Question #0a40a

1 Answer
Oct 18, 2017

Here's what I got.


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

#"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, #"NO"_2#.

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

#"solid " stackrel(color(white)(acolor(red)(Delta)aaa))(->) " solid residue" + "NO"_(2(g))# #uarr#

Now, in order for the reaction to produce nitrogen dioxide, the white salt must contain the nitrate anion, #"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, #"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, #"O"_2#,

The balanced chemical equation that describes the reaction looks like this

#2"Pb"("NO"_ 3)_ (2(s)) stackrel(color(white)(acolor(red)(Delta)aaa))(->) 2"PbO"_ ((s)) + 4"NO"_ (2(g))# #uarr + "O"_ (2(g))uarr#