Video Transcript
In which of the following chemical
reactions is the concept of conservation of elements not obeyed? (A) Mg plus CuSO4 react to produce
MgSO4 plus Cu. (B) CaO plus CO2 react to produce
CaCO3. (C) ZnCO3 decomposes into ZnO plus
CO2. (D) FeCl2 plus Zn react to produce
FeCl2 plus Cu. Or (E) NH3 plus HCl react to
produce NH4Cl.
All chemical reactions must obey
the conservation of elements. This means that atoms of elements
cannot be created or destroyed during a chemical reaction. In other words, elements which
appear on one side of the reaction arrow in a chemical equation must appear on the
other side as well. We have been given the chemical
equation for five chemical reactions. And we need to determine which
reaction does not obey the conservation of elements.
Let’s take a look at the reaction
equation provided in answer choice (A). On the left side of the reaction
arrow, we see the elements magnesium, copper, sulfur, and oxygen. On the right side of the reaction
arrow, we see the elements magnesium, sulfur, oxygen, and copper. All of the elements which appeared
on the left side of the reaction arrow also appeared on the right side of the
reaction arrow. This means that this reaction obeys
the conservation of elements. Since we need to determine which
reaction does not obey the conservation of elements, answer choice (A) cannot be the
answer to this question.
Let’s repeat this process for each
of the remaining chemical reactions. Looking at the reaction equation
provided in answer choice (B), we see that the elements calcium, oxygen, and carbon
appear on both sides of the reaction arrow. So this reaction does obey the
conservation of elements and cannot be the answer to this question.
Looking at the reaction equation
provided in answer choice (C), we see that the elements zinc, carbon, and oxygen
appear on both sides of the reaction arrow. So this reaction does obey the
conservation of elements and cannot be the answer to this question.
Now, let’s look at the reaction
equation provided in answer choice (D). On the left side of the reaction
arrow, we see the elements iron, chlorine, and zinc. On the right side of the reaction
arrow, we see the elements iron, chlorine, and copper. Looking at our list, we can see
that the elements zinc and copper do not appear on both sides of the reaction
arrow. So this reaction does not obey the
conservation of elements and is likely the answer to this question.
But just to be sure, let’s take a
look at the reaction equation provided in answer choice (E). We can see that the elements
nitrogen, hydrogen, and chlorine appear on both sides of the reaction arrow. So this reaction does obey the
conservation of elements and cannot be the answer to this question.
Therefore, the chemical reaction
that does not obey the concept of conservation of elements is the reaction shown in
answer choice (D) FeCl2 plus Zn react to produce FeCl2 plus Cu.
