# Question 9c5ae

Nov 19, 2015

$2 \text{NO"_text((g]) + "O"_text(2(g]) -> 2"NO"_text(2(g]) + "27.2 kcal}$

#### Explanation:

Your starting point here will be the balanced chemical equation for this reaction, which looks like this

$\textcolor{red}{2} {\text{NO"_text((g]) + "O"_text(2(g]) -> 2"NO}}_{\textrm{2 \left(g\right]}}$

Now, you know that when this reaction takes place, $\text{13.6 kcal}$ of heat are evolved for every mole of nitric oxide that takes part in the reaction.

The fact that heat is being evolved, which is another term used for given off, tells you that you're dealing with an exothermic reaction.

So where would you write the energy term, on the reactants' side or on the products' side?

Well, you can think of heat given off as being heat produced. In other words, the reaction produces nitrogen dioxide, ${\text{NO}}_{2}$, and heat, so it makes sense to add the nergy term on the products' side of the equation.

Now, you know that $\text{13.6 kcal}$ of heat are being produced for every one mole of nitric oxide. In this case, the reaction uses up $\textcolor{red}{2}$ moles of nitric oxide to produce $2$ moles of nitrogen dioxide.

This means that your energy term will be

color(red)(2)color(red)(cancel(color(black)("moles NO"))) * "13.6 kcal"/(1color(red)(cancel(color(black)("mole NO")))) = "27.2 kcal"#

Therefore, the chemical equation + energy term will look like this

$2 \text{NO"_text((g]) + "O"_text(2(g]) -> 2"NO"_text(2(g]) + "27.2 kcal}$