Carboxylic Acids as Acids

Introduction

• Examination of simple reactions of carboxylic acids as acids.
• Includes reactions with metals, metal hydroxides, carbonates, hydrogencarbonates, ammonia, and amines.
• Focuses on content relevant to UK syllabuses for ages 16-18.

The Acidity of Carboxylic Acids

Why are Carboxylic Acids Acidic?

• Defined as substances that donate protons (H⁺ ions).
• Carboxylic acids are acidic due to the hydrogen in the -COOH group.
• In water, a hydrogen ion transfers from the -COOH group to a water molecule:
  • Example: Ethanoic acid forms ethanoate ion and hydroxonium ion (H3O⁺).
  • Reaction is reversible, only about 1% ionization at any time, indicating they are weak acids.

The pH of Carboxylic Acid Solutions

• pH depends on acid concentration and ease of hydrogen ion loss from -COOH group.
• Ethanoic acid solutions often have pH levels between 2 and 3.
• Methanoic acid is stronger with pH about 0.5 units lower than ethanoic acid of the same concentration.

Reactions of Carboxylic Acids

With Metals

• React with more reactive metals forming a salt and hydrogen, similar to reactions with hydrochloric acid but slower.
• Example: Ethanoic acid reacts with magnesium forming magnesium ethanoate and hydrogen.
• Important to correctly write salt formulas, e.g., magnesium ethanoate.

With Metal Hydroxides

• Neutralization reactions where hydrogen ions react with hydroxide ions.
• Example: Ethanoic acid with sodium hydroxide forms sodium ethanoate and water.
• Typically involves solutions, but state symbols change if solid hydroxides are used.

With Carbonates and Hydrogencarbonates

• Reactions form a salt, carbon dioxide, and water.
• Carbonates and hydrogencarbonates are often solids; state symbols vary if solutions are used.
• Example: Ethanoic acid with sodium carbonate creates sodium ethanoate, CO₂, and water.
• Reaction vigour varies with carbonate source, e.g., slower with calcium carbonate.

With Ammonia

• Ethanoic acid transfers a hydrogen ion to ammonia, forming ammonium ions.
• Result is a solution of ammonium ethanoate.

With Amines

• Amines are derivatives of ammonia with one or more hydrogen atoms replaced by hydrocarbon groups.
• React with carboxylic acids like ammonia does.
• Example: Ethanoic acid reacts with methylamine to form methylammonium ethanoate.

Additional Notes

• Further reading and questions are available to test understanding of carboxylic acids' acidity.
• For more on amines, a separate resource is suggested.

References

• Content by Jim Clark (2004, modified November 2015).