Reactions of the hydrocarbons

Question 1

What is combustion?

Answer 1

The chemical reaction between a fuel and oxygen, releasing heat energy. All hydrocarbons are fuels that burn. When fuel burns it releases energy stored in the chemical bonds in the fuel, so it is an exothermic reaction because heat is produced. Light and heat are released.

Question 2

What is complete combustion

Answer 2

When the supply of oxygen is plentiful, fuels undergo complete combustion. Blue flame, no soot.

Question 3

Equation for complete combustion

Answer 3

Fuel + oxygen -> Carbon dioxide + water

the products are both gases however H2O condenses back into water quickly.

Question 4

What is incomplete combustion

Answer 4

When fuels burn without a plentiful supply of oxygen. Yellow flame. This is because some C will not be able to react with O atoms as H atoms will combine chemically with O atoms first, and will be released as gaseous C. Hot C atoms glow with a yellow light. Once they cool the C atoms will settle on surfaces as a black layer of soot.

Question 5

What is the equation for incomplete combustion

Answer 5

Fuel + oxygen -> CO and/or C + water

Question 6

Why do alkenes TEND to undergo incomplete combustion when compared to alkanes and alcohols?

Answer 6

Because alkenes have a higher ratio of C atoms compared to H atoms. Any available O2 is used to form water and the remaining oxygen has a high number of C atoms to react with. Some C atoms will react to form CO. Once the available O atoms have reacted, the remaining C atoms remain unbonded and appear as soot.

Question 7

How to distinguish between Alkane and Alkenes using flame?

Answer 7

Burn a sample of each in air. The alkene flame is much yellower than then alkane flame.

Question 8

Does complete combustion release more energy than incomplete combustion?

Answer 8

Yes.

Question 9

What are the dangers of incomplete combustion of soot?

Answer 9

carbon particles can affect the lungs if inhaled. They may cause respiratory problems, including asthma and even lung cancer. They can even get into the bloodstream and cause blockages in the arteries, leading to heart disease.

Question 10

What are the dangers of incomplete combustion of CO?

Answer 10

CO is colourless and odorless and very toxic because it competes with oxygen for the haemoglobin in the bloodstream.

When O2 is inhaled and enters the bloodstream it binds with haemoglobin to form OXYHAEMOGLOBIN. The oxyhaemoglobin releases oxygen when it arrives at the cells. When CO is inhaled and enters the bloodstream it also binds with the haemoglobin and forms CARBOXYHAEMOGLOBIN. Haemoglobin does not release CO easily. As more CO is inhaled, more haemoglobin molecules turn into carboxyhaemoglobin. This causes a lack of oxygen in the cells and causes body tissues and cells to die, ultimately leading to the death of the victim.

Symptoms of CO poisoning include headaches, dizziness, convulsions, respiratory arrest, unconsciousness and death.

Question 11

Explain cracking

Answer 11

Crude oil contains a mixture of hydrocarbons of different chain lengths. Long chain alkanes do not make good fuels. They do not flow easily (viscous) and are difficult to ignite. In order to produce more efficient and easier-to-use fuels, the long chain alkanes are cracked into small molecules.

Cracking is the process of producing smaller chain alkanes and alkenes from longer chain alkanes.

Shorter chain alkanes flow much easier and ignites easier due to higher volatility and lower boiling point.

Question 12

What type of reaction is cracking?

Answer 12

Thermal decomposition

Question 13

What is needed for cracking?

Answer 13

A high temperature, and presence of a catalyst

Question 14

What are the products of cracking used for?

Answer 14

The smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture.

The alkanes formed = fuels
Alkenes for plastic manufacture

eg after the cracking of hexane, butane can be used as fuel and ethene is used in the production of plastic polymers.

Question 15

What are the two types of cracking?

Answer 15

Thermal cracking - high temperatures and high pressure

Catalytic cracking - lower temp than thermal, uses catalyst. Often heated to speed up reactions.

Question 16

Why use catalytic cracking instead of thermal cracking?

Answer 16

Thermal cracking produces a high proportion of alkenes which isn’t desirable.

Question 17

What catalyst is used to produce alkanes of chain lengths 5-10?

Answer 17

ZEOLITE

Question 18

Explain fractional distillation

Answer 18

Crude oil is a mixture of hydrocarbons of various carbon-chain lengths. Different hydrocarbons have different boiling points. Short chain hydrocarbons have low boiling points, whereas longer chain hydrocarbons have higher boiling points.

So this means that crude oil can be separated into useful parts (fractions) that contain hydrocarbons of similar chain lengths.

Hot crude oil is added to the bottom of the fractional distillation column. A temp gradient exists within the column, so it’s hotter at the bottom than at the top. As the vapours pass up the tower, they are progressively cooled.

Hydrocarbons with low boiling points evaporate and rise to the top of the column where they are collected as a gas.

Hydrocarbons with higher boiling points condense at various layers throughout the column and are collected as liquids. Those with high really boiling points are collected towards the very bottom of the column.

The fractions can then be separated further using more specific distillation methods.

The smallest molecules, such as methane - butane remain as gases and are removed at the top of the tower.

Question 19

How does the carbon chain length affect intermolecular forces?

Answer 19

The shorter the chain, the weaker the intermolecular forces, meaning that less energy is required to overcome these forces, resulting in a lower boiling point.

Question 20

Describe the distillates of fractional distillation

Answer 20

Gases: 1-4 C. 20 C. More than 350°C. Used as bitumen for roads, asphalt, petroleum jelly.

Question 21

Describe polymerisation

Answer 21

Unsaturated hydrocarbons such as the alkenes are generally not used as fuels because their double-bond means that they tend to undergo incomplete combustion. However, the double bond is useful because it can be broken, allowing many small alkenes to be joined together to make extremely long molecules called polymers.

Question 22

What does polymerisation require?

Answer 22

High temps
High pressures
The presence of a catalyst.

Question 23

Properties of polymers

Answer 23

• Solid at room temp because hey are long chain hydrocarbons.
• Molecules form strong chemically resistant fibres because the carbon chain length is bonded with strong covalent bonds.

The longer the chain length, the stronger the polymer.
Straight chains can pack together closer than chains with branches so are stronger.
Sometimes side groups attract each other or bond together, making the polymer together.

Polymers are tough substances and are easy to process to form sheets, as well as thread. Since these polymers are easily moulded, they are described as plastics.

Question 24

How are polymers expressed?

Answer 24

By using/drawing the smallest repeating unit = monomer - the individual alkenes.

Question 25

What do you put outside of the bracket?

Answer 25

n

Question 26

Describe polythene

Answer 26

Uses: plastic bags, pond liners, water pipes, wire and cable insulation, moisture barrier

Flexible
Strong
Chemically inert
Can be transparent
Waterproof - insoluble

HDPE is produced at low pressure (+ heat and catalyst) and produces long linear straight chains. Strong intermolecular forces = high melting/point point
Straight chains can pack tightly together so it’s strong
Used for pipes, margarine tubs, rubbish bins and plastic bottles.

LDPE is produced under high pressure (+ heat/catalyst). Produces chains with branches. Can’t pack together as tightly, weaker intermolecular forces.
Low melting/boiling point. Flexible
Gladwrap and soft tubing.

Both are chemically unreactive because all valence e- are use in single covalent bonds. NOT biodegradable as the extremely strong covalent bonds between the C atoms require a great deal of energy to break. Will degrade eventually - may take thousands of years.

Question 27

Describe polypropylene

Answer 27

Uses: rope, chairs, textiles, reusable containers, plastic hinges and mouldings.

Flexible
Tough
Relatively lightweight
Chemically inert
Heat resistant
Waterproof - insoluble 
Does not promote growth of bacteria.  eg mats at swimming pools and medical equipment. 
Excellent resistance to concentrated acids, alcohols, bases and mineral oils. 
Can be moulded or extruded into a wide range of shapes with moderate heating
Thermal insulator
Less dense than water
Recyclable

If packed in a regular fashion, can fit closely together. Have strong intermolecular forces. High melting points

Close packing = high tensile strength - hard to snap/break

Food storage - chemically inert.

Question 28

How to turn liquid polymer into solid or gel?

Answer 28

By cross-linking the chains as cross-links lose some of their ability to move as individual polymer chains.

Question 29

What is fermentation?

Answer 29

An enzyme-controlled chemical reaction in which bacteria and fungi (esp yeasts) break down carbohydrates (eg glucose) into ethanol, CO2 and heat.

Question 30

Why are alcohols important?

Answer 30

They are used to make alcoholic drinks, to make solvents (esp for removing paint), and as fuels for cars

Question 31

Conditions for fermentation

Answer 31

• catalyst (bacteria/fungi) eg yeast
• anaerobic (without oxygen)
• temp of 30-40°C. Too cold yeast won’t metabolise. Too hot yeast die.
• too much alcohol kills the yeast.

Question 32

Equation for fermentation

Answer 32

Sugar -> alcohol + carbon dioxide

C6H12O6 -> (enzyme in yeast) 2C2H5OH + 2CO2

The CO2 makes the alcohol bubbly and in bread causes it to rise.

When alcohol is produced, it needs to be separated from the water. Alcohol has a lower boiling point and can be evaporated off and collected.

Question 33

Alkanes have higher boiling points than alkenes

Answer 33

Due to weaker intermolecular forces.

Question 34

Equations for incomplete combustion of methanol

Answer 34

CH3OH + O2 -> CO + 2H2O

2CH3OH + O2 -> 2C + 4H2O

Question 35

Uses of methanol and ethanol

Answer 35

• both are excellent fuels. Both evaporate easily (high volatility) - allows complete combustion to readily occur (known as clean burning fuels)
• used as starter chemicals to make many other carbon-containing compounds
• found as components of methylated spirits, a very useful solvent and fuel. 90% ethanol and 9.5% methanol
• ethanol is the alcohol in alcoholic drinks. In moderation, it has no lasting toxic effects on the human system. In excess, it’s a poison.
• methanol is highly toxic in humans and can cause blindness.

Question 36

Describe the synthesis of methanol from natural gas

Answer 36

The Motunui Synthetic Fuels Plant was the world’s finest commercial production facility converting natural gas to synthetic fuels.

The plant takes natural gas from both the offshore Maui and the onshore Kapuni gas fields near Taranaki.

The natural gas (mainly methane) is converted into synthetic gas (syngas) consisting of CO, CO2 and H2. This is accomplished in the PRESENCE OF STEAM at HIGH TEMPS (800°C), high pressure (40 atm) and IN THE PRESENCE OF A CATALYST.
Steam reforming (to make syngas, using reaction conditions of a nickel catalyst and 800°C)  
CH4 + H2O -> CO + 3H2

Water-gas shift reaction ( to remove some CO):
CO + H2O -> CO2 + H2

The syngas is then passed over another catalyst at a lower temp to form methanol, hydrogen and water. This reaction produces heat (exothermic)
Synthesis
2H2 + CO -> CH3OH

The overall reaction:
CO2 + CO + 5H2 -> 2CH3OH + H2O + heat