Chemistry · Chapter 28
Study notes aligned to the official NEB syllabus.
Note: This chapter is chemistry content (NEB Class 12 Chemistry, "Energetics" / chemical thermodynamics) that was filed among the physics notes. It has been rewritten faithfully to its own chemistry scope: spontaneity, entropy, Gibbs free energy, and the feasibility of reactions.
A spontaneous process is one that has a natural tendency to occur on its own, without the help of any external agent. Examples: heat flows from a hot body to a cold body, water flows from a higher level to a lower level, and water evaporates from the sea.
Characteristics of a spontaneous process:
The two natural tendencies that drive a process are the tendency towards minimum energy (enthalpy) and the tendency towards maximum disorder (entropy). The balance between them is captured by the Gibbs free energy, introduced below.
Entropy $(S)$ is a measure of the randomness or disorder of a system. The greater the molecular disorder, the greater the entropy.
The absolute value of entropy is difficult to determine, so entropy is usually expressed as a change, $\Delta S$. For heat $q_{\text{rev}}$ absorbed reversibly at absolute temperature $T$:
$$\Delta S = \frac{q_{\text{rev}}}{T}$$
The SI unit of entropy is $\text{J K}^{-1}\text{mol}^{-1}$.
Because molecular disorder increases from the solid to the liquid to the gaseous state, entropy follows the order:
$$S_{\text{gas}} \gg S_{\text{liquid}} > S_{\text{solid}}$$
Entropy is a state function, so it depends only on the state of the system and can be evaluated between two states:
$$ \begin{aligned} \Delta S &= S_{\text{final}} - S_{\text{initial}} \ &= S_2 - S_1 \end{aligned} $$
When one mole of a substance changes phase at its transition temperature, the entropy change equals the latent heat (enthalpy of the transition) divided by that temperature.
1. Entropy of fusion (solid $\rightarrow$ liquid at the melting point $T_m$):
$$\Delta S_{\text{fusion}} = \frac{\Delta H_{\text{fusion}}}{T_m}$$
where $\Delta H_{\text{fusion}}$ is the enthalpy (latent heat) of fusion.
2. Entropy of vaporization (liquid $\rightarrow$ vapour at the boiling point $T_b$):
$$\Delta S_{\text{vap}} = \frac{\Delta H_{\text{vap}}}{T_b}$$
This is always positive, since a gas is far more disordered than a liquid.
3. Entropy of sublimation (solid $\rightarrow$ vapour at the sublimation temperature $T_{\text{sub}}$):
$$\Delta S_{\text{sub}} = \frac{\Delta H_{\text{sub}}}{T_{\text{sub}}}$$
For solid $\rightarrow$ vapour, $\Delta S > 0$; for the reverse (vapour $\rightarrow$ solid), $\Delta S < 0$.
4. Entropy of transition (one allotrope $\rightarrow$ another at the transition temperature $T_t$):
$$\Delta S_{\text{transition}} = \frac{\Delta H_t}{T_t}$$
Note: This chapter is chemistry content (NEB Class 12 Chemistry, "Energetics" / chemical thermodynamics) that was filed among the physics notes. It has been rewritten faithfully to its own chemistry scope: spontaneity, entropy, Gibbs free energy, and the feasibility of reactions.
A spontaneous process is one that has a natural tendency to occur on its own, without the help of any external agent. Examples: heat flows from a hot body to a cold body, water flows from a higher level to a lower level, and water evaporates from the sea.
Characteristics of a spontaneous process:
The two natural tendencies that drive a process are the tendency towards minimum energy (enthalpy) and the tendency towards maximum disorder (entropy). The balance between them is captured by the Gibbs free energy, introduced below.
Entropy is a measure of the randomness or disorder of a system. The greater the molecular disorder, the greater the entropy.
The absolute value of entropy is difficult to determine, so entropy is usually expressed as a change, . For heat absorbed reversibly at absolute temperature :
The SI unit of entropy is .
Because molecular disorder increases from the solid to the liquid to the gaseous state, entropy follows the order:
Entropy is a state function, so it depends only on the state of the system and can be evaluated between two states:
When one mole of a substance changes phase at its transition temperature, the entropy change equals the latent heat (enthalpy of the transition) divided by that temperature.
1. Entropy of fusion (solid liquid at the melting point ):
where is the enthalpy (latent heat) of fusion.
2. Entropy of vaporization (liquid vapour at the boiling point ):
This is always positive, since a gas is far more disordered than a liquid.
3. Entropy of sublimation (solid vapour at the sublimation temperature ):
For solid vapour, ; for the reverse (vapour solid), .
4. Entropy of transition (one allotrope another at the transition temperature ):