Question #0a40a

Oct 18, 2017

Here's what I got.

Explanation:

For starters, you know that you're dealing with a thermal decomposition reaction that takes the general form

$\text{solid " stackrel(color(white)(acolor(red)(Delta)aaa))(->) " solid residue + gas}$

Now, you know that this salt decomposes to give off brown fumes. A well-known gas that matches this description if nitrogen dioxide, ${\text{NO}}_{2}$.

You can thus say that when the salt undergoes thermal decomposition, nitrogen dioxide is given off.

${\text{solid " stackrel(color(white)(acolor(red)(Delta)aaa))(->) " solid residue" + "NO}}_{2 \left(g\right)}$ $\uparrow$

Now, in order for the reaction to produce nitrogen dioxide, the white salt must contain the nitrate anion, ${\text{NO}}_{3}^{-}$. Furthermore, you can say that the solid residue will be an oxide.

So all you have to do now is look for an element that has

• a white salt when combined with the nitrate anion
• a yellow oxide

A suitable candidate here is lead, $\text{Pb}$. Lead(II) nitrate is indeed a white salt

and lead(II) oxide is a yellow salt

which means that you're dealing with the thermal decomposition of lead(II) nitrate, which produces lead(II) oxide, a yellow solid residue, and brown fumes of nitrogen dioxide. Also, keep in mind that in addition to the nitrogen dioxide, this reaction also produces oxygen gas, ${\text{O}}_{2}$,

The balanced chemical equation that describes the reaction looks like this

$2 {\text{Pb"("NO"_ 3)_ (2(s)) stackrel(color(white)(acolor(red)(Delta)aaa))(->) 2"PbO"_ ((s)) + 4"NO}}_{2 \left(g\right)}$ $\uparrow + {\text{O}}_{2 \left(g\right)} \uparrow$