Chemical Thermodynamics: Types of Standard Enthalpy of Reactions (For CBSE, ICSE, IAS, NET, NRA 2022)

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Standard Enthalpy of Reactions

It is defined as the enthalpy for a reaction when the reactants and the products are in their standard states.

Standard Enthalpy change is denoted by .

Let as denote total enthalpy of reactants as and total enthalpy products as . The difference between these enthalpies, , is the enthalpy of the reaction.

When, is greater than then is positive, and heat is absorbed in the reaction, and the reaction will be endothermic. When is less than then is negative, and heat is evolved in the reaction and the reaction will be exothermic.

Enthalpy of Formation

  • The enthalpy change when one mole of a pure compound is formed from its elements in their most stable states is called the enthalpy of formation and is denoted by .
  • When the reacting elements and the products formed are all in their standard states, the enthalpy change accompanying the chemical reaction is called the standard enthalpy of formation and is denoted by .

For example,

Enthalpy of Combustion

Enthalpy of combustion is the enthalpy change (heat evolved) accompanying the complete combustion of 1 mole of a substance in oxygen at a given temperature and 1 bar pressure.

For example,

.

Enthalpy of Neutralization

Enthalpy of neutralization is the enthalpy change (heat evolved) when one mole of hydrogen ions is neutralized by one mole of hydroxyl ions in dilute aqueous medium to form water.

For example:

Enthalpy of Atomization

It is the change in enthalpy when one mole of a substance is converted into its atoms in gaseous state at a given temperature and 1 bar pressure.

For Example,

:

:

:

Enthalpy of Transition

It is the change in enthalpy when one mole of substance changes from one phase to another at a constant temperature and under 1 bar pressure.

Phase transition/transformation is includes the following:

Phase Transition/Transformation is Includes the Following
Name of ProcessProcessSpecific symbol
(i) SublimationSolid Gas
(ii) VaporizationLiquid Gas
(iii) FusionSolid Liquid
(iv) TransitionSolid (one crystalline form) Solid (another crystalline form)

Examples,

1. :

2. :

3. :

4. :

Enthalpy of Solution

It is the change in enthalpy when one mole of a solute is dissolved in a specific amount of a solvent at a constant temperature and under 1 bar pressure.

Example,

1.

2.

Enthalpy of Ionization

It is the change in enthalpy when one mole of a weak electrolyte ionizes completely in its solution at a constant temperature and under 1 bar pressure .

Example,

:

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