# Question 81dd7

Jun 30, 2017

The first must be reversed (cool rhyme!)

#### Explanation:

When you "add together" two or more chemical equations, it goes similarly as algebra: equal quantities on opposite sides of either reaction can cancel out.

Here's what I mean by this:

Our two equations are

$\textcolor{red}{2 {\text{NO"_2(g)) rarr color(blue)(2"NO"(g) + "O}}_{2} \left(g\right)}$

$\textcolor{b l u e}{{\text{N"_2"O"_4(g)) rarr color(red)(2"NO"(g) + "O}}_{2} \left(g\right)}$

Here, there must be two duplicate quantities is a given color to cancel out. As you may see, neither red nor blue contains a duplicate species. However, if we reversed the first equation, we would then have

cancel(color(red)(2"NO"(g))) + cancel(color(red)("O"_2(g))) rarr color(blue)(2"NO"_2(g))

color(blue)("N"_2"O"_4(g)) rarr cancel(color(red)(2"NO"(g))) + cancel(color(red)("O"_2(g)))

Two quantities can cancel out, leaving behind the blue equation:

color(blue)("N"_2"O"_4(g) rarr 2"NO"_2(g)#