Question Video: Identifying Which Chemical Reaction Does Not Obey the Conservation of Elements Chemistry

In which of the following chemical reactions is the concept of conservation of elements not obeyed? [A] Mg + CuSO₄ ⟶ MgSO₄ + Cu [B] CaO + CO₂ ⟶ CaCO₃ [C] ZnCO₃ ⟶ ZnO + CO₂ [D] FeCl₂ + Zn ⟶ FeCl₂ + Cu [E] NH₃ + HCl ⟶ NH₄Cl

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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.

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