Which of the following reactions is a redox reaction?

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

Answer: (B)

Verified step by step guidance

1. Identify what a redox reaction is: a chemical reaction involving the transfer of electrons, where one species is oxidized (loses electrons) and another is reduced (gains electrons).
2. Analyze each reaction to determine if there is a change in oxidation states of any elements:
3. For the reaction \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\), check the oxidation states of Ag, Na, Cl, and N before and after the reaction to see if any change occurs.
4. For the reaction \(\mathrm{2Na + Cl_2 \rightarrow 2NaCl}\), examine the oxidation states of Na and Cl on both sides to see if electrons are transferred.
5. For the reactions \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\) and \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\), similarly check if any element changes its oxidation state, indicating a redox process.