Introduction To Acids

Definition of Acids

An acid is a compound that dissociates to produce hydrogen ions ($\text{H}^{+}$) when it is dissolved in water.

The table below shows the names, formulae and dissociation reaction in water:

Name of AcidChemical FormulaEquation
Hydrochloric Acid$\text{HCl}$$\text{HCl} \rightarrow \text{H}^{+} + \text{Cl}^{-}$
Sulphuric Acid$\text{H}_{2}\text{SO}_{4}$$\text{H}_{2}\text{SO}_{4} \rightarrow 2 \text{H}^{+} + \text{SO}_{4}^{2-}$
Nitric Acid$\text{HNO}_{3}$$\text{HNO}_{3} \rightarrow \text{H}^{+} + \text{NO}_{3}^{-}$
Ethanoic Acid$\text{CH}_{3}\text{COOH}$$\text{CH}_{3}\text{COOH} \rightarrow \text{CH}_{3}\text{CO}_{2}^{-} + \text{H}^{+}$

Important notes:

A solution of an acid will contain hydrogen ions as the only positive ions.

A compound that contains hydrogen is not always an acid.

  • E.g. Ammonia, $\text{NH}_{3}$ is not an acid. It does not produce hydrogen ions when dissolved in water.
  • Note: Ammonia is actually a basic compound.

Hence, an acid is a compound which becomes a proton ($\text{H}^{+}$) donor when dissolved in water. The properties and reactions of acid (acidic properties of acid) are due to these hydrogen ions. This means that acid only behaves as acids when they are dissolved in water.

Pure acids (without water) consist of small covalent molecules. When water is added, the pure acid molecules react with water to form ions, which is commonly referred as “Acid is ionized”.

  • Pure acids are poor electrical conductors
  • Acids diluted with water are much better conductors due to the mobile ions present.

Strong and Weak Acids

MAGNESIUM IN ACID Magnesium reacts in strong and weak acids. Magnesium reacts at different rates in a strong acid solution (left) and weak acid solution (right).
Magnesium reacts in strong and weak acids.
Magnesium reacts at different rates in a strong acid solution (left) and weak acid solution (right).

A strong acid dissociates/ionises completely into ions. All the acid molecules become ions in water.

  • E.g. Dilute hydrochloric acid is a strong acid. It only contains $\text{H}^{+} (\text{aq})$ and $\text{Cl}^{-}(\text{aq})$ ions, and no $\text{HCl}$ molecules are present.
  • $\text{HCl} (\text{aq}) \rightarrow \text{H}^{+} (\text{aq}) + \text{Cl}^{-} (\text{aq})$
  • If 4 molecules of $\text{HCl}$ are dissolved in water, all 4 molecules will dissociate into ions, producing $4 \text{ H}^{+}$ ions and $4 \text{ Cl}^{-}$ ions.
  • Sulphuric acid, nitric acid and hydrochloric acid are examples of strong acids.

Weak acids are only slightly/partially ionised. Not all the acid molecules become ions in water.

  • E.g. Ethanoic acid, $\text{CH}_{3}\text{COOH}$ is a weak acid. Most of the acid molecules remain unchanged in water as very few molecules are ionised to produce hydrogen ions.
  • $\text{CH}_{3}\text{COOH} (\text{aq}) \rightleftharpoons \text{CH}_{3}\text{COO}^{-} (\text{aq}) + \text{H}^{+} (\text{aq})$
  • The solution of ethanoic acid will contain a large number of $\text{CH}_{3}\text{COOH}$ molecules and a small number of $\text{CH}_{3}\text{COO}^{-}$ and $\text{H}^{+}$ ions.
  • Ethanoic acid, citric acid and carbonic acid are examples of weak acids.

The number of hydogen atoms in an acid that will ionize is determined by its basicity

Basicity of an acid refers to the number of moles of hydrogen ions that can be produced by one mole of acid.

  • Monobasic Acids: Hydrochloric acid ($\text{HCl}$), Nitric acid ($\text{HNO}_{3}$), Ethanoic acid ($\text{CH}_{3}\text{COOH}$), etc.
  • Dibasic Acids: Sulphuric acid ($\text{H}_{2}\text{SO}_{4}$), Carbonic acid ($\text{H}_{2}\text{CO}_{3}$), etc.
  • Tribasic Acids: Phosphproc acid ($\text{H}_{3}\text{PO}_{4}$), etc.

Two Main Classes of Acids

Inorganic acids can be prepared in the laboratory from mineral elements or inorganic matter.

  • E.g. Hydrochloric acid and nitric acid
  • Inorganic acid are usually strong acid.

Organic acids are obtained from plants and animals.

  • E.g. Ethanoic acid and citric acid
  • Organic acids are usually weak acid.

Common Uses of Acids

Acids can be used:

  • To remove rust from metals
  • In industrial processes (E.g. making of fertilizers, carbonated drinks)
  • To preserve food (Acid stops the oxidation of food)


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