What is the primary difference between the Enthalpy of Atomisation and Bond Dissociation Enthalpy for a diatomic gas like $F_2$?

Enthalpy of atomisation is the energy to form one mole of gaseous atoms ($\frac{1}{2}F_2 \rightarrow F$), whereas bond dissociation enthalpy is the energy to break one mole of bonds ($F_2 \rightarrow 2F$). Consequently, for diatomic gases, the atomisation enthalpy is exactly half the bond enthalpy.

How does the First Electron Affinity differ from the Second Electron Affinity in terms of energy change?

The first electron affinity is usually exothermic because the neutral atom attracts an electron into its outer shell. The second electron affinity is always endothermic because energy is required to overcome the electrostatic repulsion between the negative $1-$ ion and the incoming negative electron.

What is the difference between Lattice Formation Enthalpy and Lattice Dissociation Enthalpy?

Lattice formation enthalpy is the energy released when gaseous ions combine to form one mole of a solid ionic lattice (exothermic). Lattice dissociation enthalpy is the energy required to completely separate one mole of a solid ionic lattice into its constituent gaseous ions (endothermic). They have equal magnitudes but opposite signs.

What is a common error when writing the equation for the Enthalpy of Atomisation of Bromine ($Br_2$)?

A common error is using the wrong state symbol for the reactant or failing to balance for one mole of product atoms. Since bromine is a liquid at standard conditions, the correct equation is $\frac{1}{2}Br_2(l) \rightarrow Br(g)$. Using $Br_2(g)$ would incorrectly exclude the enthalpy of vaporization.

Why is it incorrect to say the Enthalpy of Formation of $O_2(g)$ is $+142\text{ kJ mol}^{-1}$?

By definition, the standard enthalpy of formation of any element in its standard state is zero. $O_2(g)$ is the standard state of oxygen at $298\text{ K}$ and $100\text{ kPa}$, so its $\Delta H_f^\ominus$ must be zero. Any non-zero value would refer to a different process, such as atomisation.

What happens if state symbols are omitted in a Lattice Enthalpy equation?

The equation becomes thermodynamically undefined and is usually marked wrong in exams. Lattice enthalpy specifically involves gaseous ions forming a solid lattice; without $(g)$ and $(s)$ symbols, the energy change could represent different processes like enthalpy of solution or simple chemical reaction.

Define Standard Enthalpy of Formation ($\Delta H_f^\ominus$).

It is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions ($298\text{ K}$ and $100\text{ kPa}$).

Define First Ionisation Enthalpy ($\Delta H_{ie}^\ominus$).

It is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous $1+$ ions. This process is always endothermic.

Define Lattice Formation Enthalpy ($\Delta H_{latt}^\ominus$).

It is the enthalpy change when one mole of a solid ionic crystal is formed from its constituent gaseous ions under standard conditions. It is a measure of the strength of ionic bonding in the crystal.

Why is the Enthalpy of Atomisation always a positive value?

Atomisation involves breaking the bonds or attractive forces (metallic, covalent, or van der Waals) that hold atoms together in the element's standard state to produce free gaseous atoms. Since bond breaking requires an input of energy, the process is always endothermic.

Library Podcasts

Courses

Referral & Rewards

Advanced Physical Chemistry

Thermodynamic Terms

Summary

Thermodynamic terms define the specific energy changes associated with the formation, breakdown, and transformation of chemical substances under standard conditions. These definitions provide a standardized framework for calculating total energy changes in complex processes, such as the formation of ionic lattices, by breaking them down into measurable individual steps like atomisation, ionisation, and electron affinity.

1. Core Definitions and Standard Conditions

Enthalpy Change ( $\Delta H$ ) is the measure of heat energy transferred during a chemical process occurring at a constant pressure. It represents the difference between the enthalpy of the products and the enthalpy of the reactants.
Standard Conditions are strictly defined to ensure consistency in data reporting, typically involving a temperature of $298\text{ K}$ ( $25^\circ\text{C}$ ) and a pressure of $100\text{ kPa}$ . All substances must be in their most stable physical state under these conditions.
The Standard Enthalpy of Formation ( $\Delta H_f^\ominus$ ) is the energy change when exactly one mole of a compound is formed from its constituent elements in their standard states. By convention, the enthalpy of formation for any element in its standard state is exactly zero.

2. Atomisation and Bond Enthalpies

Diagram showing the energy transitions from a solid element to gaseous atoms (atomisation) and then to gaseous ions (ionisation).

3. Ionisation and Electron Affinity

4. Lattice Enthalpy

5. Key Distinctions

6. Exam Strategy & Tips