Which of the following reactions is a redox reaction?

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

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, resulting in changes in oxidation states.
2. Examine each reaction and assign oxidation states to the elements involved to see if any element's oxidation state changes from reactants to products.
3. For the reaction \$2Na + Cl_2 \rightarrow 2NaCl\(, sodium (Na) starts as an element with oxidation state 0 and ends up in NaCl with +1, while chlorine (Cl) goes from 0 in \)Cl_2$ to -1 in NaCl, indicating both oxidation and reduction occur.
4. For the reaction \(CaCO_3 \rightarrow CaO + CO_2\), check if oxidation states change; typically, this is a decomposition reaction without changes in oxidation states, so it is not redox.
5. For the reactions \(HCl + NaOH \rightarrow NaCl + H_2O\) and \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\), these are acid-base neutralization and double displacement reactions respectively, with no change in oxidation states, so they are not redox reactions.