Enthalpy & Entropy

Definition

Enthalpy is a thermodynamic property of a system that represents the total heat content, calculated as the internal energy plus the product of pressure and volume ($H = U + PV$). Entropy is a measure of the degree of randomness, disorder, or uncertainty within a thermodynamic system, often described as the dispersal of energy.

Main Content

1. Enthalpy (The Heat Content)

• Enthalpy measures the total energy of a thermodynamic system, including the internal energy required to create the system and the energy required to make room for it by displacing its environment.
• It is a state function, meaning its value depends only on the current state of the system, not the path taken to reach that state.

2. Entropy (The Disorder)

• Entropy quantifies the number of microscopic configurations that correspond to a thermodynamic system in a state of specified macroscopic variables.
• According to the Second Law of Thermodynamics, the total entropy of an isolated system can never decrease over time; it can only remain constant or increase.

3. The Relationship (Gibbs Free Energy)

• Enthalpy and entropy are bridged by Gibbs Free Energy ($G = H - TS$), which determines the spontaneity of a chemical reaction.
• If the net change in Gibbs Free Energy is negative, the process is spontaneous.

Energy Relationship:
[ Enthalpy (H) ] - [ Temperature (T) * Entropy (S) ] = [ Gibbs Free Energy (G) ]
        |                   |                              |
Total Heat Content    Degree of Disorder           Spontaneity Indicator

Working / Process

1. Calculating Enthalpy Change ($\Delta H$)

• To calculate enthalpy change, subtract the enthalpy of the reactants from the enthalpy of the products: $\Delta H = H_{products} - H_{reactants}$.
• If $\Delta H$ is negative, the reaction is exothermic (releases heat); if positive, it is endothermic (absorbs heat).

2. Measuring Entropy Change ($\Delta S$)

• Entropy change is determined by the heat transferred in a reversible process divided by the absolute temperature: $\Delta S = Q_{rev} / T$.
• A positive $\Delta S$ indicates an increase in molecular disorder (e.g., solid turning into gas).

3. Determining Spontaneity

• Use the equation $\Delta G = \Delta H - T\Delta S$ to predict if a reaction will occur naturally.
• If $\Delta G < 0$, the reaction is spontaneous; if $\Delta G > 0$, the reaction is non-spontaneous and requires external energy.

Advantages / Applications

• Chemical Engineering: Used to design efficient heat exchangers and reactors by calculating energy requirements.
• Environmental Science: Entropy explains why energy conversion processes are never 100% efficient, highlighting the limit of natural resources.
• Materials Science: Helps in predicting phase transitions, such as why ice melts into water at room temperature due to increased entropy.

Summary

Enthalpy represents the total heat content of a system, while entropy measures the randomness or energy dispersal. Together, these concepts determine whether physical or chemical processes occur spontaneously.

• Enthalpy ($H$): Total energy/heat content.
• Entropy ($S$): Measure of disorder.
• Spontaneity: Determined by the Gibbs Free Energy equation.
• Important terms: Exothermic, Endothermic, Thermodynamic Equilibrium, State Function.