The Aluminium Chloride Formula is written as AlCl3, representing one aluminium atom bonded to three chlorine atoms. This compound is a key topic in NCERT Class 12 Chemistry and appears regularly in CBSE board exams. It is also tested in JEE Main and NEET under inorganic chemistry and Lewis acid-base concepts. This article covers the formula, structure, properties, preparation methods, solved examples, and exam tips in full detail.
Key Aluminium Chloride Formulas at a Glance
Quick reference for the most important formulas and expressions related to AlCl3.
- Molecular formula: \( \text{AlCl}_3 \)
- Dimerisation: \( 2\,\text{AlCl}_3 \rightarrow \text{Al}_2\text{Cl}_6 \)
- Preparation from elements: \( 2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3 \)
- Reaction with water: \( \text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl} \)
- Molar mass: \( M = 133.34 \text{ g/mol} \)
- Lewis acid behaviour: AlCl3 accepts an electron pair from a Lewis base
What is Aluminium Chloride?
The Aluminium Chloride Formula, AlCl3, represents a compound formed by the direct combination of aluminium and chlorine. It is also called aluminium trichloride or aluminium(III) chloride. Aluminium belongs to Group 13 of the periodic table and has a valency of +3. Chlorine has a valency of -1. Three chlorine atoms are needed to balance one aluminium atom, giving the formula AlCl3.
In its pure form, aluminium chloride is a white crystalline solid. Impurities such as iron(III) chloride can give it a yellowish tint. The compound is covalent in nature when solid. It has relatively low melting and boiling points compared to ionic compounds of similar size.
Aluminium chloride is covered in NCERT Class 12 Chemistry, Chapter 6 (The p-Block Elements). It is a classic example of a Lewis acid because the aluminium atom has an incomplete octet and can accept an electron pair. This property makes it highly useful as a catalyst in organic reactions such as Friedel-Crafts alkylation and acylation.
The compound dissolves readily in polar solvents and undergoes hydrolysis in water to produce hydrochloric acid and aluminium hydroxide. Its industrial importance spans aluminium metal production, pharmaceuticals, dyes, and petrochemical refining.
Aluminium Chloride Formula — Expression and Variables
The molecular formula of aluminium chloride is:
\[ \text{AlCl}_3 \]
In the vapour phase or in non-polar solvents, two AlCl3 molecules combine to form a dimer:
\[ 2\,\text{AlCl}_3 \rightarrow \text{Al}_2\text{Cl}_6 \]
This dimer, Al2Cl6, allows each aluminium atom to achieve a complete octet through coordinate (dative) bonds. The dimer structure is important for JEE and NEET questions on bonding.
| Symbol / Formula | Quantity / Meaning | Value / Unit |
|---|---|---|
| AlCl3 | Molecular formula of aluminium chloride | — |
| Al2Cl6 | Dimeric form (vapour phase) | — |
| M | Molar mass of AlCl3 | 133.34 g/mol |
| Al | Aluminium atom (oxidation state) | +3 |
| Cl | Chlorine atom (oxidation state) | -1 |
| Melting point | Temperature at which AlCl3 melts (under pressure) | 192.6 °C |
| Boiling point | Temperature at which AlCl3 sublimes | 180 °C (sublimes) |
| Density | Physical density of solid AlCl3 | 2.48 g/cm³ |
Derivation of the Formula
To derive the Aluminium Chloride Formula, we use the criss-cross method of valency.
Step 1: Identify the valency of each element. Aluminium (Al) has a valency of 3. Chlorine (Cl) has a valency of 1.
Step 2: Apply the criss-cross rule. The valency of Al (3) becomes the subscript of Cl. The valency of Cl (1) becomes the subscript of Al.
Step 3: Simplify if possible. Al1Cl3 simplifies to AlCl3. The formula is therefore \( \text{AlCl}_3 \).
Step 4: Verify charge balance. Al contributes +3. Three Cl atoms contribute \( 3 \times (-1) = -3 \). Net charge = 0. The formula is electrically neutral and correct.
Complete Inorganic Chemistry Formula Sheet
The table below lists important inorganic compound formulas related to aluminium chloride and p-block chemistry. These are frequently tested in CBSE board exams and JEE/NEET.
| Compound Name | Formula | Key Variables / Notes | Molar Mass (g/mol) | NCERT Chapter |
|---|---|---|---|---|
| Aluminium Chloride | \( \text{AlCl}_3 \) | Al = +3, Cl = -1; Lewis acid; forms dimer Al2Cl6 | 133.34 | Class 12, Ch 6 |
| Aluminium Oxide | \( \text{Al}_2\text{O}_3 \) | Amphoteric oxide; used in thermite reaction | 101.96 | Class 12, Ch 6 |
| Aluminium Hydroxide | \( \text{Al(OH)}_3 \) | Amphoteric hydroxide; antacid | 78.00 | Class 12, Ch 6 |
| Silicon Dioxide | \( \text{SiO}_2 \) | Acidic oxide; giant covalent structure | 60.08 | Class 12, Ch 6 |
| Bleaching Powder | \( \text{Ca(OCl)Cl} \) | Contains hypochlorite ion; oxidising agent | 127.00 | Class 12, Ch 7 |
| Ammonia | \( \text{NH}_3 \) | Lewis base; trigonal pyramidal geometry | 17.03 | Class 12, Ch 7 |
| Ferric Chloride | \( \text{FeCl}_3 \) | Iron(III) chloride; common impurity in AlCl3 | 162.20 | Class 12, Ch 8 |
| Sodium Chloride | \( \text{NaCl} \) | Ionic compound; Na = +1, Cl = -1 | 58.44 | Class 11, Ch 2 |
| Hydrochloric Acid | \( \text{HCl} \) | Strong acid; product of AlCl3 hydrolysis | 36.46 | Class 12, Ch 7 |
| Folic Acid | \( \text{C}_{19}\text{H}_{19}\text{N}_7\text{O}_6 \) | Vitamin B9; used in pharmaceutical synthesis involving AlCl3 | 441.40 | Class 12, Ch 14 |
Aluminium Chloride Formula — Properties
Understanding the physical and chemical properties of AlCl3 is essential for CBSE board exams and competitive tests. We have grouped these properties below for easy revision.
Physical Properties
| Property | Value / Description |
|---|---|
| Appearance | White crystalline solid (yellow if impure) |
| Odour | Pungent (due to HCl release on hydrolysis) |
| Molar Mass | 133.34 g/mol |
| Density | 2.48 g/cm³ |
| Melting Point | 192.6 °C (under pressure) |
| Boiling Point / Sublimation | Sublimes at 180 °C at 1 atm |
| Solubility | Highly soluble in water; soluble in ethanol and ether |
| Nature of Bond | Covalent (solid state); coordinate bond in dimer |
Chemical Properties
1. Hydrolysis: AlCl3 reacts vigorously with water to produce aluminium hydroxide and hydrochloric acid.
\[ \text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl} \]
2. Lewis Acid Behaviour: The aluminium atom in AlCl3 has only 6 electrons in its valence shell. It accepts an electron pair from a Lewis base (donor). This is the basis of Friedel-Crafts reactions.
3. Dimerisation: In the vapour phase, two AlCl3 molecules form a dimer Al2Cl6 through coordinate bonds. Each Al achieves an octet.
\[ 2\,\text{AlCl}_3 \rightarrow \text{Al}_2\text{Cl}_6 \]
4. Reaction with Ammonia: AlCl3 reacts with ammonia (a Lewis base) to form an adduct.
\[ \text{AlCl}_3 + \text{NH}_3 \rightarrow \text{AlCl}_3 \cdot \text{NH}_3 \]
Aluminium Chloride Preparation
AlCl3 can be prepared by several methods. NCERT Class 12 Chapter 6 describes the direct combination method as the primary route.
Method 1 — Direct Combination: Aluminium metal is heated in a stream of dry chlorine gas. The reaction is highly exothermic.
\[ 2\text{Al}(s) + 3\text{Cl}_2(g) \rightarrow 2\text{AlCl}_3(s) \]
Method 2 — Reaction of Aluminium with Hydrogen Chloride: Dry HCl gas is passed over heated aluminium metal.
\[ 2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2 \]
Method 3 — Industrial Route: In industry, AlCl3 is produced by the chlorination of aluminium oxide (Al2O3) in the presence of carbon at high temperatures.
\[ \text{Al}_2\text{O}_3 + 3\text{C} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3 + 3\text{CO} \]
All preparation methods must be carried out under anhydrous (water-free) conditions. This is because AlCl3 hydrolyses immediately in the presence of moisture to give HCl fumes.
Aluminium Chloride Formula — Solved Examples
Example 1 (Class 9-10 Level) — Molar Mass Calculation
Problem: Calculate the molar mass of aluminium chloride (AlCl3). Use atomic masses: Al = 27 g/mol, Cl = 35.5 g/mol.
Given: Atomic mass of Al = 27 g/mol; Atomic mass of Cl = 35.5 g/mol; Formula = AlCl3
Step 1: Write the formula: \( \text{AlCl}_3 \) contains 1 Al atom and 3 Cl atoms.
Step 2: Multiply each atomic mass by its subscript.
Mass from Al: \( 1 \times 27 = 27 \text{ g/mol} \)
Mass from Cl: \( 3 \times 35.5 = 106.5 \text{ g/mol} \)
Step 3: Add all contributions.
\( M = 27 + 106.5 = 133.5 \text{ g/mol} \)
Answer
Molar mass of AlCl3 = 133.5 g/mol (standard value: 133.34 g/mol).
Example 2 (Class 11-12 Level) — Moles and Mass Calculation
Problem: How many moles of AlCl3 are present in 26.7 g of aluminium chloride? Also find the number of Al3+ ions present. (Molar mass of AlCl3 = 133.5 g/mol; Avogadro’s number = \( 6.022 \times 10^{23} \))
Given: Mass = 26.7 g; M(AlCl3) = 133.5 g/mol; NA = \( 6.022 \times 10^{23} \) mol⁻¹
Step 1: Use the mole formula.
\[ n = \frac{\text{mass}}{\text{molar mass}} = \frac{26.7}{133.5} \]
Step 2: Calculate the number of moles.
\( n = 0.2 \text{ mol} \)
Step 3: Each formula unit of AlCl3 contains 1 Al3+ ion. So the number of Al3+ ions is:
\[ N = n \times N_A = 0.2 \times 6.022 \times 10^{23} \]
Step 4: Calculate.
\( N = 1.2044 \times 10^{23} \) ions
Answer
Number of moles of AlCl3 = 0.2 mol. Number of Al3+ ions = 1.2044 × 10²³.
Example 3 (JEE/NEET Level) — Lewis Acid and Hydrolysis
Problem: 0.1 mol of AlCl3 is dissolved in excess water. (a) Write the balanced hydrolysis equation. (b) Calculate the mass of HCl produced. (c) Explain why AlCl3 acts as a Lewis acid in Friedel-Crafts reactions. (Molar mass of HCl = 36.5 g/mol)
Given: Moles of AlCl3 = 0.1 mol; M(HCl) = 36.5 g/mol
Step 1: Write the balanced hydrolysis equation.
\[ \text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl} \]
Step 2: Find moles of HCl produced. From stoichiometry, 1 mol AlCl3 produces 3 mol HCl.
\( n(\text{HCl}) = 0.1 \times 3 = 0.3 \text{ mol} \)
Step 3: Calculate mass of HCl.
\[ m(\text{HCl}) = n \times M = 0.3 \times 36.5 = 10.95 \text{ g} \]
Step 4: Explain Lewis acid behaviour. In AlCl3, the aluminium atom has only 6 electrons in its valence shell (3 bond pairs with 3 Cl atoms). It has an empty p-orbital. It accepts an electron pair from a Lewis base (e.g., the pi electrons of benzene ring in Friedel-Crafts reactions). This generates a carbocation intermediate that attacks the benzene ring.
Answer
(a) \( \text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl} \)
(b) Mass of HCl produced = 10.95 g.
(c) AlCl3 acts as a Lewis acid because Al has an incomplete octet (empty p-orbital) and accepts electron pairs from Lewis bases, generating reactive electrophilic intermediates.
CBSE Exam Tips 2025-26
- Memorise the formula AlCl3 and its molar mass (133.34 g/mol). CBSE 2025-26 papers frequently ask one-mark formula-writing questions from p-Block Elements.
- Write balanced equations for preparation and hydrolysis. Always balance the hydrolysis equation: 1 AlCl3 gives 3 HCl. Unbalanced equations lose marks.
- Know both forms — monomer and dimer. AlCl3 exists as a monomer in solution and as a dimer (Al2Cl6) in the vapour phase. CBSE 3-mark questions often ask about this.
- State the Lewis acid nature clearly. When explaining Lewis acid behaviour, always mention the empty p-orbital of Al and its ability to accept an electron pair. We recommend writing one sentence on this in every answer.
- Distinguish between anhydrous and aqueous AlCl3. Anhydrous AlCl3 is covalent. In water it ionises and hydrolyses. This distinction is tested in 2-mark questions.
- Link to Friedel-Crafts reactions for bonus marks. Mentioning the role of AlCl3 as a catalyst in Friedel-Crafts alkylation or acylation can earn you extra marks in 5-mark organic chemistry questions.
Common Mistakes to Avoid
- Writing the formula as Al3Cl instead of AlCl3. This is a criss-cross error. Always write the metal symbol first, then the non-metal. Apply the criss-cross method correctly: Al (valency 3) and Cl (valency 1) give AlCl3.
- Confusing the dimer formula. Some students write Al2Cl3 as the dimer. The correct dimer formula is Al2Cl6. Two AlCl3 units combine, so all subscripts double.
- Ignoring anhydrous conditions in preparation. A common error is writing the preparation reaction without noting that dry conditions are essential. AlCl3 reacts with moisture immediately. Always state “anhydrous” or “dry” conditions.
- Incorrect hydrolysis equation. Students often write 1 HCl instead of 3 HCl in the hydrolysis product. Remember: 3 chloride ions produce 3 HCl molecules.
- Calling AlCl3 an ionic compound. In the solid state, AlCl3 is covalent, not ionic. Its low melting and boiling points confirm this. Only at very high temperatures or in polar solvents does it show ionic behaviour.
JEE/NEET Application of Aluminium Chloride Formula
In our experience, JEE aspirants encounter the Aluminium Chloride Formula in at least two to three questions per year across JEE Main and Advanced papers. NEET aspirants see it in inorganic chemistry and biomolecule-related synthesis questions. Here are the key application patterns.
Pattern 1: Lewis Acid-Base Theory
JEE Main regularly tests whether students can identify Lewis acids and bases. AlCl3 is a textbook Lewis acid. Questions ask students to identify the Lewis acid in a given reaction or to explain why AlCl3 catalyses Friedel-Crafts reactions. The key point is the empty p-orbital on aluminium. In our experience, students who explicitly mention the empty orbital score full marks, while those who only say “accepts electrons” score partial marks.
Pattern 2: Stoichiometry and Mole Concept
JEE Main frequently uses AlCl3 in mole concept problems. A typical question gives the mass of AlCl3 and asks for the number of moles of Cl⁻ ions or the volume of HCl gas produced on hydrolysis. The stoichiometry \( \text{AlCl}_3 \rightarrow \text{Al}^{3+} + 3\text{Cl}^- \) is essential. Each mole of AlCl3 produces 3 moles of Cl⁻ ions.
Pattern 3: Structure and Bonding
JEE Advanced tests the dimer structure of AlCl3. Students must know that in the vapour phase, AlCl3 dimerises to Al2Cl6 through two coordinate bonds. Each Al forms one coordinate bond by accepting a lone pair from a bridging Cl atom of the other AlCl3 unit. This gives each Al a complete octet. NEET occasionally asks about the hybridisation of Al in AlCl3 (sp2, trigonal planar) and in the dimer (sp3, tetrahedral).
Pattern 4: Industrial and Applied Chemistry
NEET questions on applied chemistry sometimes ask about the industrial uses of AlCl3. Key uses include: catalyst in Friedel-Crafts reactions, production of aluminium metal (Hall-Héroult process uses Al2O3, but AlCl3 is an intermediate in some routes), antiperspirant formulations, and as a reagent in dye and pharmaceutical manufacturing.
FAQs on Aluminium Chloride Formula
Explore more chemistry formulas on our Chemistry Formulas hub. For related compound formulas, visit our articles on the Ammonia Formula, the Silicon Dioxide Formula, and the Bleaching Powder Formula. These compounds are all covered in NCERT Class 12 Chapter 6 and 7 and are frequently tested alongside AlCl3 in CBSE board exams and JEE/NEET. For the official NCERT syllabus, refer to ncert.nic.in.