Which of the following chemical equations represents an oxidation-reduction (redox) reaction?

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

Answer: (A)

Verified step by step guidance

1. Step 1: Understand what an oxidation-reduction (redox) reaction is. A redox reaction involves the transfer of electrons between species, resulting in changes in oxidation states of elements.
2. Step 2: Examine each chemical equation and identify the oxidation states of the elements before and after the reaction to see if any element is oxidized (increase in oxidation state) or reduced (decrease in oxidation state).
3. Step 3: For the equation \$2Na + Cl_2 \rightarrow 2NaCl\(, assign oxidation states: Sodium (Na) starts as 0 and ends as +1 in NaCl; Chlorine (Cl) starts as 0 in \)Cl_2$ and ends as -1 in NaCl. This indicates electron transfer, so this is a redox reaction.
4. Step 4: For the equation \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\), check oxidation states. Silver (Ag), sodium (Na), chlorine (Cl), nitrogen (N), and oxygen (O) maintain their oxidation states, indicating no electron transfer, so this is not a redox reaction.
5. Step 5: Similarly, analyze \(HCl + NaOH \rightarrow NaCl + H_2O\) and \(CaCO_3 \rightarrow CaO + CO_2\). Both are not redox reactions because oxidation states do not change; the first is an acid-base neutralization, and the second is a decomposition reaction.