1.1 Laws of Thermodynamics and Their Application to Combustion

Thermodynamics is key to understanding combustion. The laws of energy conservation and entropy guide how fuels burn and release energy. These principles are crucial for designing efficient engines and predicting reaction outcomes.

Combustion processes involve energy transfers, work, and heat capacities. By applying thermodynamic concepts, we can analyze fuel efficiency, predict flame temperatures, and optimize combustion systems for better performance and reduced emissions.

Thermodynamic Laws and Properties

Fundamental Laws of Thermodynamics

• First Law of Thermodynamics states energy cannot be created or destroyed, only converted from one form to another
  • Mathematically expressed as $\Delta U = Q - W$
  • $\Delta U$ represents change in internal energy
  • $Q$ denotes heat added to the system
  • $W$ signifies work done by the system
• Second Law of Thermodynamics introduces concept of entropy
  • Entropy of an isolated system always increases over time
  • Defines direction of spontaneous processes
  • Imposes limitations on efficiency of heat engines

Energy-Related Properties

• Internal Energy encompasses total energy contained within a system
  • Includes kinetic energy of molecules and potential energy of molecular interactions
  • Depends on temperature, pressure, and composition of the system
• Enthalpy measures heat content of a system at constant pressure
  • Defined as $H = U + PV$
  • $U$ represents internal energy
  • $P$ denotes pressure
  • $V$ signifies volume
• Entropy quantifies degree of disorder or randomness in a system
  • Increases during spontaneous processes
  • Calculated using equation $\Delta S = Q_{rev} / T$
  • $Q_{rev}$ represents reversible heat transfer
  • $T$ denotes absolute temperature

Thermodynamic Potentials and Capacities

• Gibbs Free Energy determines spontaneity of chemical reactions
  • Defined as $G = H - TS$
  • $H$ represents enthalpy
  • $T$ denotes temperature
  • $S$ signifies entropy
  • Negative $\Delta G$ indicates spontaneous reaction
• Heat Capacity measures amount of heat required to raise temperature of a substance
  • Specific heat capacity refers to heat capacity per unit mass
  • Molar heat capacity denotes heat capacity per mole of substance
  • Varies with temperature and pressure

Thermodynamic Processes

Energy Transfer Mechanisms

• Work involves transfer of energy through force acting over a distance
  • Mechanical work done by expanding gases calculated as $W = -P\Delta V$
  • $P$ represents pressure
  • $\Delta V$ denotes change in volume
  • Electrical work calculated as $W = qV$
  • $q$ signifies charge
  • $V$ represents voltage
• Reversible Process occurs infinitely slowly, allowing system to maintain equilibrium
  • Can be reversed by infinitesimal changes in surroundings
  • Ideal concept, not achievable in practice
  • Used as reference point for maximum efficiency

Irreversible Processes

• Irreversible Process occurs in finite time, creating entropy
  • Cannot be reversed without leaving a trace on surroundings
  • All real-world processes are irreversible
  • Examples include friction, heat transfer across finite temperature difference, unrestrained expansion of gas

Reaction Types

Energy Release and Absorption in Reactions

• Exothermic Reaction releases heat to surroundings
  • Enthalpy change ($\Delta H$) is negative
  • Examples include combustion reactions (burning of fuels)
  • Releases energy in form of heat and light
• Endothermic Reaction absorbs heat from surroundings
  • Enthalpy change ($\Delta H$) is positive
  • Examples include photosynthesis, decomposition of limestone
  • Requires energy input to proceed