10.13 - Carboxylic Acids

The carboxylic acids

• Carboxylic acids are a homologous series - a family built on the same pattern: they share a general formula, each member differs from the next by a CH₂ unit, and they all have similar chemical properties.
• They all share the functional group -COOH, the carboxyl group:
  • A carbon atom joined to two oxygen atoms - one by a double bond, the other by a single bond.
  • That second oxygen carries a hydrogen atom on a single bond.
  • The carbon uses its fourth bond to join to the rest of the chain.
• It's the hydrogen atom on that oxygen that makes the compound an acid.

Naming carboxylic acids

• All names end in -oic acid, and the prefix says how many carbon atoms the molecule has: meth- (1), eth- (2), prop- (3), but- (4).
• The carbon atom in the carboxyl group is counted in - Ethanoic acid has two carbon atoms in all, one of them the carbon of the COOH.

The first four members

• Written on one line, the four atoms of the group are kept together at the end as COOH, so the group can be seen at a glance.
• The molecular formula gives the totals and hides the group. For example, C₂H₅COOH and C₃H₆O₂ are the same molecule written two different ways.
• All four acids are liquids at room temperature and pressure.

Where they turn up

• Vinegar is a dilute solution of ethanoic acid in water, which gives it its sharp taste and smell.
• Carboxylic acids are used in foods (as flavours and preservatives) and in personal care products such as soaps.

What carboxylic acids do

• Carboxylic acids are a family of organic acids. A solution of one behaves the way any other acid solution behaves.
• When dissolved in water, a carboxylic acid forms an acidic solution.
• It's acidic because the acid releases hydrogen ions, H⁺, into the water.
• Because it contains hydrogen ions, the solution has a pH below 7 and turns universal indicator red, orange or yellow.

Reaction with a carbonate

Adding a carboxylic acid to a carbonate causes fizzing: the solid disappears, leaving a colourless solution as a gas bubbles off.

carboxylic acid + carbonate → a salt + water + carbon dioxide

• The gas is carbon dioxide, which turns limewater milky.
• The salt is named from the acid, with the -oic acid ending replaced by -oate (e.g. ethanoic acid makes ethanoates). The metal comes first:

ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide

Reaction with an alcohol

A carboxylic acid reacts with an alcohol in the presence of an acid catalyst to produce an ester:

carboxylic acid + alcohol → an ester + water

Carboxylic acids are weak acids

An acid releases hydrogen ions, H⁺, when dissolved in water by ionising.

Strong versus weak

• Strong acid - Ionises completely in water. Every molecule splits up to release its H⁺ ion.
• Weak acid - Ionises only partially in water. Only a small fraction of the molecules release their H⁺ ions, forming an equilibrium mixture of ions and un-ionised molecules.

Carboxylic acids are weak acids because they only partially ionise in aqueous solution.

Why this changes the pH

• pH depends on the concentration of hydrogen ions: the higher the concentration of H⁺, the lower the pH.
• At the same concentration, a weak acid has fewer free H⁺ ions in solution than a strong acid.
• Therefore, a weak acid has a higher pH and reacts more slowly than a strong acid of the same concentration.

Strength and concentration are different things

• Concentration is the amount of acid dissolved per unit volume.
• Strength is the fraction of acid molecules that ionise in water.

Writing an equation and naming the salt

A reaction of a carboxylic acid can be written as a balanced symbol equation using six steps, shown here for ethanoic acid with sodium carbonate.

Worked example - Acid with a metal

Write the balanced symbol equation for the reaction of ethanoic acid with magnesium.