Reactions of Oxalic Acid (H₂C₂O₄) with Specific Reagents
Oxalic acid is an important organic acid studied in Class 11 and 12 Chemistry. It is a dibasic carboxylic acid and shows acidic as well as reducing properties. In this article, we will study all the important reactions of oxalic acid with specific reagents, which are frequently asked in CBSE board exams, practical exams, and competitive exams.
1. Reaction with Alkalis (Neutralization Reaction)
Oxalic acid reacts with alkalis like sodium hydroxide to form acid salt and normal salt. Since oxalic acid is dibasic, the reaction occurs in two steps.
(a) With Sodium Hydroxide (NaOH)
Step 1 (Formation of Acid Salt):
H₂C₂O₄ + NaOH → NaHC₂O₄ + H₂O
Step 2 (Formation of Normal Salt):
NaHC₂O₄ + NaOH → Na₂C₂O₄ + H₂O
This reaction proves the dibasic nature of oxalic acid.
2. Reaction with Carbonates and Bicarbonates
(a) With Sodium Carbonate (Na₂CO₃)
H₂C₂O₄ + Na₂CO₃ → Na₂C₂O₄ + CO₂ ↑ + H₂O
Carbon dioxide gas is evolved with brisk effervescence.
(b) With Sodium Bicarbonate (NaHCO₃)
H₂C₂O₄ + 2NaHCO₃ → Na₂C₂O₄ + 2CO₂ ↑ + 2H₂O
This reaction confirms the acidic nature of oxalic acid.
3. Reaction with Calcium Chloride (Confirmatory Test)
Oxalic acid reacts with calcium chloride solution to form a white precipitate.
H₂C₂O₄ + CaCl₂ → CaC₂O₄ ↓ + 2HCl
The white precipitate of calcium oxalate is insoluble in water. This reaction is used as a confirmatory test for oxalic acid.
4. Reaction with Potassium Permanganate (KMnO₄)
In acidic medium, oxalic acid acts as a strong reducing agent and decolourises potassium permanganate solution.
2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ ↑ + 8H₂O
The purple colour of KMnO₄ disappears. The reaction is slow at room temperature and becomes fast on heating.
5. Reaction with Concentrated Sulphuric Acid
When oxalic acid is heated with concentrated sulphuric acid, it decomposes to give carbon monoxide and carbon dioxide.
H₂C₂O₄ → CO + CO₂ + H₂O
Carbon monoxide burns with a blue flame. This reaction shows the reducing nature of oxalic acid.
6. Reaction with Alcohols (Esterification)
Oxalic acid reacts with alcohols in the presence of concentrated sulphuric acid to form esters.
With Ethanol
H₂C₂O₄ + 2C₂H₅OH → (COOC₂H₅)₂ + 2H₂O
The product formed is diethyl oxalate, which has a pleasant fruity smell.
7. Reaction with Metals
Oxalic acid reacts with active metals like zinc to liberate hydrogen gas.
H₂C₂O₄ + Zn → ZnC₂O₄ + H₂ ↑
This reaction confirms the acidic nature of oxalic acid.
8. Reaction with Ammonia
Oxalic acid reacts with ammonia to form ammonium oxalate.
H₂C₂O₄ + 2NH₃ → (NH₄)₂C₂O₄
9. Thermal Decomposition
On strong heating, oxalic acid decomposes into carbon monoxide, carbon dioxide and water.
H₂C₂O₄ → CO + CO₂ + H₂O
This reaction again shows the reducing nature of oxalic acid.
10. Reaction with Ferric Ions (Fe³⁺)
Oxalic acid reduces ferric ions to ferrous ions.
2Fe³⁺ + H₂C₂O₄ → 2Fe²⁺ + 2CO₂ + 2H⁺
This reaction is important in redox chemistry.
Conclusion
Oxalic acid shows a wide range of reactions due to its acidic and reducing properties. The reactions with calcium chloride and potassium permanganate are especially important for practical and board examinations. A proper understanding of these reactions helps students score well in both theory and practical chemistry.
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