Chapter 18.2: Alkanes and Alkenes

Alkanes

Hydrocarbons that contain only carbon-carbon single covalent bonds

General formula of alkanes

Cn H2n+2

Why are alkanes called saturated hydrocarbons

they contain only carbon-carbon single covalent bonds

5 general properties of any homologous series

1. have the same functional group

2. have similar chemical properties

3. show a gradual change in their physical properties

4. have the same general formula

5. each member of the series differs from the next by a CH2 unit

3 physical properties of alkanes

1. low melting and boiling points

2. insoluble in water

3. soluble in organic solvents

(Alkanes) Melting and boiling point down the series, and why

Increases down the series as their molecular size increases

This is because as alkane molecules become bigger, the intermolecular forces of attraction between the alkanes molecules increase

More energy is required to overcome the intermolecular forces between the molecules

(Alkanes) Viscosity down the series, and why

Viscosity increases as their molecular size increases

This is because as alkane molecules become bigger, the intermolecular forces of attraction between the alkane molecules increase

(Alkanes) flammability down the series, and why

Flammability decreases as when the molecular size increases, it is more difficult to burn the alkane.

as the molecular size of the alkane molecules increases, the percentage of carbon in the alkane molecules also increases

What are the 3 chemical reactions of alkanes

1. combustion

2. cracking

3. substitution reaction

Products of complete combustion

carbon dioxide and water vapour

Why are alkanes used as fuels

Complete combustion of alkanes is a highly exothermic reaction.

products of incomplete combustion

carbon monoxide and water vapour

cracking

the process where long-chain hydrocarbons are broken down into smaller hydrocarbons or hydrogen

substitution reaction

a reaction in which a hydrogen atom is replaced by a halogen

substitution reaction products

a substituted alkane and hydrogen chloride gas

substitution reaction condition

UV light

Alkenes

hydrocarbons that contain one or more carbon-carbon double bonds

Alkene general formula

Cn H2n

Name the first 3 alkenes

Ethene, Propene, Butene

Why are alkenes unsaturated

they contain carbon-carbon double covalent bonds

Solubility of alkenes

They are insoluble in water, and soluble in organic solvents

Melting and boiling point of alkenes

Increases down the group as their molecular size increases.

As alkene molecules become bigger, the intermolecular forces of attraction between the alkene molecules increase

More energy is required to overcome the intermolecular forces of attraction between the molecules

Density of alkenes

increases as their molecular sizes increases

viscosity of alkenes

the alkenes become more viscous (more difficult to pour out) as their molecular sizes increase

this is because the intermolecular forces are stronger as the alkenes molecules become bigger. this makes it difficult for the liquid to flow

flammability of alkenes

ass the molecular size of the alkene molecules increases, the percentage of carbon in the alkene molecules also increase

such alkenes also produce a smokier flame, caused by incomplete combustion

why do alkenes produce a sootier flame compared to alkanes

alkenes contain a higher percentage of carbon then the corresponding alkanes with the same number of carbon atoms

3 addition reactions for alkenes

1. hydrogenation

2. bromination

3. addition polymerisation

addition reaction

a reaction in which an unsaturated organic compound reacts with another substance to form a single new compound

why do alkenes undergo addition reactions

the carbon-carbon double bonds in alkenes are very reactive

Hydrogenation

the addition of hydrogen to alkenes

conditions for hydrogenation

temperature of 200 degree celsius, nickel catalyst

use of hydrogenation

used to make margarine from vegetable oil

bromination

the addition of aqueous bromine to alkenes

bromination observation for alkene

the reddish brown aqueous bromine decolourises rapidly to form a colourless liquid

bromination observation for alkane

aqeuous bromine remains reddish-brown

Why is bromination useful

1. test for the presence of unsaturated hydrocarbons

2. distinguish between an alkane and an alkene

addition reaction

addition of alkene molecules

conditions for addition polymerisation

high temperature and pressure, catalyst

cracking

the breaking down of long-chain hydrocarbons into smaller molecules (short-chain alkanes, alkenes, or hydrogen)

how are alkenes obtained

by cracking petroleum

conditions for catalytic cracking

aluminium oxide (Al2O3) and silicon dioxide (SiO2) catalysts, high temperature of 600

importance of cracking (3)

1. cracking produces short chain alkenes

2. cracking produces hydrogen

3. cracking converts hydrocarbons of higher molecular mass (such as lubricating oil) into smaller molecules which are in higher demand (such as petrol)

similarities between alkanes and alkenes

1. they are hydrocarbons

2. they are flammable. on complete combustion in excess air, they form carbon dioxide and water

reactants in manufacturing margarine

vegetable oil and hydrogen

conditions to manufacture margarine

temperature of 200, nickel catalyst

2 differences between fats and oils

1. at room temperature and pressure, fats are solids while oils are liquids

2. fats contain mainly saturated fat molecules, while oils contain a higher percentage of unsaturated fat molecules

polymer

a type of long chain macromolecule that is covalently linked together by many small repeat units called monomers

polymerisation

the process of joining together a large number of monomers to form a polymer

addition polymerisation

when unsaturated monomers join together without losing any molecules or atoms

Conditions for additional polymerisation

high pressure, high temperature, catalyst

Poly (ethene)

the material used for making bags, toys. It is flexible but difficult to break

why are plastics widely used

1. relatively cheaper

2. easily moulded into different shapes

3. light, tough, and waterproof

4. durable (resistant to decay, rusting, and chemical attack)