7.2.4a Carboxylic acids

Carboxylic acids: the $-COOH$ group sets their names and reactions

1. A carboxylic acid is an organic compound with the carboxyl functional group $-COOH$.
2. Carboxylic acids form a homologous series, so they share this group and react in similar ways.
3. Each member differs from the next by a ${CH}_2$ unit.
4. Names use meth-, eth-, prop- and but- followed by -anoic acid.
   1. Methanoic acid has one carbon: $HCOOH$.
   2. Ethanoic acid has two carbons: ${CH}_{3}COOH$.
   3. Propanoic acid has three carbons: ${CH}_3{CH}_{2}COOH$.
   4. Butanoic acid has four carbons: ${CH}_3{CH}_2{CH}_{2}COOH$.
5. The carbon in the $-COOH$ group is counted as part of the chain.

Carboxylic acids form acidic solutions and fizz with carbonates

1. Each of the first four carboxylic acids dissolves in water to give an acidic solution with a pH below 7.
2. A carboxylic acid reacts with a metal carbonate to give a salt, water and carbon dioxide.
3. The carbon dioxide bubbles off, so you see fizzing, also called effervescence.
4. The word equation is carboxylic acid + carbonate $\to$ salt + water + carbon dioxide.
5. The salt's name ends in -anoate, for example sodium ethanoate from ethanoic acid.

Carboxylic acids react with alcohols to make esters

1. A carboxylic acid reacts with an alcohol to make an ester and water.
2. This reaction is called esterification.
3. The mixture is warmed with concentrated sulfuric acid, which acts as a catalyst.
4. The word equation is carboxylic acid + alcohol $\to$ ester + water.
5. Ethanoic acid and ethanol make ethyl ethanoate and water.
6. In “ethyl ethanoate”, ethyl comes from ethanol and ethanoate comes from ethanoic acid.

Answering questions on the three reactions

1. Recognise a carboxylic acid by a name ending in -anoic acid or a formula containing $-COOH$.
2. For water, say it dissolves to give an acidic solution.
3. For a carbonate, describe the fizzing and name a salt, water and carbon dioxide.
4. For an alcohol, name the products as an ester and water.

What you need to recall

1. You only need the acid names methanoic, ethanoic, propanoic and butanoic acid.
2. You do not need any ester name other than ethyl ethanoate.
3. You are not expected to write balanced equations for carboxylic acid reactions.

7.2.4b Carboxylic acids as weak acids

Carboxylic acids are weak because they only partially ionise

1. A weak acid ionises only partially in water, so only a small proportion of its molecules form ions.
2. Carboxylic acids are weak acids, because most of their molecules stay un-ionised in solution.
3. Ethanoic acid is a typical carboxylic acid, and its partial ionisation is shown here.
   1. ${CH}_{3}COOH(aq)\rightleftharpoons H^{+}(aq)+{CH}_3{COO}^{-}(aq)$

Partial ionisation gives fewer hydrogen ions and a higher pH

1. The pH of a solution depends on its concentration of $H^{+}(aq)$ ions.
2. A higher $H^{+}(aq)$ concentration gives a lower pH.
3. A weak carboxylic acid makes fewer $H^{+}(aq)$ ions than a strong acid of the same concentration, because it only partially ionises.
4. So a carboxylic acid has a higher pH than a strong acid of the same concentration, though still below 7.

Strength describes ionisation, not concentration

1. Acid strength is the proportion of acid molecules that ionise in water.
2. Concentration is how much acid is dissolved in a given volume of solution.
3. A concentrated carboxylic acid is still weak, because only a proportion of its molecules ionise.
4. A dilute strong acid is still strong, because the molecules present ionise completely.
5. A weak carboxylic acid is not neutral, because partial ionisation still makes some $H^{+}(aq)$ ions.

Putting the explanation together

1. Start by saying carboxylic acids partially ionise in water.
2. Explain that partial ionisation makes fewer $H^{+}(aq)$ ions than complete ionisation at the same concentration.
3. Link the lower hydrogen ion concentration to the higher pH.
4. Do not say a weak acid does not ionise, because some molecules do form ions.