To understand how to use Hess\'s law to find the enthalpy of an overall reaction

Question

To understand how to use Hess's law to find the enthalpy of an overall reaction. The change in enthalpy, H, is the heat absorbed or produced during any reaction at constant pressure. Hess's law states that H for an overall reaction is the sum of the H values for the individual reactions. For example, if we wanted to know the enthalpy change for the reaction 3Mn+3O23MnO2 we could calculate it using the enthalpy values for the following individual steps: Step 1: 4Al+3O22Al2O3 Step 2: 3Mn+2Al2O33MnO2+4Al Overall: 3Mn+3O23MnO2 If the enthalpy change is 3352 kJ/mol for step 1 and 1792 kJ/mol for step 2, then the enthalpy change for the overall reaction is calculated as follows: H=3352+1792=1560 kJ/mol It is also important to note that the change in enthalpy is a state function, meaning it is independent of path. In other words, the sum of the H values for any set of reactions that produce the desired product from the starting materials gives the same overall H.

What is the enthalpy for the following reaction? overall: N2O42NO + O2

Explanation / Answer

Solution :-

Reaction N2O42NO + O2

using the standard enthalpy values of the formations we can calculate the enthalpy change of the reaction

Delta H rxn = sum of delta Hf product - sum of delta Hf reactant.

                 = [(NO*2)+(O2*1)] - [N2O4*1]

                 = [(90.29*2)+(0*1)]-[9.16*1]

                = 171.42 kJ

Therefore the enthalpy change of the reaction Delta H = 171.42 kJ

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