organic chemistry

Hydrocarbon

Compounds that contain only carbon and hydrogen

Homologous series

A series of compounds which have the same general formula, differ by CH2 in molecular formulae from neighbouring compounds, show a gradual variation in physical properties, and have similar chemical properties

Saturated

Only contain single bonds

Why do alkanes have low melting and boiling points?

Weak intermolecular forces between molecules needs breaking so little energy required. As molecule get bigger the forces get stronger (more electrons) and melting/boiling point increases

Complete combustion of alkanes

Hydrocarbon + oxygen → carbon dioxide + water

Incomplete combustion of alkanes

Hydrocarbon + oxygen → water (soot and CO also formed)

Alkanes

CnH2n+2

Alkenes

CnH2n

Unsaturated

Contain carbon-carbon double bonds (C=C)

Isomers

Same molecular formula but different arrangements of atoms

Bromine reaction with alkene

Bromine water turns orange to colourless

Bromine reaction with alkane

Bromine water remains orange (no reaction)

Water reaction with alkene (hydration)

Alkene + water (steam) → alcohol

What makes a fuel good?

It burns easily, produces little ash/smoke, produces a lot of energy when burnt, is easy to store and transport

Fossil fuels example

Coal, natural gas, crude oil

Crude oil- separated by fractional distillation

• Mixture of hydrocarbons

• Collects in porous rocks underneath non porous rocks

• Pumped up from the ground and taken by pipe or tankers to oil refineries

• Includes chains eg butane and rings eg cyclohexane

• A finite source

• Used as fuels and feedstock for petrochemical industry

Fractional distillation- separation according to boiling point

• Separates crude oil into simpler more useful mixture

• Crude oil is heated strongly

• Vapour rise and condense when cool enough and then piped away

As chains get longer…

Boiling point increases so the fractions become more viscous less volatile and are harder to ignite and burn with sootier flame

Cracking

Breaks down larger less useful long chain hydrocarbons into smaller more useful alkanes (fuels) and alkenes (making polymers)

Incomplete combustion (shown by yellow flame in Bunsen burner) happen in…

• Car engines where petrol burns producing CO and soot

• Poorly ventilated rooms with coal or gas producing CO

• Faulty gas appliances if air inlet is blocked

Carbon monoxide

Tasteless, odourless, colourless toxic gas. Breathing it can cause drowsiness, coma, then death. Combines with haemoglobin in blood and stops it from carrying oxygen to cells. If poisoning advances blood transfusion is the only cure

Cracking process

Large alkane is soaked in to ceramic wool and out in a boiling tube along with a catalyst (Al2O3). The tube is heated and the alkane vapours pass over the catalyst and break down. The gases are insoluble and are collected over water

Sulfur dioxide

Hydrocarbon fuel such as coal contain sulfur as natural impurities. Burning produces SO2. SO2 dissolves in clouds forming sulphurous and sulfuric acid which falls as acid rain

Effect of sulfur dioxide

Crops don't grow as well, fishes in rivers and lakes are killed, weakening of limestone and marble buildings increase.

How to solve the problems of sulfur dioxide?

Remove sulfur from fuels before burning. Can be removed from product gases by neutralising the acidic emissions with CaCO3

Nitrogen oxides

High temperature in internal combustion engines cause nitrogen and oxygen to react forming NOx which are pollutants.

Effect of nitrogen oxides

Contribute to acid rain when it dissolves in rain clouds forming nitric acid. Can cause respiratory diseases such as asthma and bronchitis.

How to solve the problems of nitrogen oxides?

Use catalytic converters to convert NOx gases into harmless nitrogen gas

Advantage of fossil fuels

• Readily available

• Lots of petrol station

• Readily stored in fuel tanks

• Easily ignited

• Release a large amount of energy

Disadvantage of fossil fuels

• Take millions of years to form

• Non renewable

• Produces CO2, contributing to global warming

• Contain sulfur compounds that contribute to acid rain

Advantages of hydrogen fuel

• Only produces water when burnt

• Renewable if made by electrolysis

• Easily ignited

• Release a larger amount of energy

Disadvantage of hydrogen fuel

• Gas is difficult to store and transport

• Expensive

• Flammable gas can easily escape

• Fewer filling stations

Alcohol

CnH2n+1OH

Carboxylic acids

CnH2n+1COOH

Polymer

A substance of high average relative molecular mass made up of small repeating units

How are alkenes polymerised?

Heated under pressure with a catalyst. One of the bonds in the double bond will break and molecules link together to form a long chain

Condensation polymeristaion

Monomers joining together and losing a small molecule

Why are polyesters polymers?

They are long chain molecules that contain many ester links.

Polyethene

Flexible, cheap, good insulators. Used for plastic bags, bottles and cling film

Polypropene

Flexible, resistant to shattering, strong. Used for plastic buckets, bowls and ropes

Polychloroethene (PVC)

Tough, good insulators, hard, flexible. Used for guttering, window frames, and electrical cable insulation

Polytetrafluoroethene (PTFE or Teflon)

Tough, slippery, unreactive, water repellent. Used for stain proofing on clothes or carpets, non stick coatings on pans