alkanes

why do alkanes do not dissolve in water

this is because they are non polar (the electronegativity of carbon and hydrogen is similar). Because the water molecules are held together by H-bonds, and alkanes by vdW forces, they do not interact with each other

boiling points of alkanes - trend 1

increasing chain length

higher boiling points due to more electrons which means stronger van der Waals’ forces between molecules

boiling points of alkanes - trend 2

More branching → lower boiling points due molecules not packing as close together and so weaker van der Waals’ forces between molecules

Petroleum/crude oil

• formed by the slow decay of marine animals and plants, over millions of years, under heat and pressure in the absence of oxygen

• composition of crude oils is a complex mixture consisting mainly of alkanes (including cycloalkanes, some aromatics and other compounds containing some S and O).

• components are separated by fractional distillation for useful products

why is Crude oil is often called a fossil fuel

because it is made from the break down of organic matter, such as plants and animals

Burning fossil fuels releases..?

carbon dioxide – a greenhouse gas.

Further more, impurities in the fossil fuels such as sulphur react with oxygen on combustion releasing sulphur dioxide.

is crude oil non renewable/renewable

non renewable fuel as, although crude oil is being formed – it takes millions of years.

process of fractional distillation

1. Crude oil is heated into vapour

2. Temp gradient in column tower or / cooler at top of column & hotter at bottom

3. Vapour cools as it rises

4. Fractions/hydrocarbons or compounds condense at different points as they have different boiling points - depends on size/ Mr

5. Longer chained molecules condense at bottom and vice versa

label the diagram of which hydrocarbons are separated in fractional distillation

what is a fraction

a mixture of hydrocarbons with similar boiling points

Boiling point:

The temperature at which something boils

• Volatility:

How easily a substance vaporises into a gas

• Flammability

The ease at which a substance ignites

• Viscosity:

How easily a substance flows

difference of hydrocarbons properties at top compared to bottom

trends down the column

as chain length increases

higher boiling point

less volatile

more viscous

harder to ignite

cracking

Cracking is a thermal decomposition reaction of long chain alkanes to

• shorter alkanes

• alkenes

.• C-C bonds are broken in the alkanes

why crack hydrocarbons?

1. High demand for short chain alkanes short supply so cracking creates more useful short chain alkanes

2. Creates ethene, an extremely useful precursor to making polyethene – polymer synthesis

Catalytic cracking Conditions

Temp: 450oC

Pressure: 1-2atm

Zeolite catalyst

what is a zeolite catalyst

Zeolite catalysts are honeycombed structure to give enormous surface area, they consist of silicon dioxide an aluminium oxide.

Catalytic cracking:Products

The products are also a mixture of alkanes mainly of short chains, used in motor fuels.

These are branched alkanes,

cycloalkanes

and aromatic compounds.

Thermal cracking:Conditions

Temp: 900oC

Pressure: 70atm

Products:

The carbon chain can break in any number of places, and hydrogen may be produced. This means that there is a high proportion of alkenes created

describe the process of thermal cracking at a molecular level

a C-C covalent bond spilts equally (homolytic fission)

each carbon atom recieves one electron, forming free radicals

these radicals form a shorter alkane and an alkene

Free radicals

highly reactive species with and unpaired electron

Complete combustion + equation

When hydrocarbon fuels are burnt (combustion) in enough oxygen (COMPLETE COMBUSTION) we produce carbon dioxide and water.

FUEL + OXYGEN → CARBON DIOXDE + WATER

Incomplete combustion + equation

If there is insufficient oxygen present combustion is incomplete – CO2 is not produced!

FUEL + OXYGEN (insufficient) → CARBON MONOXIDE + WATER

what is carbon monoxide in terms of incomplete combustion

Carbon monoxide, CO, is a toxic gas and replaces O2 in your red blood cells causing death by asphyxiation (Oxygen deficiency).

Incomplete combustion 2

FUEL + OXYGEN (insufficient) → CARBON + WATER

Carbon/soot is a carbon particulate, which can cause asthma and cancer/global dimming

Greenhouse gases

molecules with polar bonds absorb IR radiation to make bonds vibrate

• H2O, CO2 and CH4 (C-H is slightly polar)

• greenhouse gases do not absorb the sun’s higher frequency UV/visible radiation

• O2 and N2 do NOT have polar bonds so are NOT greenhouse gases

Flue Gas Desulfurisation Why is it needed?

Many fossil fuels (especially coal) contain sulfur impurities.

When sulfur burns:

S + O_2 \rightarrow SO_2

Sulfur dioxide (SO₂) is released into the atmosphere.

Problems caused by SO₂

1. Acid Rain

SO₂ dissolves in rainwater:

SO_2 + H_2O \rightarrow H_2SO_3

Sulfurous acid can then be oxidised further to sulfuric acid

what does acid rain do

• Acidifies lakes and rivers

• Kills fish and aquatic life

• Damages forests

• Erodes limestone buildings

2. Human Health affect on SO2

SO₂ irritates:

• Lungs

• Airways

Can worsen:

• Asthma

• Bronchitis

How Flue Gas Desulfurisation Works

Flue gases from power stations are passed through a scrubber.

The scrubber contains alkaline substances which neutralise acidic SO₂

Method 1: Using Calcium Oxide flue gas desulfurisation

Calcium oxide is alkaline.

Reaction:

CaO + SO_2 \rightarrow CaSO_3

What happens?

• SO₂ is acidic

• CaO is basic

• Neutralisation occurs

Product:

Calcium sulfite (CaSO₃)

Sulfur dioxide gas is an acidic oxide. Reacts with a wet mixture of calcium carbonate solid calcium sulfite and Carbon dioxide

a. CaCO 3 + SO2→CaSO3+ CO2

Method 2: Using Calcium Carbonate flue gas desulfurisation

Reaction:

CaCO_3 + SO_2 \rightarrow CaSO_3 + CO_2

Why use calcium carbonate?

• Cheap

• Readily available as limestone

Product:

• Calcium sulfite

• Carbon dioxide

Calcium sulfite then further oxidises to calcium sulfate by atmospheric oxygen, which is then dumped/used.

b. 2CaSO3 (s) + O2 (g) → 2CaSO4 (s)

[gypsum – make plasterboard

table of how substances are formed- the harms - and how to remove

Explain why calcium oxide removes sulfur dioxide.

Calcium oxide is basic and sulfur dioxide is acidic. They react in a neutralisation reaction to form calcium sulfite.

Catalytic Converters

Purpose

Used in car exhaust systems.

Reduce harmful emissions from petrol engines

have a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh) – to give a large surface area.

Pollutants Removed Catalytic Converters Carbon Monoxide (CO)

Produced by incomplete combustion.

Dangerous because:

• Toxic gas

• Binds strongly to haemoglobin

• Reduces oxygen transport in blood

Pollutants Removed Catalytic Converters Nitrogen Oxides (NO and NO₂)

Produced at high temperatures in engines:

N_2 + O_2 \rightarrow 2NO

Problems:

• Acid rain

• Respiratory problems

• Photochemical smog

Pollutants Removed Catalytic ConvertersUnburned Hydrocarbons

Produced when fuel doesn’t burn completely.

Problems:

• Air pollution

• Smog formation

Reactions in Catalytic Converters Carbon Monoxide Oxidation

Carbon monoxide is oxidised:

2CO + O_2 \rightarrow 2CO_2

What happens?

• Toxic CO converted into less harmful CO₂

Why Are Catalysts Needed?

Without catalysts:

• Reactions would be too slow

• Pollutants would leave through the exhaust

Catalysts:

• Lower activation energy

• Increase reaction rate

• Are not used up

Reactions in Catalytic Converters Nitrogen Oxides Reduction

Example:

2NO \rightarrow N_2 + O_2

Or:

2CO + 2NO \rightarrow 2CO_2 + N_2

What happens?

• Harmful NO converted into harmless nitrogen gas

Reactions in Catalytic Converters Hydrocarbon Oxidation

Example:

C_xH_y + O_2 \rightarrow CO_2 + H_2O

What happens?

• Unburned fuel converted into carbon dioxide and water

free radical

a species with an unpaired electron.

This is energetically unfavourable, so free radicals are VERY reactive.

Free Radical – formation

Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission

Free Radical representation

A free radical is represented as follows with the unpaired electron explicitly shown on the molecule/atom: E.g. Cl● , H●, CH3●

Reactions of alkanes to form Haloalkanes

Alkanes are very unreactive and will generally not react with halogens (these are diatomic! e.g. Cl2 etc) to form haloalkanes.

However under certain conditions they will - when exposed to UV light.

• A hydrogen UV light CH4 + Cl2 CH3Cl + HCl

on the alkane is replaced • This is called substitution reaction . by a Cl/halogen atom. • Since we are using free radicals it is more specifically called a free radical substitution reaction

Why bother? free radical susbstitiution

Chlorofluorocarbons (CFCs) are used refrigerants. But can also cause environmental damage. CCl4 used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent

Problems with Free Radical Substitution

1. Further Substitution

Products can continue reacting.

Example:

CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

2. Mixture of Products

Difficult to separate.

Common 2-mark question.