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The structure and reactions of alkenes, the properties and uses of alcohols, and the reactions of carboxylic acids including making esters.
Some of the most useful chemicals come from three families of carbon compounds. Alkenes help make plastics. Alcohols are used as fuels and in drinks. Carboxylic acids give us vinegar and fruity smells.
It matters because they all link together. Alkenes can be turned into alcohols. Alcohols and carboxylic acids join to make esters. Seeing the links makes the topic much easier to remember.
By the end you will be able to spot each family from its functional group. You will also test for an alkene, describe how ethanol is made and name a simple ester.
Alkenes are hydrocarbons with a carbon-carbon double bond, C=C. A hydrocarbon contains only carbon and hydrogen. Alkenes are unsaturated, which means they have a double bond. The first is ethene, C₂H₄. The general formula is CₙH₂ₙ.
The double bond makes alkenes reactive. To test for one, add bromine water, which is orange. With an alkene the orange colour disappears. With an alkane, which has no double bond, it stays orange.
This is an addition reaction: C₂H₄ + Br₂ → C₂H₄Br₂. The bromine adds across the double bond. Two molecules join to make one product.
Think of the double bond as a clip that can open. When it opens, two new atoms can join on. That is why alkenes react by addition.
With hydrogen and a nickel catalyst at about 60 °C, an alkene becomes an alkane: C₂H₄ + H₂ → C₂H₆. This is used to harden vegetable oils to make margarine.
With steam, an alkene becomes an alcohol: C₂H₄ + H₂O → C₂H₅OH. This needs a phosphoric acid catalyst, high temperature and high pressure. It is how ethanol is made from ethene.
Alcohols contain the functional group –OH. A functional group is the part of a molecule that decides how it reacts. The first four are methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH and butanol C₄H₉OH.
Alcohols dissolve in water to make neutral solutions. They burn well: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. They are used as fuels and solvents. Ethanol is the alcohol in alcoholic drinks.
The longer the chain, the higher the boiling point, as the bars show.
Fermentation uses yeast, the same living thing that makes bread rise. Yeast turns sugar into ethanol and carbon dioxide at about 30 °C, without oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Fractional distillation then separates the ethanol.
Fermentation uses renewable sugar. It is slow and gives a dilute solution. It is a batch process, which means one batch is made at a time.
Ethene and steam make ethanol too. This is fast, continuous and gives pure ethanol. But it uses a lot of energy and uses crude oil, which will run out.
Carboxylic acids contain the functional group –COOH. The first four are methanoic acid HCOOH, ethanoic acid CH₃COOH, propanoic acid C₂H₅COOH and butanoic acid C₃H₇COOH. Ethanoic acid is the acid in vinegar.
They are weak acids. This means they are only partly ionised in water. Their pH is higher than a strong acid of the same concentration.
They still behave like acids. They react with carbonates to make a salt, water and carbon dioxide. They react with reactive metals to make a salt and hydrogen.
A carboxylic acid reacts with an alcohol to make an ester and water. An acid catalyst is used. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The reaction is reversible. Esters have fruity smells and are used in perfumes and flavourings.
Names have two parts. The stem shows the carbon atoms: meth (1), eth (2), prop (3), but (4). The ending shows the family: –ane, –ene, –anol or –anoic acid.
An ester name starts with the alcohol, then the acid. Ethanol and ethanoic acid make ethyl ethanoate. Methanol and ethanoic acid make methyl ethanoate.
Alkenes have a C=C double bond. Bromine water turns from orange to colourless with an alkene.
Alkenes add hydrogen to make alkanes and steam to make alcohols. Alcohols have –OH and carboxylic acids have –COOH.
Ethanol is made by fermentation (renewable, slow) or from ethene and steam (fast, pure).
Carboxylic acids are weak acids. An alcohol and a carboxylic acid make an ester and water.
Name the test for an alkene and give the positive result.
Answer: bromine water, which turns from orange to colourless
Write a balanced symbol equation for the reaction of ethene with bromine.
Answer: C₂H₄ + Br₂ → C₂H₄Br₂
Compare making ethanol by fermentation with making ethanol from ethene and steam.
Answer: Fermentation is renewable, slow and uses lower temperatures. Hydration uses finite crude oil, is fast and gives pure ethanol.
Alkenes, alcohols and carboxylic acids practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
What is the functional group of an alcohol?
Answer: –OH
What is the colour change when an alkene is added to bromine water?
Answer: orange to colourless
Name the compound with the formula CH₃COOH.
Answer: ethanoic acid
What gas is made when ethanol is made by fermentation, as well as ethanol?
Answer: carbon dioxide
What is the name of the alkene with two carbon atoms?
Answer: ethene
Which family of compounds has the functional group –COOH?
Answer: carboxylic acids
Give the molecular formula of an alkene with 5 carbon atoms.
Answer: C₅H₁₀
Name the product when ethene reacts with hydrogen.
Answer: ethane
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Ethene and ethane are hydrocarbons. (a) Describe how to show that a gas is ethene and not ethane. [2 marks] (b) Ethene reacts with steam to make ethanol. Write a balanced equation. [1 mark] (c) State the type of reaction in part (b). [1 mark]
Answer: (a) Bubble the gas through bromine water. Ethene turns the orange solution colourless but ethane does not. (b) C₂H₄ + H₂O → C₂H₅OH (c) addition
Describe the similarities and differences between the structures and reactions of ethanol and ethanoic acid. [6 marks]
Answer: Both are organic compounds with two carbon atoms and an oxygen-containing functional group. Ethanol is an alcohol with the –OH group and ethanoic acid is a carboxylic acid with the –COOH group. Ethanol is neutral in water but ethanoic acid is a weak acid because it is partly ionised, so it has a pH below 7. Ethanoic acid reacts with carbonates to give carbon dioxide but ethanol does not. Both react with reactive metals to give hydrogen. Ethanol burns to give carbon dioxide and water. Together the two can react to make an ester, ethyl ethanoate, and water.
Saying alkenes are saturated
Alkenes are unsaturated because they contain a C=C double bond.
Saying the bromine water test makes the alkane turn colourless
Alkanes do not react. Alkenes decolourise bromine water.
Saying carboxylic acids are strong acids
Carboxylic acids are weak acids because they are only partly ionised.
Naming an ester the wrong way round
The first word comes from the alcohol and the second word from the acid.
Alkenes, alcohols and carboxylic acids is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 7.
Saying alkenes are saturated. Alkenes are unsaturated because they contain a C=C double bond.
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