ALKANES

What are alkanes?

Saturated hydrocarbons with only carbon and hydrogen, containing no double bonds.

What is the general formula for alkanes?

CnH2n+2

What is fractional distillation?

A process to separate hydrocarbons in crude oil based on different boiling points.

Describe the process of fractional distillation.

Crude oil is vaporised using a furnace to about 350 degrees, enters a column, and rises through trays, with longest hydrocarbons condensing at the bottom.

Function of the column

The column has a temperature gradient and is cooler at the top. As the vapour rises, parts of the mixture condense at different temperatures, as there are different chain lengths and different boiling points.

What are some products of fractional distillation?

Lubricating oil, paraffin wax, fuel oil, diesel, kerosene, gasoline.

Do all hydrocarbons consense?

No the shortest hydrocarbons won’t and so come off as a gas at the top

What is cracking in the context of alkanes?

A process used to break larger chain alkanes into shorter, more valuable hydrocarbons.

What are the two types of cracking?

Thermal and catalytic

What is thermal cracking?

A high temperature of 1000 degrees and pressure of 70atm process used to produce mainly alkenes.

What is catalytic cracking?

A process using lower temperatures of 450 degrees and a zeolite catalyst to produce mainly aromatic hydrocarbons.

What is an aromatic hydrocarbon?

A compound that contains a benzene ring with a delocalized electron system.

What are cycloalkanes?

Hydrocarbons that are cyclic in structure and have the same general formula as alkenes with a double bond, they are functional group isomers of alkenes

Where are alkanes found

In crude oil, as it is a mixture of different length hydrocarbons

What are alkenes used to make

Polymers like plastics, such as propane and poly(propane)

What does the zeolite catalyst do

It lowers the temperature needed for cracking to occur, and therefore lowers costs and speeds up the process

What is produced during the complete combustion of alkanes?

Carbon dioxide (CO₂) and water (H₂O).

Why are alkanes good fuels

Because people they burn readily to produce large amounts of energy

What is the combustion of alkanes used for

To power vehicles and make electricity

What is a consequence of incomplete combustion of alkanes?

Produces carbon monoxide (CO) and soot.

What to do when writing complete combustion equations

Balance carbons, hydrogens, and then oxygen

Why is carbon monoxide considered poisonous?

It binds to hemoglobin in the blood and prevents oxygen bonding.

What does soot cause

Breathing problems and clogs up engines

What can remove carbon monoxide from combustion gases?

Catalytic converters.

Where does ozone occur

In the lowest levels of the atmosphere

What creates photochemical smog (1)

When solid carbon particulates and ozone mix

What does photochemical smog do

Harms the respiratory system in animals and damages plants and is toxic to humans

What is the greenhouse effect?

The process by which carbon dioxide absorbs infrared radiation from the sun and emits some of it back to Earth, contributing to global warming.

When are oxides of nitrogen made

When nitrogen and oxygen found in the air, combines under high pressure and temperature such as in car engines

What does acid rain do

Damages plants, kills fish and erodes building

What’s type of gas is sulfur dioxide

Acidic

How is sulfur dioxide formed

When fossil fuels containing sulfur based impurities are burnt and then the sulfur reacts with the oxygen, producing sulfur dioxide

What does wet scrubbing involve

Dissolving calcium carbonate or oxide in water and spraying in acidic sulfur dioxide gas

What is the process in initiation called

Photochemical reactions where the bond breaks, producing two radicals

What will propagation do

It will keep going until termination

What causes photochemical smog? (2)

The reaction between ozone, hydrocarbons, and nitrogen dioxide in sunlight.

How do catalytic converters help with air pollution?

They reduce the amount of unburnt hydrocarbons and nitrogen oxides in the atmosphere.

What is acid rain and how is it formed?

Acid rain is caused by sulfur dioxide (SO₂) and nitrogen oxides reacting with water in the atmosphere to form sulfuric and nitric acids.

What process can be used to remove SO₂ from flue gases?

Wet scrubbing using an alkali to neutralize sulfur dioxide.

What are the three main stages of free radical chain reactions?

Initiation, Propagation, and Termination.

What occurs during the initiation stage of a free radical chain reaction?

Radicals are produced, usually using visible or ultraviolet light.

What happens during the propagation stage of a free radical chain reaction?

A radical reacts with a non-radical molecule, creating new radicals that continue the chain reaction.

What marks the termination stage of a free radical chain reaction?

When two radicals react to form a non-radical molecule, ending the reaction.

What process starts the free radical substitution in making halogenoalkanes?

Initiation occurs via photodissociation when UV light breaks the Cl-Cl bond.

What are the products of initiation in the free radical substitution process?

Two highly reactive radicals are produced.

What is the result of the first propagation step in making chloromethane?

A methyl radical (CH3•) is formed after the radical reacts with a methane molecule.

What happens during the second propagation step in the synthesis of chloromethane?

The methyl radical (CH3•) reacts with a Cl2 molecule to form chloromethane (CH3Cl) and another radical.

What happens if there is an excess of Cl2 during the reaction?

Multiple substitutions can occur, leading to the formation of di, tri, or tetra haloalkanes.

What is a key factor influencing the product when there is limited Cl2 available?

Mainly single substitution products will be formed under limited Cl2 conditions.

What to remember about free radical substitution mechanisms

The radical in the first step is always the reactant in the second step