The answer is (c) Au only.
Oxidation is the process by which an element loses electrons. The difficulty of oxidizing an element is often determined by its position in the electrochemical series, which ranks elements by their electrode potentials. Elements with higher reduction potentials are more difficult to oxidize because they have a greater tendency to gain electrons rather than lose them.
To determine which element is most difficult to oxidize, we need to consider their standard reduction potentials. The element with the highest standard reduction potential (positive value) in its most common oxidation state is the hardest to oxidize.
Here are the steps to answer the question:
1. Refer to the Standard Electrochemical Series: The standard reduction potentials for the most common oxidation states of the given elements are as follows (approximate values):
- Cu (Copper): +0.34 V
- Hg (Mercury): +0.85 V
- Au (Gold): +1.50 V
- Ag (Silver): +0.80 V
- Na (Sodium): -2.71 V
2. Compare the Potentials: A higher positive value indicates a greater tendency to gain electrons, and thus, a greater difficulty to be oxidized.
3. Determine the Most Difficult to Oxidize: Based on the standard reduction potentials, gold (Au) has the highest positive value, indicating it is the most difficult to oxidize among the given elements.