Hydrocarbons

10SCIE - Fire & Fuels

Finn Le Sueur

2024

Ngā Whāinga Ako

  • Recognise that a hydrocarbon contains only C and H atoms
  • Name and write the formula for the first six hydrocarbons
Term Description Example/Tauria
Atom/Molecule The smallest unit of an element One atom of H
Element Many of the same atoms together Hydrogen, Oxygen
Compound Two or more elements chemically joined together Water (\(H_{2}O\))

Hydrocarbon Formation

  1. Layers of dead organic matter settle on the seabed.
  2. Layers of sedimentary rock build up on top.
  3. The heat and pressure from these rocks, along with the absence of oxygen mean that oil and gas (fossil fuels) are formed over millions of years.

Crude Oil

Crude oil is what we get directly out of the ground and is made of a mixture of lots of different hydrocarbons (molecules made of only hydrogen and carbon).

Hydrocarbons

  • A hydrocarbon with 4 carbons is called butane
  • A hydrocarbon with 5 carbons is called pentane
  • A hydrocarbon with 6 carbons is called hexane
  • Can you guess what 7, 8, 9 and 10 are called?

Hydrocarbon Formula

  • Methane has 1 carbon and 4 hydrogens so it is called \(CH_{4}\)
  • Ethane has 2 carbons and 6 hydrogens so it is called \(C_{2}H_{6}\)
  • Propane has 3 carbons and 8 hydrogens so it is called \(C_{3}H_{8}\)
  • What is the pattern? Try write a general formula using n to represent the number of carbons..

Answer

\[\begin{aligned} C_{n}H_{2n+2} \end{aligned}\]

Calculating Hydrocarbons

\[\begin{aligned} C_{n}H_{2n+2} \end{aligned}\]

Using this formula you can calculate the formula for any hydrocarbon!

  1. Find the formula for the 17th hydrocarbon
  2. Find the formula for the 56th hydrocarbon
  3. Find the formula for the 117th hydrocarbon

Answers

  1. \(C_{17}H_{36}\)
  2. \(C_{56}H_{114}\)
  3. \(C_{117}H_{236}\)

Hydrocarbon Properties

A longer chain of carbons means:

  • Less ability to flow (higher viscosity)
  • Less flammable
  • Less volatile
  • Higher boiling point

Making Alkanes

IN PAIRS OR GROUPS, WRITE FORMULA AND MAKE MODELS FOR THESE ALKANES:

  1. An alkane with one carbon molecule
  2. An alkane with three carbon molecules
  3. An alkane with six carbon molecules

You may need to join groups to make the larger molecules.

Hands up for check each time!

Alkanes Formula Boiling point [°C] Melting point [°C] Density
Methane CH4 -162 -182 gas
Ethane C2H6 -89 -183 gas
Propane C3H8 -42 -188 gas
Butane C4H10 0 -138 gas
Pentane C5H12 36 -130 0.626 (liquid)
Alkanes Formula Boiling point [°C] Melting point [°C] Density
Hexane C6H14 69 -95 0.659 (liquid)
Octane C8H18 126 -57 0.703 (liquid)
Icosane C20H42 343 37 solid
Hexacontane C60H122 625 100 solid

Combustion of Hydrocarbons

Try and conmplete these word and symbol equations (refer to your notes):

Complete Combustion

\[\begin{aligned} methane + oxygen \rightarrow \hspace{1cm} + \hspace{1cm}\newline CH_{4} + \frac{1}{2}O_{2} \rightarrow \hspace{2cm} + \hspace{2cm} \end{aligned}\]

Incomplete Combustion

\[\begin{aligned} CH_{4} + \frac{1}{2}O_{2} \rightarrow \hspace{2cm} + \hspace{2cm} \end{aligned}\]