Question

What are the characteristics of entropy(s)?

Answer 1

Characteristicsof entropy 'S' :
i) The term 'S' entropy is evolved from the formulation of II law of thermodynamics as a thermodynamic state function.
ii) Entropy change $\Delta S$ of system under a process ts defined as e constant equal to the ratio of the heat change accompanying a process at constant temperature to the temperature of the system under process. The process should be reversible at that .temperature.
$\Delta S_{rev}=\frac{\Delta q_{rev}}{T\left(K\right)}$
Heat, q is not a state function, But for a reversible process $\Delta q=(q_2-q_1)$ divided by temperature (T) of the process is a state function.
iii) A spontaneous process is accompanied by inacase in the 'disorder' (or) 'randomness' of the molecules constituting the system. Entropy increases in all spontaneous processes. Hence entropy maybe regarded as a measure of disorder (or) randomness of the molecules of the system.
iv) When a system undergoes a physical (or) a chemical process, there occurs a change in the entropy of the system and also in its surroundings. This total change in the entropy of the system and its surroundings is termed as the entropy change of the universe brought about by the process. For an isothermal process (T = constant), the entropy change of the universe daring a reversible process is zero.
The entropy of the universe increases in an irreversible process.
v) The energy of the universe remains constant although the entropy of the universe tends to a maximum.
vi) For a spontaneous process, at constant
T, $\Delta S$ is positive $(\Delta S>0)$.
For an equilibrium process $\Delta S$ is zero.
For a non sponta4cous process $\Delta S$ negative or $(\Delta s<0)$
vii) Units of entropy : The dimension a entropy arc energy in terms of heat $temperatur{e}^{-1}$. The entropy is expressed as calories perdegme which is .- referred to as the entropy units (ea). Since entropy also depends on the quantity of the substance, unit of entropy is calories per degree per mote (or) eu per mole.
cgs units of entropy is cal $K^{-1}$ denoted as eta. The SI unit is $J{K}^{-1}$ and denoted as EU. 1 eu=4.184 EU.
viii) Entropy change is related to enthalpy change as follows :
For a reversible and isothermal process,
$\Delta S=\frac{\Delta q_{rev}}{T}$. Since $\Delta H$ is the heat absorbed (or)
evolved in the process at constant T and pressure P, $\Delta S$ is also calculated from $\Delta H$ as $\Delta S=\frac{\Delta H}{T}$, where I is the temperature of the process involving $\Delta H$, among of enthalpy change, at constant pressure.