What Is Oxalic Acid Definition Formula Reactions and Uses in Chemistry
Oxalic acid is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. It is a key organic acid used in labs, industry, and even in cleaning and food chemistry studies.
What is Oxalic Acid in Chemistry?
Oxalic acid refers to a simple dicarboxylic acid with the chemical formula C2H2O4. This concept appears in chapters related to carboxylic acids, redox reactions, and acid-base behavior, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula of oxalic acid is C2H2O4. It consists of two carboxyl (–COOH) groups joined together and is categorized under dicarboxylic acids. The IUPAC name is ethanedioic acid.
| Property | Oxalic Acid (Anhydrous) | Oxalic Acid (Dihydrate) |
|---|---|---|
| Molecular Formula | C2H2O4 | C2H2O4·2H2O |
| Molecular Mass (g/mol) | 90.03 | 126.07 |
| Appearance | Colourless, crystalline | Colourless crystals |
| Density (g/cm3) | 1.90 | 1.653 |
| Melting Point (°C) | 101 | 101 (decomposes) |
| Solubility | Water, alcohol, ether | Water, alcohol |
Preparation and Synthesis Methods
- Industrial oxalic acid is commonly made by oxidation of carbohydrates (like sugar, glucose, molasses) using nitric acid as an oxidizing agent.
- Another method uses catalytic oxidation of carbohydrates or heating sodium formate.
- In the lab, oxalic acid can be made by oxidizing sucrose or glucose with nitric acid or by reacting sodium oxalate with dilute sulfuric acid and crystallizing the solution.
- Always handle oxalic acid with care, as it is toxic in concentrated forms.
Physical Properties of Oxalic Acid
- Oxalic acid appears as colourless, odourless crystals in its pure form. It melts at 101°C (dihydrate) and sublimes when heated.
- It is highly soluble in water, and also dissolves in alcohol and ether. Its pKa1 is about 1.27, making it a stronger acid than acetic acid.
- The density differs between the anhydrous (1.90 g/cm3) and dihydrate (1.653 g/cm3) forms.
Chemical Properties and Reactions
- Oxalic acid is a strong organic acid and a powerful reducing agent. It reacts with bases to form oxalate salts, like calcium oxalate and sodium oxalate.
- In redox reactions, it reduces potassium permanganate (KMnO4) to Mn2+ ions while itself getting oxidized to carbon dioxide.
- It also decomposes upon strong heating to give CO and CO2, and reacts with metals to form chelates.
- Its acidic behavior is demonstrated clearly when titrated against a base in experiments.
Frequent Related Errors
- Confusing oxalic acid with acetic acid or other simple acids.
- Assuming all oxalic acid in plants is harmful (quantity and solubility matter).
- Ignoring its strong acid properties and redox behavior in school experiments.
- Mistaking oxalic acid’s crystal form as “salt” (it's the pure acid, not just a salt).
Uses of Oxalic Acid in Real Life
- Oxalic acid is widely used for cleaning, bleaching wood, removing rust, in textile processing, and in laboratories for titration experiments.
- It is also used by beekeepers (in safe controlled amounts) to control varroa mites in beehives.
- In household settings, it works as a stain remover for ink, rust, and some food stains.
- It is not added directly to foods but naturally exists in vegetables like spinach and rhubarb.
Relation with Other Chemistry Concepts
Oxalic acid is closely related to topics such as chemical reactions and redox reactions, helping students build a conceptual bridge between organic acids and analytical laboratory chemistry. It is also an example when studying organic acids and chelating agents.
Step-by-Step Reaction Example
- Start with the redox reaction between oxalic acid and potassium permanganate in acidic medium.
2 KMnO4 + 5 H2C2O4 + 6 H2SO4 → 2 MnSO4 + 10 CO2 + 8 H2O + K2SO4 - Explain each step.
Here, oxalic acid (reductant) donates electrons, getting oxidized to CO2, while Mn in KMnO4 is reduced from +7 to +2.
Lab or Experimental Tips
Remember oxalic acid by the rule that it turns potassium permanganate from purple to colourless in redox titrations. Vedantu educators often use the “KMnO4 disappears” trick to help students track oxidation visually.
Try This Yourself
- Write the IUPAC name of oxalic acid.
- Explain what happens if you add calcium chloride to an oxalic acid solution.
- List two vegetables high in oxalic acid content.
- Draw the structure of the oxalate ion (C2O42−).
Final Wrap-Up
We explored oxalic acid—its structure, properties, reactions, and real-life importance. For more in-depth explanations and easy-to-follow chemistry revision, check out live classes and study notes on Vedantu for other key topics like tartaric acid.
FAQs on Oxalic Acid Structure Properties and Preparation
1. What is oxalic acid?
Oxalic acid is a colorless, crystalline organic compound with the formula H2C2O4 and is the simplest dicarboxylic acid. It contains two carboxyl (–COOH) groups and is also written structurally as (COOH)2.
- It occurs naturally in plants like spinach and rhubarb.
- It is a relatively strong organic acid compared to most carboxylic acids.
- It is widely used in cleaning, bleaching, and laboratory chemistry.
2. What is the chemical formula and structure of oxalic acid?
The chemical formula of oxalic acid is H2C2O4, and its structural formula is HOOC–COOH.
- It contains two carbon atoms connected by a single bond.
- Each carbon is part of a carboxyl group (–COOH).
- It is classified as a diprotic acid because it can donate two H+ ions.
3. Why is oxalic acid considered a diprotic acid?
Oxalic acid is a diprotic acid because it can donate two protons (H+) in aqueous solution.
- First ionization: H2C2O4(aq) ⇌ H+(aq) + HC2O4-(aq)
- Second ionization: HC2O4-(aq) ⇌ H+(aq) + C2O42-(aq)
- The two ionizations occur stepwise, each with its own acid dissociation constant (Ka).
4. What are the common uses of oxalic acid?
Oxalic acid is commonly used as a cleaning agent, bleaching agent, and laboratory reagent.
- Removes rust by forming soluble iron oxalate complexes.
- Used in textile and leather bleaching.
- Acts as a primary standard in acid–base titrations.
- Used in metal polishing and wood bleaching.
5. How does oxalic acid react with a base?
Oxalic acid reacts with bases in a neutralization reaction to form oxalate salts and water. For example, with sodium hydroxide:
- H2C2O4(aq) + 2NaOH(aq) → Na2C2O4(aq) + 2H2O(l)
- Two moles of NaOH are required to completely neutralize one mole of oxalic acid.
- The product Na2C2O4 is sodium oxalate.
6. How is oxalic acid prepared in the laboratory?
Oxalic acid can be prepared in the laboratory by oxidizing carbohydrates like glucose with concentrated nitric acid.
- Glucose is oxidized to form oxalic acid and other by-products.
- On cooling, oxalic acid crystallizes as H2C2O4·2H2O (oxalic acid dihydrate).
- The crystals are filtered and purified by recrystallization.
7. What happens when oxalic acid reacts with potassium permanganate?
Oxalic acid is oxidized by acidified potassium permanganate to carbon dioxide and water. The balanced reaction in acidic medium is:
- 2KMnO4(aq) + 5H2C2O4(aq) + 3H2SO4(aq) → K2SO4(aq) + 2MnSO4(aq) + 10CO2(g) + 8H2O(l)
- This reaction is used in redox titration to determine concentration.
- The purple color of KMnO4 disappears at the endpoint.
8. Is oxalic acid a strong or weak acid?
Oxalic acid is a weak acid, but it is stronger than most simple carboxylic acids.
- It partially ionizes in water.
- It is classified as a weak organic diprotic acid.
- Its first ionization is stronger than its second ionization.
9. What is the molar mass of oxalic acid?
The molar mass of oxalic acid (H2C2O4) is approximately 90 g·mol-1.
- H: 2 × 1 = 2
- C: 2 × 12 = 24
- O: 4 × 16 = 64
- Total = 2 + 24 + 64 = 90 g·mol-1
- For oxalic acid dihydrate (H2C2O4·2H2O), the molar mass is 126 g·mol-1.
10. Why is oxalic acid used as a primary standard in titration?
Oxalic acid is used as a primary standard because it is pure, stable, and has a high molar mass for accurate weighing.
- Available in pure crystalline form (often as the dihydrate).
- Does not absorb moisture significantly from air.
- Reacts completely and predictably in acid–base and redox titrations.
- Its known concentration helps standardize solutions like NaOH or KMnO4.
