How to Calculate Enthalpy of Atomisation with Examples and Trends
Enthalpy of Atomisation is essential in chemistry and helps students understand various practical and theoretical applications related to chemical bonding, thermodynamics, and periodic properties. This concept is especially important for students preparing for exams and board assessments.
What is Enthalpy of Atomisation in Chemistry?
The enthalpy of atomisation is the heat energy required to break all the bonds in one mole of a substance (element or compound) in its standard state, so that it forms its constituent atoms in the gaseous state.
This value is usually expressed in kilojoules per mole (kJ/mol). It is a key concept in thermodynamics and appears in chapters related to thermodynamics, chemical bonding, and periodic trends.
Molecular Formula and Composition
Since enthalpy of atomisation is a thermodynamic property, it does not have a chemical molecular formula, but refers to the change from the original molecule (solid, liquid, or gas) to its individual atoms in the gaseous phase. For example, for hydrogen gas (H2), atomisation refers to H2(g) → 2H(g).
Preparation and Synthesis Methods
In laboratory or industry, the conversion of a substance into its atoms often involves breaking all chemical bonds by supplying energy (usually heat). For metals, this includes breaking metallic bonds in the crystal lattice.
For covalent molecules, all covalent bonds must be broken. This process can occur via direct heating, high-energy plasma, or special reactions under controlled pressure and temperature.
Physical Properties of Enthalpy of Atomisation
The enthalpy of atomisation is always a positive value (endothermic), because energy is required to break chemical bonds. Units: kilojoules per mole (kJ/mol). The magnitude depends on the type of bonding and element or compound in question. For example, iron (Fe) has a high enthalpy of atomisation because of strong metallic bonds, while chlorine (Cl2) has a lower value due to weaker intermolecular forces.
Chemical Properties and Reactions
Enthalpy of atomisation is not a reactive property itself, but it represents the amount of energy needed for reactions that completely break all bonds in a molecule or solid structure.
For diatomic molecules (like O2, Cl2), the process involves splitting the bond into individual atoms. For metals, it involves overcoming strong metallic bonding to yield atomised gas-phase atoms.
Frequent Related Errors
- Confusing enthalpy of atomisation with ionisation enthalpy (which is the energy needed to remove one electron from a gaseous atom).
- Assuming enthalpy of atomisation can be negative—it is always positive.
- Not accounting for the physical state of the starting material (solid, liquid, gas).
- Mixing up atomisation with sublimation or vaporisation enthalpies.
Uses of Enthalpy of Atomisation in Real Life
Enthalpy of atomisation is widely used in industries like metallurgy, where strong bonds in metals are broken to purify or process metals. It also helps in determining the stability and strength of chemical bonds in various materials, and is referenced in the production of synthetic chemicals and materials.
Relation with Other Chemistry Concepts
Enthalpy of atomisation is closely related to bond dissociation enthalpy, ionisation enthalpy, and enthalpy of sublimation. For some metals, the enthalpy of atomisation equals the enthalpy of sublimation. It also links thermodynamics with periodic trends—for example, transition metals (3d series) have especially high atomisation energies due to strong metallic bonding.
Step-by-Step Reaction Example
1. For hydrogen gas:H2(g) → 2H(g) ΔH0atom = +435 kJ/mol
This indicates that 435 kJ of energy is required to break one mole of H2 molecules into two moles of hydrogen atoms.
2. For metals like iron:
Fe(solid) → Fe(gas, atomised) ΔH0atom = +418 kJ/mol
This is the energy needed to convert solid iron into individual iron atoms in the gaseous state.
Lab or Experimental Tips
Remember that enthalpy of atomisation is always measured under standard conditions (usually 298 K, 1 bar pressure), and always represents the energy absorbed, not released. Vedantu educators often use the visual cue of "breaking all bonds → forming atoms" for easy recall during live classes.
Try This Yourself
- Write the atomisation reaction for 1 mole of Cl2(g).
- Determine whether the enthalpy of atomisation is higher for Na(s) or Fe(s), and explain why based on their bonding.
- State an everyday application where atomisation enthalpy affects material properties.
Final Wrap-Up
We explored enthalpy of atomisation—its key definition, calculation, trends across the periodic table, and real-world relevance. Mastering this topic is essential for excelling in chemistry exams and understanding advanced concepts. For more in-depth help, check Vedantu's live expert classes and revision notes.
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FAQs on Enthalpy of Atomisation Explained
1. What is meant by enthalpy of atomisation in Chemistry?
Enthalpy of atomisation is the energy required to convert one mole of an element in its standard state into gaseous atoms.
- It is always expressed in kJ mol-1.
- Represents the energy needed to break all bonds in one mole of the substance to form individual gaseous atoms.
2. Is the enthalpy of atomisation always positive or negative?
Enthalpy of atomisation is always positive, because energy must be supplied to break chemical bonds.
- This process is endothermic—heat energy is absorbed from the surroundings.
3. How do you calculate the enthalpy of atomisation?
The enthalpy of atomisation can be calculated using the following steps:
- Write the balanced equation converting the substance to its gaseous atoms.
- Use the appropriate enthalpy values, such as bond enthalpy or sublimation enthalpy.
- Formula for a diatomic molecule (e.g., H2):
ΔHatomisation = (Bond dissociation enthalpy) / Number of moles
4. What is the trend of enthalpy of atomisation in the 3d series of transition metals?
Enthalpy of atomisation in the 3d series rises from Scandium and peaks at Chromium and Manganese, then decreases towards Zinc.
- Highest values: Cr, Fe, Mn
- Trend reflects the maximum number of unpaired d electrons and strong metallic bonding.
5. How is enthalpy of atomisation related to bond enthalpy?
Enthalpy of atomisation and bond enthalpy both measure the energy needed to break chemical bonds, but:
- Bond enthalpy: Energy to break one specific bond in a molecule.
- Enthalpy of atomisation: Total energy to break all bonds in one mole of a substance to give gaseous atoms.
6. What is the difference between enthalpy of atomisation and ionisation enthalpy?
Enthalpy of atomisation is the energy to break a substance into gaseous atoms, while ionisation enthalpy is the energy required to remove an electron from a gaseous atom.
- Atomisation: Solid/element → gaseous atoms
- Ionisation: Gaseous atom → gaseous ion + electron
7. Why do transition metals generally have higher enthalpy of atomisation than s- or p-block elements?
Transition metals have higher enthalpy of atomisation because:
- They possess more unpaired d electrons, resulting in stronger metallic bonding.
- Their atoms are closely packed, needing more energy to separate into atoms.
8. What is the significance of enthalpy of atomisation in metallurgy?
Enthalpy of atomisation is important in metallurgy because:
- It explains the energy required to break metallic bonds during extraction and purification of metals.
- Higher atomisation enthalpy means stronger bonding and greater stability of metals.
9. Can enthalpy of atomisation be negative for any substance?
No, enthalpy of atomisation is always positive because breaking bonds to form free atoms requires energy input; the process is endothermic for all elements.
10. How does enthalpy of atomisation influence the physical properties of metals?
Enthalpy of atomisation is directly linked to the strength of metallic bonding:
- Metals with high atomisation enthalpy generally have higher melting points, hardness, and electrical conductivity.
- It indicates the stability of the metal structure.
11. How is enthalpy of atomisation different from enthalpy of sublimation?
Enthalpy of atomisation measures the energy needed to break all bonds and form gaseous atoms, while enthalpy of sublimation is the energy required to convert a solid directly to gas without breaking molecules into atoms.
- Atomisation: Always yields atoms.
- Sublimation: May yield molecules in the gas phase.
12. In which industrial processes is knowledge of enthalpy of atomisation crucial?
Understanding enthalpy of atomisation is crucial for:
- Metal extraction and refining in metallurgy
- Designing catalysts in chemical industries
- Material science research on alloys and high-strength materials
