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Describe the Hess’s law? Why is it useful?

Describe the Hess’s law? Why is it useful?

General Chemistry - Standalone book (MindTap Course List)
General Chemistry - Standalone book (MindTap Course List)
11th Edition
ISBN:9781305580343
Author:Steven D. Gammon, Ebbing, Darrell Ebbing, Steven D., Darrell; Gammon, Darrell Ebbing; Steven D. Gammon, Darrell D.; Gammon, Ebbing; Steven D. Gammon; Darrell
Publisher:Steven D. Gammon, Ebbing, Darrell Ebbing, Steven D., Darrell; Gammon, Darrell Ebbing; Steven D. Gammon, Darrell D.; Gammon, Ebbing; Steven D. Gammon; Darrell
Chapter6: Thermochemisty
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Problem 6.15QP
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Describe the Hess’s law? Why is it useful?

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Step 1: Hess's Law

Hess's Law: This is also known as Hess's law of constant heat summation.

This law states that overall enthalpy of reaction is same, whether the reaction occurs in one step or multiple steps.

Enthalpy is a state function and does not depend upon the path of reaction.

 

 

Step 2: Application and Usefulness of Hess's Law

Suppose we need to calculate the enthalpy for following reaction:

2C(s) + H2g→C2H2(g)   ∆H = Unknown---------1

This reaction takes place in multiple steps such as:

C(s) + O2(g) →CO2(g)    ∆H=∆H1------2H2(g) + 12O2(g) →H2Ol   ∆H =∆H2------3C2H2(g) + 52O2(g)→2CO2(g) + H2Ol    ∆H=∆H3----4

Reaction 1 = Reaction 2 X 2 + Reaction 3 + (Reverse of Reaction 4) 

2C(s) +2O2(g) +H2(g) + 12O2(g) + 2CO2(g) + H2O(l)→2CO2(g) + H2O(l) + C2H2(g) + 52O2(g)

Cancel common terms in reactants and products, to get overall equation:

2C(s) + H2g→C2H2(g) 

and  ∆H =∆H1 X 2 + ∆H2 + (-∆H3)

Hence, by using Hess's Law we can determine enthalpy of overall reaction 1, that occurs in three steps as shown above.

 

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