Which of the following reactions is a redox reaction?

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

Answer: (A)

Verified step by step guidance

1. Understand that a redox reaction involves the transfer of electrons, which means there is a change in oxidation states of elements between reactants and products.
2. Examine the first reaction: \$2Na + Cl_2 \rightarrow 2NaCl\(. Assign oxidation states: Na goes from 0 (elemental form) to +1 in NaCl, and Cl goes from 0 in \)Cl_2$ to -1 in NaCl. Since both oxidation states change, this is a redox reaction.
3. Look at the second reaction: \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\). This is a double displacement reaction where ions exchange partners but oxidation states remain the same, so it is not a redox reaction.
4. Check the third reaction: \(HCl + NaOH \rightarrow NaCl + H_2O\). This is an acid-base neutralization with no change in oxidation states, so it is not a redox reaction.
5. Analyze the fourth reaction: \(CaCO_3 \rightarrow CaO + CO_2\). This is a decomposition reaction where calcium carbonate breaks down, but oxidation states of elements do not change, so it is not a redox reaction.