First Law of Thermodynamics

Understanding the First Law of Thermodynamics

The First Law of Thermodynamics represents one of the most fundamental principles in physics and chemistry. This powerful law establishes that energy cannot be created or destroyed—only transformed from one form to another. Scientists and engineers worldwide rely on this principle to understand energy conservation in countless applications.

This comprehensive guide explores every aspect of the First Law of Thermodynamics, from basic thermodynamic terms to advanced applications like adiabatic expansion. You’ll master the concepts that govern energy behavior in chemical reactions, physical processes, and engineering systems.

Thermodynamic Terms: System, Boundary, Surroundings

System Definition

A thermodynamic system represents the specific portion of the universe we choose to study. Scientists define systems to analyze energy changes and apply the First Law of Thermodynamics effectively. The system contains matter and energy that interact according to thermodynamic principles.

Boundary Characteristics

The boundary separates the system from its surroundings. Boundaries can be real (like container walls) or imaginary (like mathematical surfaces). The First Law of Thermodynamics applies across these boundaries as energy flows between system and surroundings.

Surroundings Impact

The surroundings encompass everything outside the system boundary. Energy exchanges between system and surroundings follow the First Law of Thermodynamics. Understanding surroundings helps predict how external conditions affect internal energy changes.

Homogeneous and Heterogeneous Systems

Homogeneous Systems

Homogeneous systems maintain uniform properties throughout their volume. These systems simplify First Law of Thermodynamics calculations because energy distribution remains consistent. Examples include pure gases, single-phase liquids, and uniform solid solutions.

Heterogeneous Systems

Heterogeneous systems contain multiple phases or regions with different properties. The First Law of Thermodynamics applies to each phase individually and to the system as a whole. Common examples include ice-water mixtures, oil-water emulsions, and gas-solid reactions.

Types of Thermodynamic Systems

Open Systems

Open systems exchange both energy and matter with surroundings. The First Law of Thermodynamics for open systems includes terms for mass flow. Chemical reactors, living organisms, and heat exchangers exemplify open systems.

Closed Systems

Closed systems exchange energy but not matter with surroundings. The First Law of Thermodynamics simplifies for closed systems because mass remains constant. Sealed containers, piston-cylinder assemblies, and closed reaction vessels represent closed systems.

Isolated Systems

Isolated systems exchange neither energy nor matter with surroundings. The First Law of Thermodynamics states that internal energy remains constant in isolated systems. Perfect isolation rarely occurs in practice, but insulated containers approximate isolated systems.

Intensive and Extensive Properties

Intensive Properties

Intensive properties remain independent of system size or mass. Temperature, pressure, and density exemplify intensive properties. The First Law of Thermodynamics uses intensive properties to describe system states without considering system size.

Extensive Properties

Extensive properties depend on system size or mass. Internal energy, enthalpy, and volume represent extensive properties. The First Law of Thermodynamics directly relates to extensive properties like internal energy changes.

State of a System

The state of a system describes its condition at any given moment. State functions like temperature, pressure, and internal energy define system states. The First Law of Thermodynamics relates state changes to energy transfers.

State Functions

State functions depend only on current system conditions, not on how the system reached that state. Internal energy, enthalpy, and entropy are state functions. The First Law of Thermodynamics uses state functions to calculate energy changes.

Path Functions

Path functions depend on the specific process path taken. Heat and work are path functions. The First Law of Thermodynamics combines these path functions with state functions to describe energy conservation.

Equilibrium and Nonequilibrium States

Thermodynamic Equilibrium

Thermodynamic equilibrium occurs when system properties remain constant over time. Systems in equilibrium show no net energy flow. The First Law of Thermodynamics applies to equilibrium states and transitions between them.

Nonequilibrium States

Nonequilibrium states exhibit changing properties and energy flows. Real processes often involve nonequilibrium states. The First Law of Thermodynamics governs energy conservation during nonequilibrium transitions.

Thermodynamic Processes

Thermodynamic processes describe how systems change from one state to another. The First Law of Thermodynamics applies to all processes, ensuring energy conservation throughout state changes.

Isothermal Processes

Isothermal processes maintain constant temperature. The First Law of Thermodynamics for isothermal processes shows that heat added equals work done by the system (for ideal gases).

Adiabatic Processes

Adiabatic processes prevent heat transfer. The First Law of Thermodynamics simplifies to ΔU = -W for adiabatic processes, showing that work changes internal energy directly.

Isobaric Processes

Isobaric processes maintain constant pressure. The First Law of Thermodynamics for isobaric processes relates heat transfer to enthalpy changes.

Isochoric Processes

Isochoric processes maintain constant volume. The First Law of Thermodynamics shows that heat added equals internal energy change in isochoric processes.

Reversible and Irreversible Processes

Reversible Processes

Reversible processes can be reversed without leaving traces in the surroundings. These idealized processes maximize work output and minimize energy dissipation. The First Law of Thermodynamics applies equally to reversible processes.

Irreversible Processes

Irreversible processes cannot be reversed without permanent changes to surroundings. Real processes are irreversible due to friction, heat conduction, and other dissipative effects. The First Law of Thermodynamics governs energy conservation in irreversible processes.

Nature of Heat and Work

Heat Transfer

Heat represents energy transfer due to temperature differences. Heat flows spontaneously from high to low temperature regions. The First Law of Thermodynamics treats heat as energy crossing system boundaries.

Work Definition

Work represents energy transfer through organized motion or force application. Mechanical work, electrical work, and expansion work are common types. The First Law of Thermodynamics includes work as energy transfer mechanism.

Sign Conventions

The First Law of Thermodynamics uses consistent sign conventions. Heat added to the system is positive; work done by the system is positive. These conventions ensure proper energy accounting in thermodynamic calculations.

Internal Energy

Internal energy represents the total energy contained within a system. This fundamental concept in the First Law of Thermodynamics includes kinetic energy of molecular motion and potential energy of intermolecular forces.

Components of Internal Energy

Internal energy consists of multiple energy forms. Translational, rotational, and vibrational kinetic energies contribute to internal energy. Intermolecular potential energies also affect total internal energy values.

Internal Energy Changes

The First Law of Thermodynamics relates internal energy changes to heat and work transfers. ΔU = Q – W represents the fundamental equation connecting these quantities. Internal energy changes depend only on initial and final states.

Units of Internal Energy

The First Law of Thermodynamics uses various units for internal energy measurements. Consistent unit usage ensures accurate calculations and meaningful results in thermodynamic analyses.

Specific Internal Energy

Specific internal energy represents internal energy per unit mass. The First Law of Thermodynamics often uses specific properties for intensive calculations. Units include J/kg, cal/g, and BTU/lbm.

Molar Internal Energy

Molar internal energy represents internal energy per mole of substance. Chemical applications of the First Law of Thermodynamics frequently use molar quantities. Units include J/mol, cal/mol, and BTU/lbmol.

First Law of Thermodynamics Statement

Physical Interpretation

The First Law of Thermodynamics embodies energy conservation principles. Energy cannot be created or destroyed, only converted between forms. This fundamental law governs all energy transformations in the universe.

Alternative Forms

The First Law of Thermodynamics appears in various mathematical forms. For closed systems: dU = δQ – δW. For open systems, additional terms account for mass flow and associated energy transport.

Enthalpy of a System

Enthalpy represents a thermodynamic property combining internal energy and flow work. The First Law of Thermodynamics uses enthalpy to simplify calculations for constant-pressure processes. H = U + PV defines enthalpy mathematically.

Enthalpy Significance

Enthalpy changes equal heat transfer in constant-pressure processes. The First Law of Thermodynamics shows that ΔH = Q for isobaric processes. This relationship makes enthalpy particularly useful in chemical thermodynamics.

Enthalpy Applications

Chemical reactions, phase changes, and heating processes commonly use enthalpy. The First Law of Thermodynamics relates enthalpy changes to energy conservation in these processes. Enthalpy tables provide data for engineering calculations.

Molar Heat Capacities

Molar heat capacities quantify energy required to raise temperature per mole of substance. The First Law of Thermodynamics relates heat capacities to internal energy and enthalpy changes during heating processes.

Constant Volume Heat Capacity

Cv represents molar heat capacity at constant volume. The First Law of Thermodynamics shows that Cv = (∂U/∂T)v for closed systems. This relationship connects heat capacity to internal energy changes.

Constant Pressure Heat Capacity

Cp represents molar heat capacity at constant pressure. The First Law of Thermodynamics demonstrates that Cp = (∂H/∂T)p for constant-pressure processes. Cp values exceed Cv values for gases due to expansion work.

Temperature Dependence

Heat capacities vary with temperature for real substances. The First Law of Thermodynamics requires integration over temperature ranges for accurate energy calculations. Polynomial expressions often represent temperature dependence.

Joule-Thomson Effect

The Joule-Thomson effect describes temperature changes during constant-enthalpy expansion. This phenomenon demonstrates the First Law of Thermodynamics in throttling processes where enthalpy remains constant but temperature changes.

Joule-Thomson Coefficient

The Joule-Thomson coefficient μ = (∂T/∂P)H quantifies temperature change per pressure change at constant enthalpy. The First Law of Thermodynamics relates this coefficient to thermodynamic properties and intermolecular forces.

Practical Applications

Refrigeration systems, gas liquefaction, and pressure reduction valves utilize the Joule-Thomson effect. The First Law of Thermodynamics governs energy conservation in these applications while temperature changes occur.

Adiabatic Expansion of an Ideal Gas

Adiabatic expansion occurs without heat transfer between system and surroundings. The First Law of Thermodynamics simplifies to ΔU = -W for adiabatic processes, showing that work comes from internal energy decrease.

Adiabatic Process Equations

For ideal gases undergoing adiabatic processes, PVᵞ = constant, where γ = Cp/Cv. The First Law of Thermodynamics combined with ideal gas behavior yields these relationships for adiabatic processes.

Temperature Changes

Adiabatic expansion causes temperature decrease as internal energy converts to work. The First Law of Thermodynamics quantifies this temperature change through the relationship between internal energy and temperature for ideal gases.

Work Done in Adiabatic Reversible Expansion

The First Law of Thermodynamics determines work done during adiabatic reversible expansion. Since Q = 0 for adiabatic processes, all work comes from internal energy decrease: W = -ΔU.

Work Calculation

For ideal gases, work in adiabatic reversible expansion equals W = (P₁V₁ – P₂V₂)/(γ-1). The First Law of Thermodynamics ensures this work equals the internal energy decrease during expansion.

Maximum Work

Adiabatic reversible expansion produces maximum work output for given initial and final states. The First Law of Thermodynamics shows that irreversible adiabatic processes produce less work due to energy dissipation.

Explore More Science Topics