7.2.1 Structure and formulae of alkenes
Alkenes
- Hydrocarbons with double C=C bond
- General formula: CnH2n
Why are alkene molecules unsaturated?
- They contain 2 fewer hydrogen atoms than alkane with same no of carbon atoms
What are alkenes used?
- Produce polymers
- As starting materials for production of other chemicals
What are the first 4 members of alkenes? (Money eat proper big peanut)
Ethene (C2H4) | Propene (C3H6) |
![]() |
![]() |
Butene (C4H8) | Pentene (C5H10) |
![]() |
![]() |
7.2.2 Reactions of alkenes
Reactions of alkenes
- Alkenes are more reactive than alkanes coz they contain a C=C double bond
- Most of their reactions are addition reactions, where C=C bond is broken to form C-C bond
Why do alkenes burn in air with smoky flames when they react with oxygen in combustion reactions?
- Incomplete combustion
Describe reactions & conditions for addition of hydrogen, water & halogens to alkenes.
- C=C bond is broken to form C-C bond
Describe the colour change when alkenes react with bromine water. (1)
- Orange to colourless
Why alkenes react with bromine water but alkanes don’t?
- Alkenes’ double bond make them more reactive than alkanes
Burning alkanes
- Burn with smoky flames when react with O2 in combustion reactions coz incomplete combustion
To make polymers and many other chemicals
7.2.3 Alcohols
Alcohols
- Contain functional group -OH
- General formula: CnH2n+1OH
What are the first 4 members of homologous series of alcohols?
Describe what happens when any first four alcohols react with sodium, burn in air, are added to water, react with an oxidising agent
Sodium | Alcohols react with sodium producing bubbles of H2(g) |
Burn in air | Burn well with O2 and are good fuels |
Added to water | Alcohols mix with water & form a neutral solution with pH7 |
React with oxidising agent | Oxidised to carboxylic acids if they are left standing in air or treated with a mild oxidising agent |
Carboxylic acids | Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters |
What are the main uses of these alcohols?
- Alcohols used as solvents & fuels
- Ethanol in alcoholic drinks
Fermentation
What are produced when sugar solutions are fermented using yeast?
- Aq of ethanol
What are the conditions used for fermentation of sugar using yeast?
- Sugar dissolved in water
- Yeast is added
- Mixture kept at 30°C
- Air kept out of mixture
7.2.4 Carboxylic acids
Carboxylic acids have the functional group –COOH
What are the first 4 members of a homologous series of carboxylic acids?
Describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, react with alcohols
- React with carbonates to form salt eg sodium carbonate, water & carbon dioxide
- Dissolves in water to form weak acidic solutions (pH4-6)
- Alcohol + carboxylic acid → ester + water
Reaction | What happens |
Water |
|
Carbonates | Alcohol + acid → ester + water
Ethanol + ethanoic acid → ethyl ethanoate + water |
Alcohols | Produce bubbles of CO2(g) |
Explain why carboxylic acids are weak acids in terms of ionisation and pH?
- Partially ionised in aq solution
- pH4-6