state and explain Hess law of constant heat summation with suitable example
Answers
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Hess's law states that, “The amount of heat evolved or absorbed in a
chemical change is the same whether the process takes place in one step
or in several steps”. (i.e. it follows 1st Law of Thermodynamics)
Applications of Hess’s law:
(i) Determination of transition
It helps in the determination of enthalpy of transition during allotropic modification.
According to Hess’s law this difference in the enthalpy of reaction represents the change.
(ii) Determination of enthalpy of formation
It helps in the determination of enthalpy of
formation which cannot be determined experimentally e.g. it is not
possible to calculate enthalpy of formation of CO experimentally, but
can be calculated by Hess’s law.
(iii) Bond Energy
It may be defined as, “The quantity of heat evolved when a bond
is formed between two free atoms in a gaseous state to form a molecular
product in a gaseous state”. It is also known as enthalpy of formation of the bond.
It may also be defined as, “The average quantity of heat required to break (dissociate) bonds of that type present in one mole of the compound”.
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❤.Hess law : "The total heat change in a reaction is the same whether the chemical reaction takes place in a single step or in several steps".
❤.It is based on the I law of thermodynamics .
