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

• A. Zn + Cu^{2+} → Zn^{2+} + Cu
• B. CaCO_3 → CaO + CO_2
• C. AgNO_3 + NaCl → AgCl + NaNO_3
• D. HCl + NaOH → NaCl + H_2O

Answer: (A)

Verified step by step guidance

1. Understand that an oxidation-reduction (redox) reaction involves the transfer of electrons, where one species is oxidized (loses electrons) and another is reduced (gains electrons).
2. Examine each equation to identify changes in oxidation states of elements from reactants to products.
3. For the equation \(\mathrm{Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu}\), note that zinc changes from 0 to +2 (oxidation), and copper changes from +2 to 0 (reduction), indicating electron transfer.
4. For the other equations, check if any element changes its oxidation state. For example, in \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\), no change in oxidation states occurs; it is a decomposition reaction.
5. Conclude that the equation where oxidation states change for at least two elements, such as \(\mathrm{Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu}\), represents a redox reaction.