Chemical Thermodynamics: Laws of Thermochemistry and Bond Enthalpies (For CBSE, ICSE, IAS, NET, NRA 2022)

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Laws of Thermochemistry

Lavoisier- Laplace Law

According to this law, when a chemical equation is reversed, the sign of is changed.

For example,

:

:

Hess՚s Law of Constant Heat Summation

  • Hess՚s law states that the enthalpy of reaction is independent of the number and the nature of the intermediate steps.
  • The result of Hess՚s law is that thermochemical equations can be added and subtracted just like algebraic equations to get the desired reaction.
  • To obtain the enthalpy change for the reactions, we can algebraically combine known values of r H0 in the same way as the reactions themselves. Thus,

:

:

:

Bond Enthalpies

Bond enthalpy is defined as the average amount of enthalpy change involved in the dissociation of one mole of bonds present in different gaseous compounds. For example,

:

:

In case of a molecule, it is called as bond dissociation energy which is defined as the enthalpy change of the reaction in which one mole of the gaseous molecules are dissociated into gaseous atoms. For example,

:

Bond dissociation enthalpy refers to breaking a particular bond in a particular molecule whereas bond enthalpy is the average value of bond dissociation energies for a given type of bond. The bond enthalpies of some bonds are listed in below table:

Average Bond Enthalpies

Average Bond Enthalpies

By using bond enthalpies (B. E.) it is possible to estimate the energy released when a gaseous molecule is formed from its gaseous atoms. For example,

The energy released at constant pressure for this reaction is the sum of the energies of three bonds, and one bond, all values taken with a negative sign because energy is released. Thus,

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