6.1.1 Acids
Acids
Acids
- Acids form a class of chemical substances which contain hydrogen ions in aqueous solution, H+ (aq), as the only positive ion.
- Acids are usually classified into mineral or organic acids.
Mineral Acids
- generally much stronger
- most do not occur naturally
- usually have simpler molecules
Example
- Sulphuric Acid
- Nitric Acid
- Hydrochloric Acid
- Phoshoric Acid
- Carbonic Acid
Organic Acids
- naturally occurring
- found in vegetables, fruit and other foodstuffs
- usually weaker and less corrosive
Example
- Ethanoic Acid
- Citric Acid
- Lactic Acid
- Tartaric Acid
- Acetic Acid
- Acids are substances that form hydrogen ions when dissolved in water.
- 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
- Basicity of an acid is the number of hydrogen ions which can be produced by one molecule of the acid.
- There are 3 common types of Basicity of an acid
- monoprotic
- diprotic
- 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
- Acids have the following physical properties:
- Tastes sour
- Turns moist blue litmus to red
- pH value < 7
- Can conduct electricity
- Corrosive
Colour of Litmus in Acids
- Litmus can be used as acid/alkali indicator.
- Image below shows the colour of litmus paper when immerse in acid and alkali.
- The litmus turn red in acids and turn blue in alkali.
pH value of acids
- 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.
- Pure water has pH value of 7.
- All acids has pH value lower than 7.
Electrical Conductivity of Acids
- Acids are covalent compounds.
- However, when acids dissolve in water, they ionise to form ions in the solution.
- Since there are free moving ions in the solution, hence the solution can act as an electrolyte to conduct electricity.