Which of the following reactions is a redox reaction?

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

Answer: (C)

Verified step by step guidance

1. Identify what a redox reaction is: it involves the transfer of electrons, meaning one species is oxidized (loses electrons) and another is reduced (gains electrons).
2. Analyze each reaction by assigning oxidation states to the elements involved to see if any changes occur from reactants to products.
3. For the reaction \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\), check the oxidation states of Ag, Na, Cl, N, and O to see if any element changes its oxidation number.
4. For the reaction \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\), determine if the oxidation states of Ca, C, and O change during the decomposition.
5. For the reaction \(\mathrm{2\ Na + Cl_2 \rightarrow 2\ NaCl}\), observe that sodium goes from 0 to +1 and chlorine goes from 0 to -1, indicating electron transfer and thus a redox reaction.