Topic 6 - OC Alkanes

Question 1

General Formula of Alkanes

Answer 1

CnH2n+2

Question 2

Saturated

Answer 2

Alkanes and cycloalkanes are saturated because they do not contain a double bond

Question 3

Where are alkane fuels obtained from?

Answer 3

Fractional Distillation, Cracking and Reforming

Question 4

What is Fractional distillation of Alkanes

Answer 4

Crude oil is a complex mixture of hydrocarbons which can be seperated into different fractions. Crude oil is first heated in a furnace to convert it into vapour which is passed into a distillation columb via the bottom. There is a temperature gradient along the column and therefore as the vapor passes through different fractions of molecules will condense at different heights depending on their melting points
–> The bottom of the column is hotter and therefore larger molecules with longer chains and higher melting points will condense
–> The top of the column is cooler and therefore smaller molecules with shorter chains and lower melting points will condense

Question 5

What is Cracking of Alkanes

Answer 5

There is a surplus of longer chain hydrocarbons because the world has fewer uses for them. However, demand for shorter chain hydrocarbons is high because they are better fuels and can be used to make other substances such as polymers. There are not enough to satisfy demand and therefore we convert longer chain hydrocarbons into shorter chains via the process of cracking

–> Cracking is completed by passing hydrocarbons throuh a heated catalyst (usually zeolite) which causes larger molecules to break into smaller ones

Question 6

What is Reforming of Alkanes

Answer 6

Converts striaght chain alkanes into branched chain alkanes and cyclic hydrocarbons by heating with a catalyst (usually platinum)
- because straight chained hydrocarbons burn less efficiently than branched or those with rings (cyclic compound)

Question 7

What are the products of complete Combustion of Alkane Fuels

Answer 7

Produces water and carbon dioxide

Question 8

Incomplete combustion

Answer 8

When there is insufficient oxygen present, all the hydrogen atoms in the alkane molecule are converted into water, but some of the carbon atoms can form gaseous carbon monoxide ot solid carbon

Question 9

Carbon formed as a byproduct of incomplete combustion

Answer 9

Can often be seen as smoke in the air or soot on the burner
- tiny particles of carbon in the air can be harmful

Question 10

Carbon monoxide formed as a byproduct of incomplete combustion

Answer 10

Toxic gas that can cause death. Prevents the transport of oxygen around the body since it binds to haemoglobin and takes the place of oxygen. It is colourless and odourless

Question 11

Unburned hydrocarbons form as a byproduct of incomplete combustion

Answer 11

Hydrocarbon does not burn at all and are released into the atmopshere unchanged

Question 12

Oxides of sulfur formed as a byproduct of incomplete combustion

Answer 12

Some of the molecules in crude oil contain atoms of sulfur and are not removed via fractional distillation, cracking or reforming
- during the combustion of alkanes, these atoms of sulfur form sulfur dioxide which reacts with the atmopshere to form sulfur trioxide
- These gases are both acidic oxides and therefore when they dissolve in the atmosphere, they form sulfurous acid and sulfuric acid
- These acids contribute to the formation of acid rain which is responsible for damage to aquatic life in lakes and rives, and damage to crops and forests

Question 13

Oxides of nitrogen formed as a byproduct of incomplete combustion

Answer 13

Few molecules contain atoms of nitrogen - however at high temperatures, like in an engine, nitrgoen atoms can react with oxygen in the air to form oxides of nirtogen
- Nitrogen dioxide is acidic and can dissolve in water ion the atmopshere, forming nitrous acid and nitric acid

Question 14

Catalytic converters

Answer 14

Designed to reduce pollution emissions - can monitor level of oxygen in exhaust stream via an oxygen sensor

Contains both a reduction and oxidation catalyst - both consist of a ceramic structure coated with a metal catalyst - usually platinum, rhodium and palladium - usually formed in a honeycomb arranged to allow for maximum surface area

reduction catalyst causes NO bond to break to form an 02 molecule
oxidation catalyst oxidises the unburnt hydrocarbons and CO

Question 15

Why do we need alternative fuels?

Answer 15

Relying on combustion of fossil fuels for energy causes many cocnerns. It causes pollution, global warming and climate change, while also reducing natural resources.

Question 16

Carbon neutrality

Answer 16

Carbon neutrality means having a balance between emitting carbon and absorbing carbon from the atmosphere in carbon sinks.

Question 17

Biofuels

Answer 17

Biofuels are clsoer to being carbon neutral than fossil fuels - however they are not compeltely carbon neutral because plants have to be harvested, transported and processed in a factory which uses up more carbon dioxide than it releases

Question 18

Biodiesel vs Bioalcohol

Answer 18

Biodiesels can be made from vegetable oils such as sunflower or rapeseed

Bioalcohol (most common one is bioethanol) - can be produced via the fermentation of sugars to produce alcoholic drinks - ethanol then has to be seperated from large amounts of water which requires lots of energy
- Now bioethanol refers to ethanol produced via bacteria rather than enzymes.

Question 19

Natural Gas vs Biodiesel and bioethanol

Answer 19

Natural Gas
1. no land is needed because it comes from undergground sources, however may destroy land
2. very high yield
3. exploration and drilling costs are very high, processing and transport costs are low
4. not carbon neutral

Biodiesel
1. A lot of land is needed - may replace land which would otherwise be used to grow food
2. Low but gradually increasing yield
3. No exploration or drilling costs
4. Closer to being carbon neutral

Question 20

Substitution reaction of alkanes with Halogens

Alkanes are non polar and very stable so do not undergo many reactions,

no curly arrows

Answer 20

One atom or a group is replaced by another atom of group

1. Initiation - one molecule becomes two radicals
   When alkanes and halogens (diatomic molecules) are mixed and heated with UV light. the mixture will get very energetic and The halogen molecules will turn into individual halogeno atoms (this bond breaking is known as homolytic fission)
   Cl2 –> CL + Cl*
2. Propagation - a molecule and a radical become a different radical and molecule and there are two reactions in this step
   Chlorine radicals are very reactive species and when they collide with the hydrocarbon they react by breaking the bonds between the C-H and replace the H
3. Termination - two radicals become one molecule

Question 21

Radical

Answer 21

• A species with an unpaired electron and is represented in mechanisms by a single dot
• Formed by homolytic fission of a covalent bond (eahc of the bonding electron leaves with one species)

Question 22

Understand the limitations of the use of radical substitution reactions in the synthesis of organic molecules, in terms of further substitution reactions and the formation of a mixture of products

Answer 22

Since the collisions are uncontrollable you can’t use this method to make one particular product.

Exam questions often ask you to draw the first two of these propagation reactions to form CH3Cl.

But once that has been successfully done more collisions are inevitable because the remaining Chlorine radicals can’t be stopped from colliding with this desired product.

So further substitutions will happen and dichloro-, trichloro and tetrachloromethane will form, creating a mixture of products.

This would mean that messy and expensive separation would be required.