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

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

Answer: (D)

Verified step by step guidance

1. Step 1: Understand what defines a redox reaction. A redox (oxidation-reduction) reaction involves the transfer of electrons between species, resulting in changes in oxidation states of elements involved.
2. Step 2: Analyze each given reaction to check for changes in oxidation states of elements. For example, in the reaction \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\), identify the oxidation states of Ag, Na, Cl, N, and O on both sides to see if any element is oxidized or reduced.
3. Step 3: Repeat the oxidation state analysis for the reaction \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\). Determine if calcium, carbon, or oxygen changes oxidation states during the decomposition.
4. Step 4: Examine the acid-base neutralization reaction \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\). Since this is typically a proton transfer without electron transfer, check if oxidation states remain constant.
5. Step 5: Finally, analyze the reaction \(\mathrm{2Na + Cl_2 \rightarrow 2NaCl}\). Sodium starts as a neutral atom and ends as \(\mathrm{Na^+}\) in \(\mathrm{NaCl}\), and chlorine goes from \(\mathrm{Cl_2}\) (oxidation state 0) to \(\mathrm{Cl^-}\) in \(\mathrm{NaCl}\). This change in oxidation states confirms it is a redox reaction.