According to hess’s law, if a series of intermediate reactions are combined, the enthalpy change of the overall reaction is

the difference between the enthalpies of the intermediate reactions.
the sum of the enthalpy changes of the intermediate reactions.
the product of the enthalpy changes of the intermediate reactions.
the fraction of the individual enthalpies of the intermediate reactions.

Answer : The correct option is, the sum of the enthalpy changes of the intermediate reactions.

Explanation :

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

For example :

The formation reaction of CO will be,

The intermediate balanced chemical reaction will be,

(1)    

(2)    

The expression for enthalpy of formation of CO is,

where,

n = number of moles

By adding reaction 1 and reverse reaction of reaction 2, we get the enthalpy of formation of CO.

Hence, the correct option is, the sum of the enthalpy changes of the intermediate reactions.

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