# Enthalpy of A Reaction Using Hess' Law

This solution will discuss how to determine the enthalpy of a chemical reaction primarily using Hess's law. The use of the general formula for enthalpy will also be addressed. These problems will be solved using both Hess's law and the general formula for enthalpy. A step by step example and answers are included.

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Calculating Entropy using Hess's law

Find the enthalpy of the following reactions:

NOg + Â½ o2 NO2

Cgraphite C diamond

5C(s) +6H2(g) C5H12

1. There are 2 ways to solve these problems. One can solve them using Hess's law or one can solve them using the following formula:

âˆ‘ { (Î”H products) x (mol products)} - âˆ‘{ ( Î”H reactants) x ( mol reactants)}

I will solve them using hess's law and I will use the above formula to check my answers for some of the problems.

Formation of NO2

Add Î”H of each component reaction. Don't forget to reverse the reaction if necessary to obtain the desired product. If one reverses the reaction, one needs to assign the opposite sign to the Î”H value.

Write the equation for the formation of NO as follows:

Â½ N2(g) + Â½ o2(g) NO(g) Î” H = 90.29 kj

We ...

#### Solution Summary

This solution will find the enthalpy of the following reactions using Hess' Law

NO_g + Â½ o_2 -> NO_2

C graphite -> C diamond

5C_(s) +6H_2_(g) -> C_5H_12

The general formula for enthalpy is also illustrated in less detail.