Which one of the following reactions is a redox reaction?

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

Answer: (A)

Verified step by step guidance

1. Identify what a redox reaction is: a reaction where oxidation and reduction occur simultaneously, meaning there is a transfer of electrons between species.
2. For each reaction, assign oxidation states to all elements in the reactants and products to see if any element changes its oxidation state.
3. In the reaction \$2Na + Cl_2 \rightarrow 2NaCl\(, sodium (Na) goes from 0 in elemental form to +1 in NaCl, and chlorine (Cl) goes from 0 in \)Cl_2$ to -1 in NaCl, indicating both oxidation and reduction occur.
4. In the reaction \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\), check the oxidation states and notice that all elements retain their oxidation states, so no redox occurs; this is a double displacement reaction.
5. Similarly, for \(CaCO_3 \rightarrow CaO + CO_2\) and \(HCl + NaOH \rightarrow NaCl + H_2O\), verify oxidation states and observe no changes, indicating these are not redox reactions.