Which of the following balanced equations represents a redox reaction?

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

Answer: (B)

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. Examine each equation to determine if there is a change in oxidation states of elements from reactants to products.
3. For the equation \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\), check the oxidation states of Ag, Na, Cl, N, and O. Since all species retain their oxidation states, this is not a redox reaction.
4. For the equation \(\mathrm{2Na + Cl_2 \rightarrow 2NaCl}\), note that sodium (Na) goes from 0 in elemental form to +1 in NaCl, and chlorine (Cl) goes from 0 in \(\mathrm{Cl_2}\) to -1 in NaCl. This indicates electron transfer, so this is a redox reaction.
5. For the other equations, \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\) and \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\), check oxidation states and observe that no changes occur, so these are not redox reactions.