hess's law Enthalpy of reaction

Question

Applying Hess's Law

Consider the chemical equations shown here.

What is H_rxn for the reaction shown below?

Answer 1

Start with NO+O3->NO2+O2 because that's the closest to the last reaction notation. Our goal is to manipulate all of the reactions and then add them together to create the last reaction.

We can add the second reaction to the first directly because it will add an O3 to the right side. That is good because it will cancel out the O3 on the left side of the 1st reaction.

NO + O3 + 3/2O2-> NO2 + O2 + O3 H1+H2 = -56.6

NO + 1/2O2 -> NO2

We can also add the second reaction directly. The 1/2 O2 on both sides will cancel out.

NO +1/2O2 +O -> NO2 + 1/2O2 H1+H2+H3 = -304.1

NO + O -> NO2 Hreaction = -304.1

This one was pretty straightforward. The process usually involves reversing some of the reactions or manipulating the values of the coefficients.