1.2 Thermodynamics Fundamentals

Thermodynamic properties and laws form the backbone of chemical engineering. They help us understand how energy flows and changes in systems, from simple reactions to complex industrial processes.

Enthalpy, entropy, and free energy changes are key concepts for predicting reaction feasibility. By calculating these values, engineers can determine if a reaction will occur spontaneously or require external input, crucial for designing efficient chemical processes.

Thermodynamic Properties and Laws

Thermodynamic properties and equilibrium

• Thermodynamic properties measure or calculate a system's characteristics (temperature, pressure, volume, internal energy, enthalpy, entropy)
• State variables depend only on the current state, not the path taken to reach it (pressure, temperature, volume) independent of the system's history
• Equilibrium occurs when a system's properties remain constant over time with no net change in macroscopic properties
  • Thermal equilibrium: no net heat transfer between system and surroundings
  • Mechanical equilibrium: no net change in pressure or volume
  • Chemical equilibrium: no net change in the system's composition

First and second laws in chemistry

• First Law of Thermodynamics states energy cannot be created or destroyed, only converted between forms
  • Change in internal energy ($\Delta U$) equals heat added ($Q$) minus work done by the system ($W$): $\Delta U = Q - W$
• Second Law of Thermodynamics states entropy of an isolated system always increases or remains constant
  • Heat flows spontaneously from hot to cold bodies
  • Impossible to completely convert heat into work in a cyclic process
• Apply these laws to analyze heat transfer and work in chemical reactions and phase changes, determine process efficiency, and assess reaction feasibility and spontaneity

Thermodynamic Calculations and Reaction Feasibility

Enthalpy, entropy, and free energy changes

• Enthalpy ($H$) measures a system's total heat content
  • Change in enthalpy ($\Delta H$) equals heat absorbed or released at constant pressure: $\Delta H = Q_p$
• Entropy ($S$) measures a system's disorder or randomness
  • Change in entropy ($\Delta S$) equals heat absorbed or released divided by absolute temperature for reversible processes: $\Delta S = \frac{Q}{T}$
• Gibbs free energy ($G$) measures maximum useful work obtainable from a system
  • Change in Gibbs free energy ($\Delta G$) equals change in enthalpy minus the product of temperature and change in entropy: $\Delta G = \Delta H - T\Delta S$

Reaction feasibility and spontaneity

• Spontaneous reactions occur without external intervention and have negative change in Gibbs free energy ($\Delta G < 0$)
• Feasible reactions can occur under given conditions and have negative or zero change in Gibbs free energy ($\Delta G \leq 0$)
• Relationship between $\Delta G$, $\Delta H$, and $\Delta S$ determines reaction spontaneity:
  1. $\Delta H < 0$ and $\Delta S > 0$: always spontaneous
  2. $\Delta H > 0$ and $\Delta S < 0$: never spontaneous
  3. $\Delta H < 0$ and $\Delta S < 0$ or $\Delta H > 0$ and $\Delta S > 0$: spontaneity depends on temperature