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)

Rationale

CaCO3 → CaO + CO2 is a decomposition reaction.

Decomposition reactions involve a single compound breaking down into two or more simpler substances. In this case, calcium carbonate (CaCO3) is breaking down into calcium oxide (CaO) and carbon dioxide (CO2), clearly illustrating the characteristic of a decomposition reaction.

A (1) YOUR ANSWER

The reaction 2H2 + O2 → 2H2O is a synthesis reaction, where two reactants (hydrogen and oxygen) combine to form a single product (water). This is not a decomposition reaction, as it does not involve breaking down a compound into simpler substances.

B (2 and 3) YOUR ANSWER

The reaction 2Na + Cl2 → 2NaCl is also a synthesis reaction, where sodium and chlorine combine to form sodium chloride. While reaction 3 (CaCO3 → CaO + CO2) is indeed a decomposition reaction, the presence of the synthesis reaction in this option makes it incorrect as a choice for decomposition reactions.

C (4) CORRECT

The reaction PbCO3(s) → PbO(s) + CO2(g) is another example of a decomposition reaction, as lead(II) carbonate (PbCO3) decomposes into lead(II) oxide (PbO) and carbon dioxide (CO2). This confirms PbCO3 as a reactant breaking down into simpler products, aligning with the definition of a decomposition reaction.

D (All of them are decomposition reactions.) YOUR ANSWER

This choice is incorrect because not all listed reactions are decomposition reactions. Specifically, reactions 1 and 2 are synthesis reactions, which do not fit the criteria for decomposition. Therefore, this option fails to recognize the correct classification of the reactions.

Conclusion

In the context of the question, options 1, 2, and 4 do not represent decomposition reactions, while both reactions 3 and 4 do. Thus, the most accurate answer highlighting the decomposition reactions is option C, as it correctly identifies the breakdown of compounds into simpler substances. Understanding these classifications is essential in chemistry for predicting reaction behavior and outcomes.

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)

CaCO3 → CaO + CO2 is a decomposition reaction.

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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)