Hydrocarbons and cracking

• Saturated - there are singular bonds between carbon atoms
• Cracking is a reaction when a large hydrocarbon molecule is broken into small hydrocarbon molecules which are more useful
• The size of a hydrocarbon affects it’s properties
• Properties of hydrocarbons Include:

  • Boiling point
  • Viscosity
  • Flammability
• Viscosity is a measure of how sticky (or thick) a liquid is.
• Flammability is a measure of how easy it is to burn something.
• The differences in properties explain why different hydrocarbons have different uses.
• Shorter hydrocarbons have low boiling points, low viscosity and high flammability.
• Longer hydrocarbons have a highlet boiling point, higher viscosity and low flammability.
• Hydrocarbons make good fuels
• The chemical term for burning is combustion
• When hydrocarbons combust they release energy making them useful as fuels
• During the combustion reaction of hydrocarbons, carbon and hydrogen are oxidised.
• If there is plenty of oxygen present in the combustion reaction, complete combustion occurs
• The products of complete combustion are carbon dioxide and water.
• Hydrocarbon + oxygen -> carbon dioxide + water
• Smaller hydrocarbons are more useful for fuel than long hydrocarbons.
• There are 2 different methods of cracking: catalytic cracking and steam cracking.
• Catalytic cracking involves passing the hydrocarbon over a hot catalyst at low pressure
• Steam cracking involves long hydrocarbons reacting with steam at high temperatures.
• The products of cracking are a mixture of smaller alkanes and alkenes
• Alkanes and alkenes are different types of hydrocarbon
• The number of carbon and hydrogen atoms are the same on both sides of the equation of cracking.
• Alkenes are more reactive than alkanes because they have the presents of a double bond
• Bromine water is orange.

  When alkenes react with bromine water, the solution changes from orange to colourless.
• Alkanes do not react with bromine in this way meaning this is a useful chemical test to tell the difference between alkanes and alkenes.
• Alkenes can be used to make polymers (plastics) and a range ofother chemical products.
• Larger hydrocarbons are less flammable than smaller hydrocarbons molecules. This is because larger hydrocarbons have stronger intermolecular forces which require more energy to break apart. This means they have higher boiling points and are less easy to burn.
• Bromine water changes from orange to colourless when reacted with an unsaturated compound such as an alkene. This is because the C=C double bond can 'open up' and the bromine atoms can form bonds to the carbon atoms in an addition reaction.
• There is greater demand for smaller hydrocarbons as fuels. This is because they are more flammable than longer hydrocarbons.
• Alkanes do not react with bromine water, so the colour remains an orange colour.
• A catalyst is used to speed up the cracking process.
• Cracking by heating to a very high temperature is an example of a thermal decomposition reaction
• Viscosity refers to the thickness of a liquid. Longer molecules have a greater viscosity than smaller molecules.
• A homologous series is a group of organic compound that have similar chemical properties, due to them having the same general formula/functional group
• What is the general formula for alkanes?

  CnH2n+2.
• An alkane only has single covalent bonds
• Increasing the chain length (number of carbons) of alkanes leads to a higher boiling point because there are more bonds needed to break.
• What is the word equation for complete combustion of a hydrocarbon?

  Hydrocarbon + Oxygen -> Carbon Dioxide + Water
• Combustion is an exothermic reaction