Question

In which of the following reactions is a combination reaction taking place?
I. CuO + H$_2$ $\xrightarrow{\Delta}$ Cu + H$_2$O
II. ZnO + C → Zn + CO
III. 4Na + O$_2$ → 2Na$_2$O
IV. CH$_4$ + 2O$_2$ → CO$_2$ + 2H$_2$O

• A. Only III
• B. Only IV
• C. II and III
• D. I, III and IV

Hint:

In a combination reaction, two or more reactants combine to form a single product. Always look for reactions with a single compound formed on the right-hand side.

Answer 1

The Correct Option is A

A combination reaction is characterized by two or more substances combining to form a single product. Let's evaluate each given reaction to determine if it fits this definition:

1. I. CuO + H₂ → Cu + H₂O
   This reaction involves a compound (CuO) reacting with an element (H₂) to form new products (Cu and H₂O). Two products are formed, so this is not a combination reaction.
2. II. ZnO + C → Zn + CO
   Similarly, this reaction involves a compound (ZnO) reacting with an element (C) to produce two new substances (Zn and CO). This also does not qualify as a combination reaction.
3. III. 4Na + O₂ → 2Na₂O
   In this reaction, elements Na and O₂ combine to form a single compound, Na₂O. This matches the definition of a combination reaction.
4. IV. CH₄ + 2O₂ → CO₂ + 2H₂O
   This is a combustion reaction where methane (CH₄) reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O). Since multiple products are formed, it's not a combination reaction.

Based on the analysis above, only reaction III is a combination reaction.

Answer 2

We are asked to identify which of the given reactions is a combination reaction. A combination reaction is one in which two or more reactants combine to form a single product.

Let us analyze each reaction:

I. CuO + H₂ → Cu + H₂O
This is a reduction reaction, where CuO is reduced to Cu and hydrogen is oxidized to H₂O. Since it produces more than one product, it is not a combination reaction.

II. ZnO + C → Zn + CO
This is also a reduction reaction. Zinc oxide is reduced to zinc. Carbon is oxidized to carbon monoxide. It is not a combination reaction.

III. 4Na + O₂ → 2Na₂O
This is a classic example of a combination reaction, as sodium and oxygen combine to form a single compound: sodium oxide.

IV. CH₄ + 2O₂ → CO₂ + 2H₂O
This is a combustion reaction. It produces more than one product and hence is not a combination reaction.

Thus, only reaction III is a combination reaction.

Correct Answer: (A) Only III