7.5 Chemical Properties

  Hydrogen

1. pH of pure water is $7$ at $25^°C$. It has the ability to act as an acid as well as a base i.e., it is amphoteric in nature. In the Bronsted sense it acts as an acid with $NH_3$ and a base with $H_2S$.

$${H_2}O\left( l \right) + N{H_3}\left( {aq} \right) \Leftrightarrow O{H^ - }\left( {aq} \right) + N{H_4}^ + \left( {aq} \right)$$$${H_2}O\left( l \right) + {H_2}S\left( {aq} \right) \Leftrightarrow {H_3}{O^ + }\left( {aq} \right) + H{S^ - }\left( {aq} \right)$$

The auto-protolysis (self-ionization) of water takes place as follows:

$$\begin{equation} \begin{aligned} {H_2}O\left( l \right) + {H_2}O\left( l \right) \to {H_3}{O^ + }\left( {aq} \right) + O{H^ - }\left( {aq} \right) \\ {\text{acid - 1 base - 2 acid - 2 base - 1}} \\ {\text{(acid) (base) (conjugate acid) (conjugate base)}} \\\end{aligned} \end{equation} $$

2. Reacts with active metals and evolve hydrogen. $$2Na + 2{H_2}O \to 2NaOH + {H_2}$$ Thus, it is a great source of dihydrogen. Water is oxidised to $O_2$ during photosynthesis.

3. Hydrides, Carbides, Nitrides, Phospides are decomposed by water. $$Ca{H_2} + 2{H_2}O \to Ca{\left( {OH} \right)_2} + 2{H_2}$$

4. Hydrates Formation: From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz.,

• coordinated water e.g., ${[Cr{({H_2}O)_6}]^{(3 + )}}3C{l^ - }$
• interstitial water e.g., $BaC{l_2}.2{H_2}O$
• hydrogen-bonded water e.g., ${[Cr{({H_2}O)_4}]^{(2 + )}}S{O_4}^{2 - }.{H_2}O\,in\,CuS{O_4}.5{H_2}O$

Note: How many hydrogen-bonded water molecule(s) are associated in $CuS{O_4}.5{H_2}O$?

Only one water molecule, which is outside the brackets (coordination sphere), is hydrogen-bonded. The other four molecules of water are coordinated.