Master Thermochemistry! Every chemical reaction and physical change is accompanied by an exchange of heat. By defining specific types of Enthalpy Changes (ΔH), chemists can calculate the energy requirements for complex reactions using Hess's Law.

Different Types of Enthalpies of Reaction

Enthalpy of reaction (denoted as Δ_rH) is the total amount of heat absorbed or evolved during a chemical reaction at constant pressure. To standardize these measurements, scientists evaluate reactions under standard conditions (298 K and 1 bar pressure), giving us the Standard Enthalpy of Reaction (Δ_rH°).

To make studying chemistry easier, we classify these enthalpies based on the specific type of reaction or physical change taking place. Let's do a quick, high-yield revision of all the major types!

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Figure 1: Summary chart of standard enthalpies in chemical reactions.

1. Primary Chemical Enthalpies

Standard Enthalpy of Formation (Δ_fH°)

The enthalpy change that occurs when exactly 1 mole of a compound is formed from its constituent elements in their most stable, standard states.

• Rule: The Δ_fH° of naturally occurring elements in their standard state (like O₂(g), C(graphite), Br₂(l)) is exactly zero.
• It can be either exothermic (-) or endothermic (+).

Standard Enthalpy of Combustion (Δ_cH°)

The enthalpy change when exactly 1 mole of a substance is completely burnt in the presence of excess oxygen.

• Rule: Combustion is a burning process, so this value is always negative (highly exothermic).
• Used extensively to calculate the calorific value of fuels and foods.

Standard Enthalpy of Neutralization (Δ_neutH°)

The heat evolved when 1 gram equivalent of an acid is completely neutralized by 1 gram equivalent of a base in a dilute aqueous solution.

• For any Strong Acid + Strong Base, the value is universally constant: -57.1 kJ/mol (because it is simply the reaction of H⁺ + OH^- → H₂O).
• If either the acid or base is weak, the heat evolved is less than 57.1 kJ/mol because some energy is consumed to fully dissociate the weak electrolyte.

Figure 2: Types of enthalpies related to state changes, atomization, and dissolution.

2. Phase Changes and Atomization

Enthalpy of Atomization (Δ_aH°)

The enthalpy change when exactly 1 mole of a given substance is completely broken down into individual, independent atoms in the gaseous phase.

Since breaking bonds requires energy, this process is always endothermic (+).

Enthalpies of Phase Transition

When matter changes state without breaking chemical bonds, the heat exchange is captured by these parameters (all are usually endothermic as they overcome intermolecular forces):

• Enthalpy of Fusion (Δ_fusH°): Heat required to melt 1 mole of a solid into a liquid at its melting point.
• Enthalpy of Vaporization (Δ_vapH°): Heat required to boil 1 mole of a liquid into a gas at its boiling point.
• Enthalpy of Sublimation (Δ_subH°): Heat required to convert 1 mole of a solid directly into a gas. According to Hess's Law: Δ_subH = Δ_fusH + Δ_vapH.

Enthalpies of Solution and Hydration

• Enthalpy of Solution (Δ_solH°): The heat change when 1 mole of a solute is completely dissolved in a specified amount of solvent. It can be exothermic (like dissolving NaOH) or endothermic (like dissolving NH₄NO₃).
• Enthalpy of Hydration (Δ_hydH°): The enthalpy change when 1 mole of completely isolated gaseous ions is dissolved in water to form hydrated ions. This process involves the formation of ion-dipole bonds with water and is always exothermic (-).

Frequently Asked Questions (FAQs)

What is the standard enthalpy of formation of an element?

By strict thermodynamic convention, the standard enthalpy of formation (Δ_fH°) of any element in its most stable, naturally occurring reference state (e.g., O₂ gas, Carbon graphite, Br₂ liquid) is taken as exactly zero.

Why is the enthalpy of combustion always negative?

Combustion is an oxidative process of burning a substance (fuel) in the presence of oxygen. Because this process breaks relatively weaker bonds in the reactants and forms highly stable, strong bonds in the products (like CO₂ and H₂O), it always releases excess heat energy into the surroundings. Thus, the enthalpy change (Δ_cH) is universally negative (exothermic).

What is the enthalpy of neutralization for strong acids and bases?

The standard enthalpy of neutralization for any strong acid (like HCl) reacting with any strong base (like NaOH) is universally constant at approximately -57.1 kJ/mol (or -13.7 kcal/mol). This is because both are completely ionized in water, so the only actual reaction occurring is the formation of 1 mole of water from H⁺ and OH^- ions.

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