Question

The condensation of any gas to a liquid is expected to have:

• A. $\Delta H=-ve,\Delta S=+ve$
• B. $\Delta H=+ve,\Delta S=+ve$
• C. $\Delta H=-ve,\Delta S=-ve$
• D. $\Delta H=+ve,\Delta S=-ve$

Answer 1

Answer: (C)

$\Delta H=-ve,\Delta S=-ve$

$Enthalpy$ is the amount of heat consumed or released in a system at constant pressure. In a $Endothermic$ reaction, heat is added to a system from the surroundings due to a decrease in heat in the reaction. In a $Exothermic$ reaction, heat from a system is given off into the surroundings due to the increase in heat in the reaction.
Here, In condensation, heat is absorbed in a reaction and must be given off from the system into the surroundings. Hence, $\Delta H_{condensation}$ is an exothermic process, its value is always $negative$.
$\Delta H=-ve$
$Entropy$ is a measure of molecular disorder within a macroscopic system. A phase change is accompanied by the change in Entropy.
Here, a gas is condensed into a liquid. This will normally reduce the molecular disorder of the system. So, there will be an entropy decrease that accompanies condensation. Therefore, $\Delta S$ is $negative$.
$\Delta S=-ve$
$\Delta H=-ve$ and $\Delta S=-ve$

So, the correct answer is $C$.