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Unit 2 —Nature’s Chemistry Revision Notes
Alkanes
General formula: CnH2n+
Combustion: hydrocarbon + oxygen carbon dioxide + water, e.g. propane + oxygen^ ^ carbon dioxide + water
C 3 H 8 + O 2 CO 2 + H 2 O
C 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O
Homologous series : a group of compounds with the same general formula and similar chemical
properties that show a gradual change in physical properties.
Examples include the alkanes , alkenes , cycloalkanes , alkanols and alkanoic acids.
Name Structural formula
Shortened structural formula
Molecular formula
methane CH 4 CH 4
ethane CH 3 CH 3 C 2 H 6
propane CH 3 CH 2 CH 3 C 3 H 8
butane CH 3 CH 2 CH 2 CH 3 C 4 H 10
pentane CH 3 CH 2 CH 2 CH 2 CH 3 C 5 H 12
hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 C 6 H 14
heptane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 C 7 H 16
octane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 C 8 H 18
Alkenes
General formula: CnH2n
Name Structural formula
Shortened structural formula
Molecular formula
ethene CH 2 =CH 2 C 2 H 4
propene CH 3 CH=CH 2 C 3 H 8
butene CH 3 CH 2 CH=CH 2 C 4 H 8
pentene CH 3 CH 2 CH 2 CH=CH 2 C 5 H 10
hexene CH 3 CH 2 CH 2 CH 2 CH=CH 2 C 6 H 12
heptene CH 3 CH 2 CH 2 CH 2 CH 2 CH=CH 2 C 7 H 14
octene CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH=CH 2 C 8 H 16
Alkenes are unsaturated hydrocarbons and can undergo addition reactions.
In this reaction bromine decolourises rapidly. This reaction is used as a test for unsaturation.
Hydrogenation - the addition of hydrogen, can convert alkenes into alkanes.
Systematic names
Structural formulae can be drawn and molecular formulae written from systematic names and vice versa.
Rules
- Identify and name the longest chain of carbon atoms.
- Identify the branch and name it according to the number of carbon atoms in the branch.
- Number the branch so that it has the lowest possible number.
3 - ethylhexane
3 - methylheptane
- Alkenes are named by numbering the carbon atoms from the end that gives the carbon of the double bond the lowest number.
- Where there are branches, the double bond takes priority over the branch.
but- 1 - ene but- 2 - ene
CH 3 CH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 3
CH 3 CH 2 CH(CH 3 )CH 2 CH 2 CH 2 CH 3
CH=CHCH 2 CH 3 CH 3 CH=CHCH 3
4 - methylpent- 2 - ene
CH 3 CH(CH 3 )CH=CHCH 3
Alcohols
Functional group: hydroxyl group (-OH)
Alkanols: homologous series of alcohols
General formula: CnH2n+1OH
Name Structural formula
Shortened structural formula
Molecular formula
methanol CH 3 OH CH 3 OH
ethanol CH 3 CH 2 OH C 2 H 5 OH
propanol CH 3 CH 2 CH 2 OH C 3 H 7 OH
butanol CH 3 CH 2 CH 2 CH 2 OH C 4 H 9 OH
pentanol CH 3 CH 2 CH 2 CH 2 CH 2 OH C 5 H 11 OH
hexanol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH C 6 H 13 OH
heptanol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH C 7 H 15 OH
Isomers
e.g. butanol
butan- 1 - ol butan- 2 - ol
CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH(OH)CH 3
Alcohols are effective solvents, highly flammable, and burn with very clean flames resulting in their use as
fuels.
Energy from fuels
Alkanes and alcohols can be used as fuels.
Combustion reactions are exothermic reactions.
Exothermic reactions release heat energy to the surroundings.
Endothermic reactions take in heat energy from the surroundings.
Calculations based on equations
When a substance is combusted the reaction can be represented using a balanced formulae equation.
The quantities of reactants and products in these reactions can be calculated.
E.g. Calculate the mass of oxygen required to burn 50g of butan- 1 - ol.
C 4 H 9 OH(l) + 6O 2 (g) → 4CO 2 (g) + 5H 2 O(l)
1mol reacts with 6mol
74g 192g
50g 50 x 192
= 130g
Energy calculations
Different fuels provide different quantities of energy and this can be measured experimentally and
calculated using Eh = cmΔT.
Eh = cmΔT
Energy (kJ)
Specific heat capacity of water (kJkg-1oC-^1 ) = 4.
Mass of water heated (kg) 1cm^3 = 0.001kg
Change in temperature (oC)
Energy per gram (kJg-^1 ) = Eh
Mass of fuel burned (g)
water
fuel
thermometer
copper can
