What is Oleum?
Oleum (also known as fuming sulfuric acid) is a solution of sulfur trioxide (SO₃) dissolved in concentrated sulfuric acid (H₂SO₄). It is one of the most important industrial chemicals and a crucial intermediate in sulfuric acid production.
1. Basic Oleum Formulas
| Formula Type | Formula | Name | Explanation |
|---|---|---|---|
| Chemical Formula | H₂SO₄ · nSO₃ | Oleum (General) | Where ‘n’ represents the number of SO₃ molecules per H₂SO₄ molecule |
| Molecular Formula | H₂S₂O₇ | Disulfuric Acid | Most common form when n=1 |
| Structural Formula | HO-SO₂-O-SO₂-OH | Disulfuric Acid | Shows the bonding arrangement |
| Alternative Representation | H₂SO₄ + SO₃ | Oleum Solution | Shows composition as mixture |
2. Common Oleum Concentrations
| Oleum Type | Formula | SO₃ Content | Description |
|---|---|---|---|
| 20% Oleum | H₂SO₄ · 0.25SO₃ | 20% free SO₃ | Light fuming sulfuric acid |
| 40% Oleum | H₂SO₄ · 0.67SO₃ | 40% free SO₃ | Medium concentration oleum |
| 65% Oleum | H₂SO₄ · 1.84SO₃ | 65% free SO₃ | High concentration oleum |
| 100% Oleum | SO₃ | 100% SO₃ | Pure sulfur trioxide |
3. Formation and Preparation Formulas
| Reaction Type | Chemical Equation | Description |
|---|---|---|
| Direct Dissolution | H₂SO₄ + SO₃ → H₂S₂O₇ | SO₃ dissolves in concentrated H₂SO₄ |
| Contact Process | 2SO₂ + O₂ → 2SO₃ (then SO₃ + H₂SO₄) | Industrial production method |
| Pyrosulfuric Acid Formation | 2H₂SO₄ → H₂S₂O₇ + H₂O | Dehydration of sulfuric acid |
4. Molecular Structure and Bonding
| Aspect | Formula/Description | Details |
|---|---|---|
| Molecular Weight | H₂S₂O₇ = 178.14 g/mol | For disulfuric acid |
| Bond Angles | S-O-S ≈ 123° | Angular structure |
| Hybridization | sp³ (Sulfur atoms) | Tetrahedral around sulfur |
| Lewis Structure | [Complex structure with double bonds] | Contains S=O and S-OH bonds |
5. Chemical Properties Formulas
Hydrolysis Reactions
| Reaction | Equation | Type |
|---|---|---|
| With Water | H₂S₂O₇ + H₂O → 2H₂SO₄ | Highly exothermic |
| Stepwise Hydrolysis | SO₃ + H₂O → H₂SO₄ | Liberation of SO₃ first |
Acid-Base Properties
| Property | Formula/Value | Significance |
|---|---|---|
| pH | < 0 (extremely acidic) | Stronger than concentrated H₂SO₄ |
| Dissociation | H₂S₂O₇ ⇌ H₂SO₄ + SO₃ | Releases free SO₃ |
6. Physical Properties Formulas
| Property | Formula/Value | Units |
|---|---|---|
| Density | ρ = 1.9-2.0 | g/cm³ |
| Boiling Point | 317°C (for H₂S₂O₇) | °C |
| Melting Point | 35°C (for H₂S₂O₇) | °C |
| Vapor Pressure | High (fumes readily) | mmHg |
7. Concentration Calculations
Formula for Oleum Strength
% Free SO₃ = (Mass of free SO₃ / Total mass of oleum) × 100
Converting Between Different Expressions
| From | To | Formula |
|---|---|---|
| % SO₃ | H₂SO₄ equivalent | % H₂SO₄ = 100 + (0.225 × % SO₃) |
| Molarity | % SO₃ | Use density and molecular weights |
8. Industrial Applications Formulas
| Application | Relevant Formula | Process |
|---|---|---|
| Sulfonation | R-H + SO₃ → R-SO₃H | Organic compound sulfonation |
| Dehydration | R-OH → R + H₂O (with oleum) | Removal of water from alcohols |
| Nitration | Mixed acid preparation | HNO₃ + H₂SO₄ system |
9. Safety and Handling Formulas
| Hazard Type | Related Chemistry | Safety Note |
|---|---|---|
| Heat of Dilution | ΔH < 0 (highly exothermic) | Always add oleum to water, never reverse |
| Corrosivity | Dehydration reactions | Destroys organic matter |
| Fuming | SO₃ vapor pressure | Handle in well-ventilated areas |
Important Points for Students
Formulas to Remember:
- Basic Formula: H₂S₂O₇ (disulfuric acid)
- General Formula: H₂SO₄ · nSO₃
- Hydrolysis: H₂S₂O₇ + H₂O → 2H₂SO₄
Study Tips:
- Structure: Remember the S-O-S bridge in disulfuric acid
- Concentration: Higher % SO₃ means stronger oleum
- Safety: Always extremely careful with water contact
Common Exam Questions:
- Molecular formula and structure of oleum
- Hydrolysis reactions and products
- Industrial preparation methods
- Concentration calculations
Quick Reference
| Parameter | Value/Formula |
|---|---|
| Most Common Formula | H₂S₂O₇ |
| Alternative Name | Fuming sulfuric acid |
| Key Reaction | SO₃ + H₂SO₄ → H₂S₂O₇ |
| Molecular Weight | 178.14 g/mol |
| Industrial Importance | Sulfonation, dehydration agent |
Frequently Asked Questions (FAQs) About Oleum Formulas
1. What is the exact chemical formula of oleum?
The chemical formula of oleum depends on its concentration:
- Most common form: H₂S₂O₇ (disulfuric acid or pyrosulfuric acid)
- General formula: H₂SO₄ · nSO₃ (where n = moles of SO₃ per mole of H₂SO₄)
- Alternative representation: H₂SO₄ + SO₃
For standard oleum, H₂S₂O₇ is the accepted molecular formula, which represents one molecule of SO₃ dissolved in one molecule of H₂SO₄. In exams, if asked for “the chemical formula of oleum,” H₂S₂O₇ is the most appropriate answer.
2. How do you calculate the percentage strength of oleum?
Oleum strength is expressed as % free SO₃:
Formula: % Free SO₃ = (Mass of free SO₃ / Total mass of solution) × 100
Example Calculation:
- If 100g oleum contains 20g free SO₃
- % Free SO₃ = (20/100) × 100 = 20% oleum
Converting to H₂SO₄ equivalent:
- % H₂SO₄ equivalent = 100 + (0.225 × % SO₃)
- For 20% oleum: 100 + (0.225 × 20) = 104.5% H₂SO₄
This means 20% oleum has the same strength as 104.5% sulfuric acid (theoretical concentration).
3. What happens when oleum reacts with water? Write the formula.
When oleum reacts with water, it undergoes vigorous hydrolysis:
Main Reaction: H₂S₂O₇ + H₂O → 2H₂SO₄
Or in two steps:
- H₂S₂O₇ → H₂SO₄ + SO₃
- SO₃ + H₂O → H₂SO₄
Important Points:
- This reaction is highly exothermic (releases tremendous heat)
- Can cause explosive boiling if water is added to oleum
- SAFETY RULE: Always add oleum slowly to water, never water to oleum
- Final product is concentrated sulfuric acid (H₂SO₄)
4. What is the difference between oleum and sulfuric acid formula?
Main differences in formulas and composition:
| Aspect | Sulfuric Acid | Oleum |
|---|---|---|
| Formula | H₂SO₄ | H₂S₂O₇ or H₂SO₄·SO₃ |
| Composition | Pure sulfuric acid | SO₃ dissolved in H₂SO₄ |
| % Strength | Up to 98% (concentrated) | 100%+ (as H₂SO₄ equivalent) |
| State | Oily liquid, non-fuming | Fuming liquid (releases SO₃) |
| Reactivity | Strong acid | Even stronger oxidizing agent |
| Water Content | Contains ~2% water | Contains excess SO₃, no free water |
Note: Oleum is essentially sulfuric acid with extra SO₃ dissolved in it, making it “super-concentrated.”
5. How is oleum prepared? What is the formation formula?
Oleum is prepared by dissolving sulfur trioxide in concentrated sulfuric acid:
Primary Formation Reaction: H₂SO₄ (conc.) + SO₃ → H₂S₂O₇ (oleum)
Industrial Preparation (Contact Process):
Step 1: Sulfur dioxide formation
- S + O₂ → SO₂
Step 2: Catalytic oxidation
- 2SO₂ + O₂ → 2SO₃ (V₂O₅ catalyst, 450°C)
Step 3: Absorption in H₂SO₄
- SO₃ + H₂SO₄ → H₂S₂O₇
Alternative Method (Laboratory):
- Dehydration: 2H₂SO₄ → H₂S₂O₇ + H₂O (requires heating and P₂O₅)
The Contact Process is the standard industrial method for commercial oleum production.
6. What is the structural formula of oleum? How to draw it?
The structural formula shows the arrangement of atoms in oleum (H₂S₂O₇):
Linear Representation: HO—SO₂—O—SO₂—OH
Detailed Structure:
O O
║ ║
H—O—S—O—S—O—H
║ ║
O OStructural Features:
- Two sulfur atoms connected by an oxygen bridge (S—O—S)
- Each sulfur has:
- Two double-bonded oxygen atoms (S=O)
- One bridging oxygen (S—O—S)
- One hydroxyl group (—OH)
- Bond angles: Approximately 109.5° around sulfur (sp³ hybridization)
- Central bridge: S—O—S angle ≈ 123°
Drawing Tips for Exams:
- Show all oxygen atoms clearly
- Indicate double bonds (S=O) with double lines
- Mark the bridging oxygen connecting two sulfur atoms
- Add OH groups at both ends