Nature of Oxides & Hydroxides
Periodic Trends & Chemical Behavior | Classification of Elements
1. Classification of Oxides
Oxides are binary compounds of oxygen with another element. Based on their acid-base behavior, they are classified into four types:
A. Acidic Oxides
- Generally oxides of Non-metals.
- React with water to form acids.
- React with bases to form salt and water.
- Examples: $CO_2, SO_2, SO_3, P_4O_{10}, Cl_2O_7, N_2O_5$.
- Reaction: $SO_3 + H_2O \rightarrow H_2SO_4$.
B. Basic Oxides
- Generally oxides of Metals (especially Group 1 & 2).
- React with water to form bases (alkalis).
- React with acids to form salt and water.
- Examples: $Na_2O, CaO, MgO, K_2O$.
- Reaction: $Na_2O + H_2O \rightarrow 2NaOH$.
C. Amphoteric Oxides
- Oxides that show both acidic and basic characteristics.
- They react with both acids and bases.
- Important Examples (Memorize): $BeO, Al_2O_3, ZnO, SnO_2, PbO_2, As_2O_3$.
- Reaction (as base): $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$.
- Reaction (as acid): $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$.
D. Neutral Oxides
- Oxides which are neither acidic nor basic. They do not react with acids or bases.
- Examples: $CO$ (Carbon Monoxide), $NO$ (Nitric Oxide), $N_2O$ (Nitrous Oxide/Laughing Gas), $H_2O$.
2. Periodic Trends
Along a Period (Left to Right):
Basic nature decreases $\rightarrow$ Amphoteric $\rightarrow$ Acidic nature increases.
$Na_2O$ (Basic) $\dots$ $Al_2O_3$ (Amphoteric) $\dots$ $Cl_2O_7$ (Strongly Acidic).
$Na_2O$ (Basic) $\dots$ $Al_2O_3$ (Amphoteric) $\dots$ $Cl_2O_7$ (Strongly Acidic).
Down a Group (Top to Bottom):
Basic nature increases (Metallic character increases).
$Li_2O < Na_2O < K_2O < Rb_2O < Cs_2O$ (Basicity increases).
$Li_2O < Na_2O < K_2O < Rb_2O < Cs_2O$ (Basicity increases).
3. Effect of Oxidation State
For the same element forming multiple oxides, Acidity increases as the Oxidation State increases (due to higher charge density/electronegativity).
| Element | Oxide | Oxidation State | Nature |
|---|---|---|---|
| Chromium | $CrO$ | +2 | Basic |
| $Cr_2O_3$ | +3 | Amphoteric | |
| $CrO_3$ | +6 | Acidic | |
| Manganese | $MnO$ | +2 | Basic |
| $Mn_2O_7$ | +7 | Strongly Acidic |
4. Nature of Hydroxides
The trends for hydroxides mirror those of oxides.
- Alkali/Alkaline Earth Metals: Hydroxides are strong bases ($NaOH, KOH$). Basicity increases down the group.
- Non-metals: Hydroxides are actually acids (oxyacids). Example: $B(OH)_3$ is Orthoboric acid ($H_3BO_3$).
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