Minus the standard enthalpies of formation of the reactant.

And for H product and reactant,

we have to multiply by the coefficient in our balanced chemical equation.

So expanding on this a little bit, I'll see that delta

H of reaction equals two times the delta H of formation of NO2.

Because I have a two in front of NO2, I'm going to have a balanced

chemical equation minus two times the delta H of formation of NO.

Because I also have a two in front of the NO

minus the enthalpy of formation of oxygen.

No coefficients there,

because I don't have a coefficient in front of oxygen in my balanced equation.

Now, I can simplify this and put in some values.

So I have delta H of reaction equals two

times 33.2 kilojoules per mole.

And I got this information right here form our table.

Minus two times 91.3 kilojoules per mole.

And that information also came from the tabulated values.

And for oxygen, I have an enthalpy of formation of zero.

Because elements in their standard state have an enthalpy of formation

of zero kilojoules per mole.