Hydrocarbons

A hydrocarbon is any compound that is only made up of carbon and hydrogen atoms.

Alkanes are the simplest type of hydrocarbon- they have all C-C single bonds. Their general formula is CnH2n+2. Alkanes are a homologous series: a group of organic compounds which react in a similar way. They are also saturated compounds: each carbon atom forms 4 single covalent bonds.

The first 4 alkanes are methane, ethane, propane and butane.

As the length of the hydrocarbon changes, so do the properties:

• The longer the hydrocarbon, the more viscous it is.
• The longer the chain, the higher the boiling point.
• The shorter the hydrocarbon, the more flammable it is.
  

hydrocarbon + oxygen --> carbon dioxide + water (+ energy)

During combustion, both carbon and hydrogen from the hydrocarbon are oxidised. Hydrocarbons are used as fuels due to the vast amount of energy they release when they combust completely.

Crude oil is a fossil fuel, formed from the remains of plants and animals. Crude oil is a mixture of lots of different hydrocarbons, most of which are alkanes. The different compounds in crude oil are separated by fractional distillation

How crude oil is separated:

• The oil is heated until most of it has turned into a gas. These gases enter a fractionating column and the remaining liquid is drained off.
  
• In the column, it is hottest at the bottom and coolest at the top.
• The longer hydrocarbons have high boiling points, therefore they condense near the bottom and drain off early on. The shorter hydrocarbons have lower boiling points, so they condense near the top and drain out much later on.

approximate hydrocarbon length- use

40- heavy fuel oil (heating oil, lubricating oil)

20- diesel 

15- kerosene (aeroplane fuel)

8- petrol

3- LPG (liquefied petroleum gas- mostly propane and butane).

The petrochemical industry uses some of the hydrocarbons from crude oil as a feedstock to make new compounds for use in polymers, solvents, lubricants and detergents.

Short-chain hydrocarbons are flammable, so are good fuels and are in high demand. However, long-chain hydrocarbons are viscous and form things such as tar, which isn't as useful.

Cracking is the process by which longer alkane molecules produced by fractional distillation are turned into smaller, more useful ones.

There are 2 main methods of cracking: catalytic cracking and steam cracking.

Catalytic cracking is where long-chain hydrocarbons are heated and vapourised. Then the vapour is passed over a hot powdered aluminium oxide catalyst. The long-chain molecules split apart on the catalyst's surface.

Steam crackingis where the hydrocarbons are vapourised, mixed with steam and then heated to a very high temperature.

Alkenes are hydrocarbons which have a double bond between 2 of the carbon atoms in their chain. The C=C double bond means that alkenes have 2 fewer hydrogen atoms than alkanes with the same amount of carbons. This means that alkenes are unsaturated.

The first 4 alkenes are ethene, pentene, butene and pentene. The general formula for alkenes is CnH2n. 

In a large amount of oxygen, alkenes combust completely to produce only water and carbon dioxide. However, when burned in air, alkenes tend to undergo incomplete combustion which also produces carbon monoxide, a poisonous gas. Incomplete combustion results in a smoky yellow flame and less energy being released compared to complete combustion.

A functional group is a group of atoms in a molecule that determines how that molecule typically reacts. All alkenes have the functional group 'C=C'.

Mostly, alkenes react by addition reactions. Here, the C=C double bond will open up to leave a single bond and a new atom is added to each carbon.

When hydrogen is reacted with an alkene, it is known as hydrogenation. When an alkene reacts with H2, it forms the equivalent, saturated alkane.

Alkenes will also react in addition reactions with halogens such as bromine (Br2), chlorine (Cl2) and iodine (I2). The C=C bonds open up and each one becomes bonded to a halogen atom. For example, bromine and ethene react to form dibromoethane. 

Therefore, when orange bromine water is added to a saturated compound such as an alkane, the water will remain orange. However, when added to an unsaturated compound such as an alkene, the bromine will react with the alkene and form a colourless dibromo- compound. 

When alkenes react with steam, water is added across the double bond, and an alcohol is formed. For example, when ethene reacts with water and is passed over a catalyst, it forms ethanol. After this reaction, the mixture is passed into a condenser. Because ethanol and water have a higher boiling point than ethene, they condense and any leftover ethene is recycled. Then the ethanol can be separated from the water by fractional distillation.

Alcohols have an '-OH' functional group, and end in '-ol'. The general formula for alcohols is CnH2n+1OH.  The first 4 alcohols are methanol, ethanol, propanol and butanol. 

Alcohols are flammable: they undergo complete combustion in air. Methanol, ethanol, propanol and butanol are all soluble in water. They also react with sodium, which produces hydrogen.

When alcohols are oxidised, it produces a carboxylic acid.

Methanol and ethanol are used as solvents in industry, as they can dissolve most things that water can dissolve, but also things water can't dissolve. The first four alcohols are also used as fuels, such as ethanol in spirit burners.

Ethanol can be made by reacting ethene with steam, but also by fermentation.

Fermentation is where an enzyme in yeast converts sugars into ethanol. This also produces carbon dioxide.

sugar --> ethanol + carbon dioxide

Fermentation works best at around 37 Celsius, in a slightly acidic solution and under anaerobic conditions.

Carboxylic Acids are a homologous series of compounds that all have the '-COOH' functional group. Their names end in -anoic acid. For example, methanoic acid has the formula: 'CHCOOH', and ethanoic acid has the formula: 'CH3COOH'.

Carboxylic acids react with carbonates to produce a salt, water and carbon dioxide, like any other acid. These salts end in -anoate. 

Carboxylic acids dissolve in water, and when they dissolve they release H+ ions, resulting in an acidic solution. But because they don't ionise completely, the solution is only weakly acidic. 

Esters are produced from carboxylic acids and alcohols, with an acid catalyst such as sulfuric acid.

They have the functional group '-COO-'.

alcohol + carboxylic acid --> ester + water

ethanoic acid + ethanol --> ethyl ethanoate + water