Hess's Law

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Dhamnekar Winod
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Hess's Law

Post by Dhamnekar Winod »

Germain Henri Hess reported calorimetric analyses of a number of reactions of sulfuric acid and several of its hydrates. Among them were
\((9) H_2SO_4 + 2 H_2O \rightarrow H_2SO_4.2H_2O\)
\((10) H_2SO_4.H_2O + H_2O \rightarrow H_2SO_4.2H_2O\)

(The numbering is from Hess's paper.) He reported that the heat evolved in reaction (9) was 116.7 and that of reaction (10) 38.56. (The units are not important, but they refer to a fixed quantity of H2SO4.) Use Hess's law to predict the heat evolved in the following reaction:

\((7) H_2SO_4 + H_2O \rightarrow H_2SO_4.H_2O\)

How to answer this question?
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
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ChenBeier
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Re: Hess's Law

Post by ChenBeier »

You have to do the difference between the two energies.
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Dhamnekar Winod
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Re: Hess's Law

Post by Dhamnekar Winod »

So, we get 116.7 - 38.56 = 78.14. (Unit of energy is unknown).
How can we say confidently without any experiment that this aforesaid answer is correct? :? :?: :o
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
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ChenBeier
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Re: Hess's Law

Post by ChenBeier »

Mr. Hess found out 150 years ago. So we can be confident its correct.
The units are something like joule/mol
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Dhamnekar Winod
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Re: Hess's Law

Post by Dhamnekar Winod »

I got detailed answer to this question given by chemistry professor as follows:

\((9) H_2SO_4 + 2H_2O \rightarrow H_2SO_4\cdot 2H_2O \Delta H= 116.7 kJ/mol\)
\(Reverse(10)H_2SO_4\cdot2H_2O \rightarrow H_2SO_4\cdot H_2O + H_2O \Delta H =-38.56 kJ/mol\) Change sign


Adding these two reactions to obtain...
\(H_2SO_4 + 2H_2O + H_2SO_4\cdot 2H_2O \rightarrow H_2SO_4\cdot 2H_2O + H_2SO_4\cdot H_2O + H_2O\)

The net reaction is \(H_2SO_4 + H_2O \rightarrow H_2SO_4 \cdot H_2O\)

\(\Delta H =116.7-38.56=78.14 kJ/mol\)
Last edited by Dhamnekar Winod on Fri Feb 05, 2021 5:45 am, edited 1 time in total.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
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ChenBeier
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Re: Hess's Law

Post by ChenBeier »

Its no new information. Its the law from Hess.
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