12. Which of the following reactions is a decomposition reaction? 1. 2H2 + O2 → 2H2O 2. 2Na + Cl2 → 2NaCl 3. CaCO3 → CaO + CO2 4. PbCO3(s) → PbO(s) + CO2(g)
Answer: C
CaCO3 → CaO + CO2 is a decomposition reaction.
Decomposition reactions involve a single compound breaking down into two or more simpler substances. In the case of CaCO3 → CaO + CO2, calcium carbonate decomposes into calcium oxide and carbon dioxide, making it a clear example of a decomposition reaction.
A) 1
The reaction 2H2 + O2 → 2H2O is a combination reaction, not a decomposition reaction. In this process, hydrogen and oxygen gases combine to form water, which does not fit the definition of decomposition where a single substance breaks down into multiple products.
B) 2 and 3
This option incorrectly identifies the reactions. While 2Na + Cl2 → 2NaCl is a combination reaction, 2NaCl does not result from a single compound breaking down. Although 2 (reaction not applicable) and 3 (CaCO3 → CaO + CO2) includes a decomposition reaction, the presence of the combination reaction in 2 makes this option incorrect.
C) 4
The reaction PbCO3(s) → PbO(s) + CO2(g) is indeed a decomposition reaction, as lead(II) carbonate breaks down into lead(II) oxide and carbon dioxide. This reaction perfectly fits the criteria for decomposition, where one compound yields multiple products.
D) All of them are decomposition reactions.
This statement is incorrect because not all reactions listed are decomposition reactions. Specifically, options 1 and 2 are combination reactions, while options 3 and 4 are the only valid decomposition reactions.
Conclusion
Option C is definitively correct as it identifies a genuine decomposition reaction, where a single compound decomposes into simpler substances. The other options either misclassify the reactions or incorrectly group them, failing to adhere to the criteria of decomposition reactions. Thus, options A, B, and D do not align with the definition, reinforcing the correctness of option C.