Acids

Acids

  1. Acids form a class of chemical substances which contain hydrogen ions in aqueous solution, H+ (aq), as the only positive ion.
  2. Acids are usually classified into mineral or organic acids.

Mineral Acids

  1. generally much stronger
  2. most do not occur naturally
  3. usually have simpler molecules

Example

  • Sulphuric Acid
  • Nitric Acid
  • Hydrochloric Acid
  • Phoshoric Acid
  • Carbonic Acid
Organic Acids
  1. naturally occurring
  2. found in vegetables, fruit and other foodstuffs
  3. usually weaker and less corrosive

Example

  • Ethanoic Acid
  • Citric Acid
  • Lactic Acid
  • Tartaric Acid
  • Acetic Acid
  1. Acids are substances that form hydrogen ions when dissolved in water.
  2. A hydrogen ion is actually a proton. Therefore, acids are called the proton donors.

Examples
\[HCl\xrightarrow{{{H_2}O}}{H^ + } + C{l^ – }\]
\[HN{O_3}\xrightarrow{{{H_2}O}}{H^ + } + N{O_3}^ – \]
\[{H_2}S{O_4}\xrightarrow{{{H_2}O}}2{H^ + } + S{O_4}^{2 – }\]

The hydrogen ions produced will combine with the water molecule to form hydroxonium ions (H3O+)
\[{H^ + } + {H_2}O \to {H_3}{O^ + }\]
Therefore the reaction can also be written as
\[HCl + {H_2}O \to {H_3}{O^ + } + C{l^ – }\]
\[HN{O_3} + {H_2}O \to {H_3}{O^ + } + N{O_3}^ – \]
\[{H_2}S{O_4} + 2{H_2}O \to 2{H_3}{O^ + } + S{O_4}^{2 – }\]

Basicity of Acids

  1. Basicity of an acid is the number of hydrogen ions which can be produced by one molecule of the acid.
  2. There are 3 common types of Basicity of an acid
    1. monoprotic
    2. diprotic
    3. triprotic

Monoprotic Acids

The monoprotics acids are the acids that produced 1 H+ ion from each acid molecule.
Example
\[\begin{gathered}
HCl\xrightarrow{{{H_2}O}}{H^ + } + C{l^ – } \hfill \\
HN{O_3}\xrightarrow{{{H_2}O}}{H^ + } + N{O_3}^ – \hfill \\
C{H_3}COOH\xrightarrow{{{H_2}O}}{H^ + } + C{H_3}CO{O^ – } \hfill \\
\end{gathered} \]

Diprotic Acids

The diprotics acids are the acids that produced 2 H+ ion from each acid molecule.
Example
\[\begin{gathered}
{H_2}S{O_4}\xrightarrow{{{H_2}O}}2{H^ + } + S{O_4}^{2 – } \hfill \\
{H_2}S{O_3}\xrightarrow{{{H_2}O}}2{H^ + } + S{O_3}^{2 – } \hfill \\
{H_2}C{O_3}\xrightarrow{{{H_2}O}}2{H^ + } + C{O_3}^{2 – } \hfill \\
\end{gathered} \]

Triprotic Acids

The triprotics acids are the acids that produced 3 H+ ion from each acid molecule.
Example
\[{H_3}P{O_4}\xrightarrow{{{H_2}O}}3{H^ + } + P{O_4}^{3 – }\]

Physical Properties of Acids

  1. Acids have the following physical properties:
    1. Tastes sour
    2. Turns moist blue litmus to red
    3. pH value < 7
    4. Can conduct electricity
    5. Corrosive

Colour of Litmus in Acids

  1. Litmus can be used as acid/alkali indicator.
  2. Image below shows the colour of litmus paper when immerse in acid and alkali.
  3. The litmus turn red in acids and turn blue in alkali.

pH value of acids

  1. pH value is quantity to measure the concentration of hydrogen ions in a solution. The higher the concentration of hydrogen, the lower the pH value of the solution.
  2. Pure water has pH value of 7.
  3. All acids has pH value lower than 7.

Electrical Conductivity of Acids

  1. Acids are covalent compounds.
  2. However, when acids dissolve in water, they ionise to form ions in the solution.
  3. Since there are free moving ions in the solution, hence the solution can act as an electrolyte to conduct electricity.