3.2 Alkanes

Question 1

What is the c-c-c angle of an unbranched chain?

Answer 1

109.5 degrees

Question 2

Give an example of an unbranched chain

Answer 2

Pentane

Question 3

Give an example of a branched chain

Answer 3

Methylbutane which is an isomer of pentane

Question 4

What is the general molecular formula of ring alkanes and explain why

Answer 4

CnH2n because the end hydrogens are not required

Question 5

Name the first six alkanes

Answer 5

Methane Ethane Propane Butane Pentane Hexane

Question 6

How do you name straight chain alkanes?

Answer 6

Alkanes are named from the root, which tells us the number of carbon atoms, and the suffix -ane, denoting an alkane.

Question 7

How do you name branched chain alkanes

Answer 7

You must first find the longest unbranched chain which gives the root name. Then name the branches as prefixes and add numbers to say which carbon atoms the side chains are attached to.

Question 8

What happens to the number of isomers when the chain length increases

Answer 8

The number of isomers rise rapidly with chain length

Question 9

Panay is the general formula for all chain alkanes

Answer 9

CnH2n+2

Question 10

What is the polarity of alkanes and why

Answer 10

They are almost non-polar because the electronegativities of carbon and hydrogen are so similar. So the only intermolecular forces are between the molecules are weak van der waals and the larger the molecule the stronger the van der waals.

Question 11

Explain the boiling points of alkanes

Answer 11

The shorter chains are gases at room temperature. At a chain length of about 18 carbons, the alkanes become solids at room temperature. Alkanes with branched chains have a lower melting point than straight chain alkanes with the same number of carbon atoms. This is because they cannot be packet together as closely as unbranched chains so the van der waals forces are not as effective.

Question 12

Explain the solubility of alkanes

Answer 12

They are insoluble in water because the water molecules are held together by hydrogen bonds which are much stronger than the van der waals forces that act between the alkane molecules. However alkanes do mix with other relatively non-polar liquids.

Question 13

How do alkanes react

Answer 13

They are relatively unreactive. They have strong carbon-carbon and carbon-hydrogen bonds. They do not react with acids,bases,oxidising agents and reducing agents. However they burn and react with halogens under suitable conditions. They burn in a plentiful supply oxygen to form carbon dioxide and water (in a restricted supply of oxygen they form carbon monoxide or carbon)

Question 14

What are alkanes

Answer 14

Alkanes are saturated hydrocarbons which contain only carbon-carbon and carbon-hydrogen single bonds. They are among the least reactive organic compounds.

Question 15

What is the main source of alkanes

Answer 15

Crude oil

Question 16

Why are alkanes very important to industry

Answer 16

They are used as fuels and lubricants and as starting materials for a range of other compounds.

Question 17

Why are long chain hydrocarbons shortened?

Answer 17

Shorter hydrocarbons are more useful so sell for more. Therefore long chain hydrocarbons are often ‘cracked’ into smaller hydrocarbons to meet demand and sell for more money?

Question 18

How does thermal cracking work?

Answer 18

Between 700K and 1200K and 7000kPa. C-C bonds get broken to form 2 free radicals. One forms an alkene and loses a hydrogen. The other takes a hydrogen and becomes a smaller alkane.

Question 19

How does catalytic cracking work?

Answer 19

720K, lower pressure than thermal cracking but more than atmospheric. Zeolite catalyst (honeycomb structure of silicon and aluminium dioxides). Produces motor fuels, branched alkanes and some cycloalkanes.

Question 20

Why do sulfur impurities in hydrocarbon fuels make it particularly bad for the environment?

Answer 20

The sulfur reacts with water vapour and oxygen to form sulfuric acid, which makes acid rain.

Question 21

What is flue gas desulfurisation?

Answer 21

The process of removing sulfur from the gases given out by power stations.

Question 22

What are the 2 ways in which flue gas desulfurisation is done?

Answer 22

1) with CaO, water and oxygen to make gypsum which can be sold. CaO+2H₂O+1/2O₂+SO₂–> CaSO₄-2H₂O 2)With calcium carbonate CaCO₃+1/2O₂+SO₂–> CaSO₄+CO₂

Question 23

What is the essential condition needed for free radical substitution?

Answer 23

It needs to be under UV light.

Question 24

What is the initiation step for C₂H₆ +F₂?

Answer 24

F₂–>2F^. with UV light as a catalyst.

Question 25

What are tthe 2 propogation steps for C₂H₆ +F₂?

Answer 25

C₂H₆+F^.–> C₂H₅^.+HF C₂H₅^. + F₂ C₂H₅F +F^.

Question 26

What are two possible termination steps for C₂H₆ +F₂?

Answer 26

F^. + C₂H₅^. –> C₂H₅F C₂H₅^. + C₂H₅^. –> C₄H₁₀

Question 27

What is the general formula for an alkane?

Answer 27

C2H2n+2

Question 28

Are alkanes saturated or unsaturated?

Answer 28

Saturated because no more bonds can be made

Question 29

What are the first six alkanes (length of carbon chains)?

Answer 29

Methane, ethane, propane, butane, pentane, hexane

Question 30

Alkanes are useful as fuels. Write a balanced symbol equation for the complete combustion of propane

Answer 30

C3H8+5O2 -> 3CO2+4H2O

Question 31

Write a balanced symbol equation for the incomplete combustion of methane.

Answer 31

CH4+1.5O2 -> CO+H2O

Question 32

What are the three main greenhouse gases?

Answer 32

Carbon dioxide, water vapour, methane

Question 33

What are the two types of cracking called?

Answer 33

Catalytic cracking and thermal cracking

Question 34

Give three features of thermal cracking

Answer 34

-high temperature -high pressure -produces lots of alkenes

Question 35

Give three features of catalytic cracking

Answer 35

-uses a zeolite catalyst -therefore there is a low temperature and pressure -makes motor fuels and aromatic hydrocarbons

Question 36

Write a molecular formula for hexane

Answer 36

C6H14