Chemical Properties of Water
Amphoteric Auto-protolysis, Redox Duality, and the Hydrolysis Traps.
Water ($H_2O$) is a highly stable molecule due to its massive enthalpy of formation and strong $O-H$ bonds ($492.7 \text{ kJ/mol}$). However, its unique bent structure and polar nature allow it to participate in a vast array of critical chemical reactions. It acts as an acid, a base, an oxidizing agent, a reducing agent, and a powerful hydrolyzing agent.
1. Amphoteric Nature & Auto-protolysis
According to the Brønsted-Lowry concept, water is amphoteric: it can act as both an acid (proton donor) and a base (proton acceptor).
A. Acting as an Acid and Base with Solutes:
- Acting as an Acid: When reacting with a stronger base like Ammonia ($NH_3$), water donates a proton ($H^+$).
$H_2O_{(l)} + NH_{3(aq)} \rightleftharpoons OH^-_{(aq)} + NH_{4(aq)}^+$ - Acting as a Base: When reacting with a stronger acid like $HCl$ or $H_2S$, water accepts a proton.
$H_2O_{(l)} + H_2S_{(aq)} \rightleftharpoons H_3O^+_{(aq)} + HS^-_{(aq)}$
B. Auto-protolysis (Self-Ionization) of Water:
Even in pure water, molecules constantly collide and transfer protons among themselves. One molecule acts as an acid, and the other as a base.
At $298\text{K}$ ($25^\circ\text{C}$), the concentration of $H_3O^+$ and $OH^-$ are both exactly $10^{-7}\text{ M}$.
$K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14} \text{ M}^2$
JEE Trap: Auto-protolysis is an endothermic process. According to Le Chatelier's principle, if you increase the temperature of pure water, the reaction shifts forward. $K_w$ increases as temperature increases (e.g., at $90^\circ\text{C}$, $K_w \approx 10^{-12}$, making the neutral pH $\approx 6$).
2. Redox Reactions: The Dual Agent
Water contains Hydrogen in the $+1$ oxidation state and Oxygen in the $-2$ state. This allows it to act as an oxidizing agent (reducing $H^+$ to $H_2$) or a reducing agent (oxidizing $O^{2-}$ to $O_2$).
A. Water as an Oxidizing Agent:
When water reacts with highly electropositive metals (like Na, K, Ca), it oxidizes the metal and is itself reduced, liberating Hydrogen gas ($H_2$).
B. Water as a Reducing Agent:
When water reacts with exceptionally strong oxidizing agents, like highly electronegative Fluorine ($F_2$), water is oxidized to Oxygen gas ($O_2$) (or sometimes Ozone, $O_3$).
Biological Example: During Photosynthesis, water acts as a reducing agent. It is oxidized by sunlight (with chlorophyll's help) to provide electrons to reduce $CO_2$, releasing $O_2$ into the atmosphere:
$6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2$
3. The Hydrolysis Traps (JEE Goldmine)
Water has a very high dielectric constant and strong hydrating ability, making it an exceptional hydrolyzing agent. It easily breaks down covalent and ionic bonds of non-metal compounds. Memorize these specific carbide and nitride products!
| Compound Type | Hydrolysis Reaction & Product |
|---|---|
| Nitrides |
Yields Ammonia ($NH_3$). $Mg_3N_2 + 6H_2O \rightarrow 3Mg(OH)_2 + \mathbf{2NH_3}$ |
| Phosphides |
Yields Phosphine ($PH_3$). $Ca_3P_2 + 6H_2O \rightarrow 3Ca(OH)_2 + \mathbf{2PH_3}$ |
| Carbides (Acetylides) |
Calcium carbide yields Acetylene (Ethyne). $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + \mathbf{C_2H_2}$ |
| Carbides (Methanides) |
Aluminum/Beryllium carbides yield Methane. $Al_4C_3 + 12H_2O \rightarrow 4Al(OH)_3 + \mathbf{3CH_4}$ |
| Non-Metal Oxides |
Yields their respective acids. $P_4O_{10} + 6H_2O \rightarrow \mathbf{4H_3PO_4}$ (Orthophosphoric acid) |
| Halides |
Yields oxoacids and hydrogen halides. $SiCl_4 + 4H_2O \rightarrow \mathbf{Si(OH)_4} + 4HCl$ |
4. Hydrates and Water of Crystallization
From aqueous solutions, many salts crystallize as hydrated salts. The water molecules can associate with the salt in three completely different ways.
Figure 1: The structure of Blue Vitriol, demonstrating the single hydrogen-bonded water molecule.
- 1. Coordinated Water: Water molecules form dative (coordinate) bonds directly with the central metal cation. Example: In $[Cr(H_2O)_6]^{3+} 3Cl^-$, all six water molecules are directly coordinated to Chromium.
- 2. Interstitial Water: Water molecules occupy empty voids (interstices) within the crystal lattice without forming direct coordinate bonds to the metal. Example: $BaCl_2 \cdot 2H_2O$.
- 3. Hydrogen-Bonded Water (The Trap): Water molecules are held outside the coordination sphere via hydrogen bonding to the anions and the coordinated water molecules.
Classic JEE Example: Blue Vitriol ($CuSO_4 \cdot 5H_2O$)
Despite the formula, only 4 water molecules are coordinated to Copper. The exactly 1 remaining water molecule is hydrogen-bonded to the sulfate ion outside the coordination sphere: $\mathbf{[Cu(H_2O)_4]SO_4 \cdot H_2O}$.
Mastery Check: Water Chemistry
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