Organic chemistry (chemistry only)

Alkenes are one example of a homologous series. They:

• have the general formula of C_nH_2n
  this means that for every 1 carbon atom in an alkane, there are two times the amount of hydrogens
  
• differ by CH₂ in molecular formulae from neighbouring compounds
  
  increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2
  
• show a gradual variation in physical properties
  
  boiling points increase with increased carbon chain length
  viscosity increases with increased carbon chain length
  
• have similar chemical properties
  

Because alkenes contain at least one double bond between carbon atoms, we describe them as unsaturated compounds. Think of when a sponge cannot hold any more water, it is "full". A saturated molecule can't fit any more single bonds into it. Alkenes can fit more single bonds in (once the double bonds are broken).

Addition reactions

The functional group (double carbon bond, C=C) allows alkenes to undergo addition reactions. The reaction is an 'addition' reaction because one molecule combines with another molecule, forming one larger molecule and no other products.

The test for alkenes makes use of the addition reaction, and involves adding bromine water to a sample of hydrocarbon. If an alkane is present the solution stays orange-brown, but an alkene will turn the solution colourless. This is because the bromine is added across the carbon-carbon double bond.

Complete combustion

The complete combustion of alkanes and alkenes involves the oxidation of both the carbon and hydrogen atoms in hydrocarbons, producing carbon dioxide and water. For example:

ethane + oxygen → carbon dioxide + water

2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l)

ethene + oxygen → carbon dioxide + water

C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)

Alcohols are one example of a homologous series. They:

• have the general formula of C_nH_2n+1OH
  this means that for every 1 carbon atom in an alcohol, there are two times the amount of hydrogens... plus one more, and an OH group
  
• differ by CH₂ in molecular formulae from neighbouring compounds
  
  increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2
  
• show a gradual variation in physical properties
  
  boiling points increase with increased carbon chain length
  viscosity increases with increased carbon chain length
  
• have similar chemical properties
  

Alcohols have the functional group of -OH (hydroxyl) which gives rise to the ending of alcohol. It is responsible for the general reactions that all alcohols can do.

The longer the carbon chain of an alcohol, the more energy the molecule stores. When burned, this energy is released in the form of heat and can be measured using a calorimeter.

Carboxylic acids are one example of a homologous series. They:

• have the general formula of C_nH_2n+1COOH
  this means that for every 1 carbon atom in a carboxylic acid, there are two times the amount of hydrogens... plus one more, and a COOH group
  
• differ by CH₂ in molecular formulae from neighbouring compounds
  
  increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2
  
• show a gradual variation in physical properties
  
  boiling points increase with increased carbon chain length
  viscosity increases with increased carbon chain length
  
• have similar chemical properties
  

Carboxylic acids have the functional group of -COOH (carboxyl) which gives rise to the ending of carboxylic acid. It is responsible for the general reactions that all carboxylic acids can do.

Alcohols

• colourless liquids that dissolve in water to form neutral solutions
  
• can be dehydrated to form alkenes (produces a water molecule)
  
• will react with the alkali metals to produce hydrogen and a metal hydroxide (like sodium would with water)
  
• burn in air to produce carbon dioxide and water (complete combustion)
  
• can be oxidised to make a carboxylic acid in the presence of an oxidising agent (shown as [O])
  

Carboxylic acids

• dissolve in water to form acidic solutions (pH values less than 7)
  
• react with metals to form a salt and hydrogen
  
• react with bases to form a salt and water
  
• react with carbonates to form a salt, water and carbon dioxide
  

A polymer is a substance of high average relative formula mass, made up of small repeating units called monomers.

Addition Polymers

Ethene molecules can combine together in a polymerisation reaction to make poly(ethene). This type of reaction is called addition polymerisation, as we are adding lots of the same monomer together to make one product. The C=C double bond in ethene breaks open to allow ethene molecules to join together.

We don't waste our time drawing out a whole polymer chain, as it contains far too many atoms. Instead, we show the structure of the repeating unit. 

When we draw an equation to show an addition polymerisation, it is important that we write n next to both the monomer and the polymer to show large numbers of each are made. We must also draw square brackets around the polymer structure.

Biological polymers are made naturally by living organisms.

DNA is a polymer made from four different monomers called nucleotides. Most DNA molecules are made from two polymer chains. These polymer chains join together in the shape of a double helix. Each nucleotide is made of a sugar molecule, a base (A, C, T or G), and a phosphate group.

Carbohydrates include substances such as sugar, starch, and cellulose. These molecules are made from carbon, hydrogen and oxygen. Simple sugars, like glucose, are called monosaccharides. Glucose monomers can join together in long chains to make a polysaccharide called starch.​

Proteins are another example of polymers. There are 21 different amino acids that can be arranged to make proteins we need to survive. Many proteins are enzymes, and/or are essential to our biological functions. 

Fuel cells will produce a voltage continuously, provided they have a constant supply of fuel and oxygen (from the air).

Hydrogen-oxygen fuel cells

In this type of fuel cell, hydrogen and oxygen are used to produce a voltage. The only product from this reaction is water.

A hydrogen-oxygen fuel cell and electric motor are much quieter, and need less maintenance, than a petrol or diesel engine, but the hydrogen still needs to be stored in a container - like a tank.

hydrogen + oxygen → water
2H₂ + O₂ → 2H₂O

Hydrogen, diesel and petrol are all highly flammable fuels. Fuel cells have their advantages and disadvantages depending on the use.

Fuel cells in spacecraft

Fuel cells in vehicles