Chemistry only
Time to meet another homologous series of organic compounds, the carboxylic acids. Carboxylic acids are a family of weak acids which contain the carboxyl group (-COOH). Their general formula is CnH2n+1COOH. They are named by taking the corresponding alkane and replacing the -e on the end with -oic acid e.g.
| alkane | carboxylic acid |
|---|---|
| methane | methanoic acid |
| ethane | ethanoic acid |
| propane | propanoic acid |
| butane | butanoic acid |
The first 4 carboxylic acids are shown below:
As before when we draw out the formula for these compounds we always draw them with the carboxyl functional group shown e.g. Methanoic acid is HCOOH and never CO2H. Ethanoic acid is CH3COOH and never C2H402.
Higher tier only
Carboxylic acids with less than 6 carbon atoms are soluble in water, they dissolve to form weak acids. If you remember a weak acid forms when a solid or liquid acidic compound only partly ionises when it is added to water. The dissociation of a carboxylic acid in water is shown below. Here the carboxylic acid molecule ionises to form a carboxylate anion (RCOO-) and a hydrogen ion (H+). It is the presence of the hydrogen ion (H+) which gives the molecule its acidic properties. However very few of the carboxylic acid molecules actually ionise, most remain intact. This means that the position of equilibrium lies very much to the left hand side, that is the reactants, hence the reason why carboxylic acids are weak acids with fairly high pH values of between 3-5.
As an example consider the simplest carboxylic acid , methanoic acid (HCOOH). When methanoic acid is added to water a few of molecules will ionise to form methanoate ions and hydrogen ions. Most of the methanoic acid molecules stay intact and do not dissociate or break down in water, we can show this as:
similarly with ethanoic acid we have:
| carboxylic acid | molecular formula | ion formed | molecular formula of ion |
|---|---|---|---|
| methanoic | HCOOH | methanoate | HCOO- |
| ethanoic | CH3COOH | ethanoate | CH3COO- |
| propanoic | C2H5COOH | propanoote | C2H5COO- |
| butanoic | C3H7COOH | butanoate | C3H7COO- |
A reaction you have seen before is when hydrochloric acid reacts with a metal carbonate such as sodium carbonate or calcium carbonate to form a salt, water and carbon dioxide gas. equations for these reactions are shown below:
swapping the hydrochloric acid, a strong acid for a weak carboxylic acid will produce a similar reaction but it will be much slower e.g.
Example 3:The apparatus to compare the rate of reaction of a weak carboxylic acid and a strong acid with chalk (calcium carbonate) is shown below. Using this simple set-up we can compare the rate of reaction of a strong and a weak acid with calcium carbonate (chalk). All we have to do is count the number of bubbles or modify the apparatus to include a gas syringe and measure the volume of carbon dioxide gas released in a given time.
