Question

Which of the following processes are reversible? I. Vaporization of a liquid at its boiling point. II. Expansion of gas into vacuum. III. Transformation of a solid substance into liquid at its melting point. IV. Neutralization of an acid by a base.

• A. I & III
• B. II & III
• C. II & IV
• D. I & IV

Hint:

In thermodynamics, reversible processes are idealized processes where the system is always infinitesimally close to equilibrium. Phase changes at the melting or boiling point are classic examples of reversible processes. Spontaneous, rapid processes like free expansion or chemical reactions are generally irreversible.

Answer 1

The Correct Option is A

We are asked to identify the reversible processes among the given options. Step 1: Definition of a reversible process A reversible process is a process that can be reversed by infinitesimal changes in a system's properties, without increasing the entropy of the universe. The system remains in thermodynamic equilibrium with its surroundings throughout the process.
Step 2: Analyze each process I. Vaporization of a liquid at its boiling point:

• This is a phase change occurring at constant temperature and pressure (boiling point).
• An infinitesimal decrease in external pressure causes vaporization, and an infinitesimal increase causes condensation.
• The system remains in equilibrium during the transition.
• Conclusion: Reversible process.

II. Expansion of gas into vacuum (Free Expansion):

• The gas expands spontaneously without opposing pressure.
• The process is rapid and irreversible; the system is not in equilibrium.
• Reversing requires external work and changes to surroundings.
• Conclusion: Irreversible process.

III. Transformation of a solid substance into liquid at its melting point:

• Occurs at constant temperature (melting point).
• Infinitesimal increase in temperature melts the solid; infinitesimal decrease freezes the liquid.
• System remains in equilibrium at the melting point.
• Conclusion: Reversible process.

IV. Neutralization of an acid by a base:

• Example: HCl + NaOH $\rightarrow$ NaCl + H$_2$O
• Highly exothermic and spontaneous.
• Cannot be reversed by infinitesimal changes; requires significant external intervention.
• Conclusion: Irreversible process.

Step 3: Conclusion The reversible processes are: \[ \text{I and III: Vaporization at boiling point and melting at melting point} \]