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4.34 Carboxylic acids 2

 4.34 - 4.37 From wine to vinegar

Vinegar is a dilute solution of ethanoic acid in water.

Carboxylic acids are a homologous series of compounds with the functional group -CO2H.

They are all organic acids and are regarded as weak acids which means they do not ionise completely in solution

Carboxylic acids are formed when alcohols are oxidised by the air or by using an appropriate oxidising agent.  Methanol can be oxidised to methanoic acid, ethanol to ethanoic acid and so on.    

Assumed background knowledge:

4.29 - 4.33 Alcohols

Ethanol is the alcohol in alcoholic drinks  4.29. Handle with care The alcohols are anot...
https://www.mychem.co.uk/index.php/igcse-chemistry/organic-chemistry/ethanol

 Students should:

  • 4.34C know that carboxylic acids contain the functional group COOH
  • 4.35C understand how to draw structural and displayed formulae for unbranched-chain carboxylic acids with up to four carbon atoms in the molecule, and name each compound

 4.34 Activity 1 The functional group - a sting in the tail

Ant stings contain methanoic acid
Nettle stings also contain methanoic acid
​No. Carbon atoms ​Name ​Formula
​1 ​Methanoic acid ​HCO2H
​2​Ethanoic acid​CH3CO2H
​3​Propanoic acid​CH3CH2CO2H
​4​Butanoic acid​CH3CH2CH2CO2H

​On the "reading" tab, the names, displayed formulae and structural formulae of the first four carboxylic acids are given. 

  1. Use this information to produce an attractive display of all the above information as well as the molecular and empirical formula . 
  2. For each carboxylic acid you should give its name, molecular formula, structural formula, displayed formula and empirical formula and something interesting about where it can be found, its uses or its properties. On each displayed formula you should highlight with a circle, the carboxylic acid functional group.
  3. Can you work out the general formula for these carboxylic acids?
Methanoic acid is the simplest carboxylic acid
Propanoic acid is mainly used in food preservatives
Ethanoic acid is a colourless acid that is found in vinigar
Butanoic acid is found in milk

 Activity 2. 4.36C Carboxylic acid reactions

 Students should:

Carboxylic acids react in a very similar way to other acids like :

  • hydrochloric acid HCl(aq)
  • nitric acid HNO3(aq) and 
  • sulfuric acid H2SO4(aq)

In this video we see citric acid reacting with a carbonate to produce bubbles of carbon dioxide. Notice how the acid does not react until the water is added. Acids need to be aqueous to show their acidic properties.

This video shows magnesium being added to citric acid. Read the questions then watch the video all the way through and answer them carefully.

  1.  What do the colour changes suggest has happened to the Citric acid in this reaction?
  2. What is the name gas produced ?
  3. How would you test this gas to identify it?
  4. What happens to the magnesium ribbon in this reaction?
  5. Suggest a name for the salt produced
  6. How could you obtain crystals from the solution produced?
M+A+S+H
  1. The citric acid is neutralised by the reaction with magnesium.
  2. The reaction produces hydrogen gas
  3. Insert a lighted splint into the mouth of a tube of the gas. If it is hydrogen it will burn with a squeaky pop.
  4. The magnesium ribbon dissolves.
  5. The salt formed is called magnesium citrate.
  6. Crystals of magnesium citrate can be obtained by evaporating the water off.
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