In which of the following processes will ΔS° be negative?
a) C2H5OH(l)→C2H5OH(g)
b) NaCl(s)→NaCl(l)
c) CO2(s)→CO2(g)
d) Cl2(g)→Cl2(l)
The Correct Answer and Explanation is:
The correct answer is d) Cl₂(g) → Cl₂(l).
Explanation:
Entropy (ΔS°) is a thermodynamic property that measures the disorder or randomness of a system. A negative ΔS° indicates a decrease in entropy, meaning the system is becoming more ordered. Conversely, a positive ΔS° indicates an increase in entropy, which corresponds to greater randomness.
Let’s analyze each of the options:
a) C₂H₅OH(l) → C₂H₅OH(g):
This process involves the vaporization of ethanol from a liquid state to a gas. When a substance transitions from liquid to gas, the molecules are moving from a more ordered state (liquid) to a more disordered state (gas). Therefore, this transition increases entropy, resulting in a positive ΔS°. Hence, this option does not represent a process with negative ΔS°.
b) NaCl(s) → NaCl(l):
This is the melting of sodium chloride (NaCl) from a solid to a liquid. In the solid state, NaCl molecules are closely packed in a highly ordered lattice structure. Upon melting, the molecules gain more freedom of movement in the liquid phase, leading to an increase in entropy. Therefore, this transition has a positive ΔS°, not a negative one.
c) CO₂(s) → CO₂(g):
This is the sublimation of carbon dioxide (CO₂) from a solid to a gas. In the solid state, CO₂ molecules are arranged in a rigid, ordered structure. As it transitions to a gas, the molecules spread out and move more freely, increasing the disorder. This process also results in a positive ΔS°, indicating an increase in entropy.
d) Cl₂(g) → Cl₂(l):
This process represents the condensation of chlorine gas into a liquid. As chlorine gas condenses, the molecules move from a highly disordered state (gas) to a more ordered state (liquid), which decreases entropy. Therefore, this transition corresponds to a negative ΔS° because the system becomes more ordered as the gas molecules lose their freedom of movement and organize into a liquid state.
Thus, the correct answer is d) Cl₂(g) → Cl₂(l), where the process involves a reduction in entropy, as the gas phase becomes more ordered in the liquid phase.