Reactions of the hydrocarbons Flashcards
(36 cards)
What is combustion?
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.
What is complete combustion
When the supply of oxygen is plentiful, fuels undergo complete combustion. Blue flame, no soot.
Equation for complete combustion
Fuel + oxygen -> Carbon dioxide + water
the products are both gases however H2O condenses back into water quickly.
What is incomplete combustion
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.
What is the equation for incomplete combustion
Fuel + oxygen -> CO and/or C + water
Why do alkenes TEND to undergo incomplete combustion when compared to alkanes and alcohols?
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.
How to distinguish between Alkane and Alkenes using flame?
Burn a sample of each in air. The alkene flame is much yellower than then alkane flame.
Does complete combustion release more energy than incomplete combustion?
Yes.
What are the dangers of incomplete combustion of soot?
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.
What are the dangers of incomplete combustion of CO?
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.
Explain cracking
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.
What type of reaction is cracking?
Thermal decomposition
What is needed for cracking?
A high temperature, and presence of a catalyst
What are the products of cracking used for?
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.
What are the two types of cracking?
Thermal cracking - high temperatures and high pressure
Catalytic cracking - lower temp than thermal, uses catalyst. Often heated to speed up reactions.
Why use catalytic cracking instead of thermal cracking?
Thermal cracking produces a high proportion of alkenes which isn’t desirable.
What catalyst is used to produce alkanes of chain lengths 5-10?
ZEOLITE
Explain fractional distillation
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.
How does the carbon chain length affect intermolecular forces?
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.
Describe the distillates of fractional distillation
Gases: 1-4 C. 20 C. More than 350°C. Used as bitumen for roads, asphalt, petroleum jelly.
Describe polymerisation
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.
What does polymerisation require?
High temps
High pressures
The presence of a catalyst.
Properties of polymers
- 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.
How are polymers expressed?
By using/drawing the smallest repeating unit = monomer - the individual alkenes.
