Which ionic equation represents a spontaneous reaction that can occur in a voltaic cell? 1. Cu(s) + Zn(s) → Cu2+(aq) + Zn2+(aq) 2. Cu(s) + Zn2+(aq) → Cu2+(aq) + Zn(s) 3. Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq) 4. Cu2+(aq) + Zn2+(aq) → Cu(s) + Zn(s)
The Correct Answer and Explanation is:
The correct answer is Option 3: Cu²⁺(aq) + Zn(s) → Cu(s) + Zn²⁺(aq).
Explanation:
In a voltaic (galvanic) cell, a spontaneous redox reaction occurs to produce electrical energy. The reaction involves the transfer of electrons from the anode (where oxidation takes place) to the cathode (where reduction takes place). For a reaction to be spontaneous, it must have a positive cell potential (E°cell > 0), indicating that the redox reaction releases energy.
Let’s break down each option to understand why Option 3 is correct:
- Cu(s) + Zn(s) → Cu²⁺(aq) + Zn²⁺(aq)
- This is a non-spontaneous reaction. If both copper and zinc are solid, it would require external energy (such as a power source) to drive the reaction. This reaction doesn’t fit the definition of a spontaneous voltaic cell reaction, as it does not involve the movement of electrons that would generate an electric current.
- Cu(s) + Zn²⁺(aq) → Cu²⁺(aq) + Zn(s)
- This reaction is also non-spontaneous. In this case, copper is oxidized to Cu²⁺, and zinc is reduced. However, zinc would not spontaneously lose electrons to reduce copper. This reaction would need to be forced by external energy.
- Cu²⁺(aq) + Zn(s) → Cu(s) + Zn²⁺(aq)
- This is the correct spontaneous redox reaction. Here, zinc metal (Zn) undergoes oxidation at the anode, releasing electrons and turning into Zn²⁺. These electrons flow through an external circuit to the cathode, where Cu²⁺ ions are reduced to copper metal (Cu). This reaction is spontaneous because zinc is more easily oxidized than copper, and the overall cell potential is positive.
- Cu²⁺(aq) + Zn²⁺(aq) → Cu(s) + Zn(s)
- This is not a valid voltaic reaction because both Cu²⁺ and Zn²⁺ are already in their ionic forms. No solid metals are involved as reactants or products, which makes it non-spontaneous in the context of a voltaic cell.
In summary, Option 3 represents the correct spontaneous reaction in a voltaic cell, where zinc metal loses electrons (oxidation), and copper ions gain electrons (reduction), resulting in a flow of electric current.