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Reaction Enthalpy

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Reaction Enthalpy - Lesson Summary

The enthalpy change that accompanies a reaction is called reaction enthalpy or enthalpy of reaction. The enthalpy of a reaction is defined as the amount of heat evolved or absorbed in a chemical reaction, when the number of moles of the reactants as expressed in the chemical equation has completely reacted. Enthalpy change of a reaction is denoted by ΔrH.

Enthalpy of reaction is expressed as the
                   Reactants → Products + ΔrH

                ΔrH = ∑i  ai Hproducts - ∑i  bi Hreactants

Where, ai and bi are the stoichiometric coefficients of the products and reactants respectively in the balanced chemical equation.

The standard enthalpy of a reaction is the enthalpy change accompanying the reaction when all the reactants and products are taken in their standard states.

A substance is said to be in a standard state when it is present in its most stable state under a pressure of 1bar and at a specified temperature.

Ex: The standard state of liquid ethanol is pure liquid at 298 Kelvin and 1bar pressure.

The intermolecular forces in the three states of matter are different. Therefore, the amount of energy required to convert matter from one state to another also differs.

Standard enthalpy is represented as ΔH°.

The process by which matter is converted from one state into another is known as phase change or phase transition.

The enthalpy or heat changes accompanying these phase changes are enthalpy of fusion, enthalpy of vaporization, and enthalpy of sublimation.

Standard Enthalpy of fusion is the enthalpy change accompanying the melting of one mole of a solid substance in standard state into its liquid state and is also called as molar enthalpy of fusion and is represented as ΔfusH°.            

   Substance      ΔfusH°/KJmol-1           N 2               0.72         NH 3               5.65         HCl               1.992          CO               6.836    CH 3COCH 3               5.72       CCl 4               2.5        H 2O              6.01       NaCl              28.8        C 6H 6              9.83

Ex: The molar enthalpy of fusion of ice is +6 kJ per mole.

The amount of heat required to convert 1mole of a liquid into its vapor state at constant temperature and under standard pressure is called its standard enthalpy of vaporization or molar enthalpy of vaporisation, it is represented as ΔvapH°.

Ex: The molar enthalpy of vaporization of water is +40.7kJ per mole.

Enthalpy of sublimation is defined as the enthalpy change accompanying the conversion of one mole of a solid directly into vapor phase at constant temperature and under standard pressure.

Ex: The molar enthalpy of sublimation of iodine is +62.4kJ per mole.

The magnitude of the enthalpy change in phase transformations depends on the strength of the intermolecular interactions between the molecules in the substance undergoing phase change.


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