To determine which statements are correct regarding galvanic cells and electrolysis, we will analyze each statement step by step.
### Step 1: Analyze the First Statement
**Statement:** In a galvanic cell, the salt bridge is not necessary for the start of the cell reaction.
**Solution:**
- A galvanic cell can indeed start its reaction without a salt bridge. However, the salt bridge is essential for maintaining electrical neutrality in the half-cells during the reaction. Without it, the reaction can only proceed for a short time before the buildup of charge stops it.
**Conclusion:** This statement is correct.
**Hint:** Remember that while the reaction can start without a salt bridge, it cannot continue indefinitely without one.
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### Step 2: Analyze the Second Statement
**Statement:** The standard hydrogen electrode is assumed to have zero electrode potential at all temperatures.
**Solution:**
- The standard hydrogen electrode (SHE) is defined to have an electrode potential of 0 volts at standard conditions (1 M concentration, 1 atm pressure, and 25°C). However, its potential can vary with temperature. Thus, it is not accurate to say it has zero potential at all temperatures.
**Conclusion:** This statement is incorrect.
**Hint:** Consider the definition of standard electrode potential and how it may change with temperature.
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### Step 3: Analyze the Third Statement
**Statement:** In the electrolysis of dilute aqueous NaCl, the products obtained at the anode and cathode are O2 and H2 respectively.
**Solution:**
- During the electrolysis of dilute NaCl, at the anode, hydroxide ions (OH-) are oxidized to produce oxygen gas (O2), and at the cathode, hydrogen ions (H+) are reduced to produce hydrogen gas (H2). Therefore, the products are indeed O2 at the anode and H2 at the cathode.
**Conclusion:** This statement is correct.
**Hint:** Recall the reactions that occur at the anode and cathode during electrolysis involving water and salt.
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### Step 4: Analyze the Fourth Statement
**Statement:** An inert electrolyte having comparable velocities of cation and anion is taken in the salt bridge to maintain almost constant potential difference between the two half cells.
**Solution:**
- An inert electrolyte, such as KCl, is used in the salt bridge to maintain electrical neutrality. The velocities of the cations (K+) and anions (Cl-) being comparable helps to ensure that the potential difference remains stable during the operation of the galvanic cell.
**Conclusion:** This statement is correct.
**Hint:** Think about the role of the salt bridge in maintaining charge balance and how ion mobility affects this.
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### Final Conclusion
- **Correct Statements:** 1, 3, and 4 are correct.
- **Incorrect Statement:** 2 is incorrect.
