrocarbon fuel happens when there is a good supply of air. Carbon and hydrogen atoms in the fuel react with oxygen in an exothermic reaction:

carbon dioxide and water are produced
the maximum amount of energy is given out
In a Bunsen burner, this occurs when the air hole is fully open.

Question 16

What’s the general formula for complete combustion

Answer 16

hydrocarbon + oxygen → carbon dioxide + water

Question 17

What is incomplete combustion

Answer 17

Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced. Less energy is released than during complete combustion.

In a Bunsen burner, incomplete combustion occurs when the air hole is closed.

Question 18

What is cracking

Answer 18

Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

the original starting hydrocarbons are alkanes
the products of cracking include alkanes and alkenes, members of a different homologous series

Question 19

What are the rules for cracking

Answer 19

The starting compound will always fit the rule for an alkane, CnH2n+2. The first product will also follow this rule. The second product will contain all the other C and H atoms. The second product is an alkene, so it will follow the rule CnH2n.

Question 20

What methods can be used for cracking

Answer 20

catalytic cracking uses a temperature of approximately 550 °C and a catalyst known as a zeolite which contains aluminium oxide and silicon oxide
steam cracking uses a higher temperature of approximately 850 °C and no catalyst

Question 21

What are the reasons for cracking

Answer 21

it helps to match the supply of fractions with the demand for them
it produces alkenes, which are useful as feedstock for the petrochemical industry

Question 22

What is the supply and demand like for fractions

Answer 22

The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, fractional distillation of crude oil produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.

Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.

Question 23

What is an alkene

Answer 23

Alkanes and alkenes both form homologous series of hydrocarbons, but:

alkanes are saturated, their carbon atoms are only joined by C-C single bonds
alkenes are unsaturated, they contain at least one C=C double bond
As a result, alkenes are more reactive than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a polymer.

Question 24

How can u test for an alkene

Answer 24

Alkenes will react with bromine water and turn it from orange/brown to colourless. This is the way to test for a double C=C bond in a molecule.