Alkanes

Crude oil

-Mixture composed mainly of alkane hydrocarbons

These hydrocarbons vary in molecular size and boiling points, which allows them to be separated into useful fractions, What process is used?

Fractional distillation

The Process of Fractional Distillation

1. Heating the Crude Oil:

-Crude oil is heated to approximately 350°C. Most of the hydrocarbons are vaporised at this temperature, except for the largest hydrocarbons, which remain in liquid form due to their high boiling points.

2. Entering the Fractionating Column:
   -The vaporised crude oil enters the bottom of a fractionating column.

-This column has a temperature gradient: it is hottest at the bottom and gradually cools as you move up the column.

3. Separation by Boiling Point
   Larger hydrocarbons:
   -with higher boiling points condense back into liquids near the bottom of the column, where it is hotter.

-Smaller hydrocarbons with lower boiling points rise higher in the column and condense near the top, where the temperature is cooler.

4. Formation of Fractions:

-As the hydrocarbons condense at different levels of the column, they are separated into fractions.

-Each fraction contains hydrocarbons with similar boiling points.

5. Hydrocarbons that Don't Condense:
   -The hydrocarbons with the lowest boiling points (e.g., methane, ethane) remain as gases and exit the top of the column without condensing.

Why Is Fractional Distillation Important

-Fractional distillation is crucial because it allows crude oil, a complex mixture of hydrocarbons, to be separated into components that are useful for various applications.

-Without this process, crude oil would be far less valuable as it would be impossible to isolate the specific hydrocarbons needed for products like gasoline, diesel, and lubricants.

What is Cracking?

Cracking is a process used to break the carbon-carbon (C−C) bonds in long-chain alkanes, converting them into shorter-chain alkanes and alkenes.

Why is cracking important?

This process is crucial because there is a higher demand for shorter hydrocarbons, which are more useful in industries such as fuel production and plastics.

Types of Cracking

-Thermal Cracking

-Catalytic Cracking

Thermal Cracking- Conditions, products and mechanism

-Conditions: High pressure and very high temperature (around 700-1200 K).

-Products: A high percentage of alkenes (e.g., ethene, which is used to produce polymers).

-Mechanism: Involves breaking the bonds in a random manner, resulting in a variety of smaller hydrocarbons.

Catalytic Cracking- Conditions, Products and Mechanism

-Conditions: Lower pressure, high temperature (around 720 K), and the presence of a zeolite catalyst.

-Products: Motor fuels (e.g., gasoline) and aromatic hydrocarbons.

-Mechanism: The catalyst speeds up the breaking of C−C bonds, focusing on producing branched alkanes and aromatic compounds, which are important in motor fuel production.

What is the economic reasons for cracking

High Demand for Short-Chain Hydrocarbons:

-Short-chain alkanes are better fuels because they are more volatile and burn more easily. For example, propane and butane are widely used in heating and cooking.

-Short-chain alkenes are more reactive than alkanes and are used as feedstock for producing polymers like plastics.

Imbalance in Supply:

-Fractional distillation of crude oil naturally produces a surplus of long-chain hydrocarbons and fewer short-chain hydrocarbons than are needed.

-Long-chain hydrocarbons are less useful and harder to ignite, making them less desirable for fuel.

What are alkanes used as?

Fuels

Why is alkanes used as fuels

This is because they undergo combustion reactions that release significant amounts of energy

Two types of combustion

complete and incomplete (this depends on the concentration of oxygen

Complete Combustion

-In the presence of excess oxygen, alkanes undergo complete combustion, producing carbon dioxide (CO2​) and water (H2O).

Why is complete combustion desirable?

-Complete combustion is desirable because it releases the maximum amount of energy and avoids the production of harmful by-products like carbon monoxide (CO)

Incomplete Combustion

When oxygen is limited, alkanes undergo incomplete combustion, leading to the production of carbon monoxide (CO) or carbon (soot) in addition to carbon dioxide and water.

Why is incomplete combustion less efficient?

-Carbon monoxide (CO): A colourless, odourless, and poisonous gas that can be fatal if inhaled in large quantities.

-Carbon (soot): Particles that can cause respiratory issues and dirty engine parts.

Why is combustion of alkanes bad?

It produces pollutants

What harmful pollutants are created? Why are they harmful?

-Carbon dioxide (CO2​):
A greenhouse gas that contributes to global warming.

-Carbon monoxide (CO):
Formed during incomplete combustion and is toxic when inhaled.

Unburnt hydrocarbons:

These can react with nitrogen oxides to form photochemical smog.

Nitrogen oxides (NOx​):

-Produced at the high temperatures in car engines when nitrogen and oxygen from the air react.

-These gases contribute to the formation of acid rain and smog.

Sulfur dioxide (SO2​):
-Formed when hydrocarbons containing sulfur are combusted.

-Sulfur dioxide reacts with water in the atmosphere, forming acid rain.

Catalytic Converters, why are they useful and explain how

-Catalytic converters are devices installed in cars to reduce harmful emissions.

-This is because they convert CO is converted to CO2:

2CO + O2 → 2CO2

-Unburnt hydrocarbons are converted to CO2 and H2O

-Nitrogen Oxides are reduced to nitrogen and oxygen:

2NO → N2 + O2

What is the structure of catalytic converters

They contain a ceramic honeycomb structure coated with a thin layer of catalyst metals such as platinum, palladium, or rhodium

What is the honeycomb structure importance

The honeycomb provides a large surface area for reactions.

Why is SO2 bad?

Acid rain

Flue-Gas Desulfurisation

-The waste gases pass through a scrubber containing an alkaline slurry of calcium oxide (CaO) or calcium carbonate (CaCO3​) mixed with water.

-The sulfur dioxide reacts with the calcium compounds to form calcium sulfate (CaSO4), a harmless salt:
So2 + CaO → CaSO3

Chlorination of alkanes mechanism