Which of the following reactions is classified as an oxidation-reduction (redox) reaction?

• A. CaCO_3 → CaO + CO_2
• B. 2Na + Cl_2 → 2NaCl
• C. AgNO_3 + NaCl → AgCl + NaNO_3
• D. HCl + NaOH → NaCl + H_2O

Answer: (B)

Verified step by step guidance

1. Step 1: Understand what an oxidation-reduction (redox) reaction is. A redox reaction involves the transfer of electrons between species, resulting in changes in oxidation states of elements.
2. Step 2: Analyze each reaction by assigning oxidation numbers to the elements involved to see if any element changes its oxidation state during the reaction.
3. Step 3: For the reaction \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\), check the oxidation states of calcium, carbon, and oxygen on both sides to determine if any changes occur.
4. Step 4: For the reaction \(\mathrm{2Na + Cl_2 \rightarrow 2NaCl}\), assign oxidation numbers to sodium and chlorine before and after the reaction to see if electron transfer occurs.
5. Step 5: For the reactions \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\) and \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\), verify if any oxidation states change, indicating redox, or if these are simply double displacement or acid-base neutralization reactions.