8.10 - Carboxylic Acids

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.

The salts these acids make, and how they are named

• Reactions of carboxylic acids produce a salt, where the acidic hydrogen is replaced by a metal ion:
  • The first part of the name is the metal.
  • The second part comes from the acid, with -oic acid replaced by -oate (e.g. ethanoic acid makes ethanoates).
• The salt is an ionic compound containing a carboxylate ion, such as the ethanoate ion, CH₃COO^-:
  • Sodium ethanoate is CH₃COONa (or NaCH₃COO) - one Na⁺ balances one CH₃COO^-.
  • Magnesium ethanoate is (CH₃COO)₂Mg - One Mg²⁺ requires two CH₃COO^- ions.

Reaction with a metal

Adding a metal to a carboxylic acid solution produces bubbles of gas and an increase in temperature:

carboxylic acid + metal → a salt + hydrogen

ethanoic acid + magnesium → magnesium ethanoate, (CH₃COO)₂Mg, + hydrogen

The hydrogen gas burns with a squeaky pop.

Reaction with a carbonate

Adding a carbonate to a carboxylic acid solution produces fizzing as the solid dissolves:

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

ethanoic acid + sodium carbonate → sodium ethanoate, CH₃COONa, + water + carbon dioxide

The carbon dioxide gas turns limewater milky.

Reaction with a base

Neutralising a carboxylic acid with a metal oxide or metal hydroxide releases heat without producing gas:

carboxylic acid + metal oxide (or hydroxide) → a salt + water

ethanoic acid + sodium hydroxide → sodium ethanoate, CH₃COONa, + water

What the three reactions have in common

• A metal gives a salt and hydrogen. A base gives a salt and water. A carbonate gives a salt, water and carbon dioxide.
• These are the standard reactions of any acid.