5.4

First Law of Thermodynamics

The principle that energy cannot be created or destroyed, only transformed or transferred.

Internal Energy (ΔU)

The total energy contained within a system.

Heat (q)

The energy transferred into or out of a system due to temperature differences.

Work (w)

The energy transferred when a force is applied over a distance.

P–V Work

Work associated with the expansion or compression of a gas.

Enthalpy (ΔH)

A measure of the total heat content of a system at constant pressure.

Endothermic Process

A process that absorbs heat from the surroundings (q > 0).

Exothermic Process

A process that releases heat to the surroundings (q < 0).

State Function

A property whose value depends only on the state of the system, not on how it reached that state.

Path Function

A property that depends on the specific way in which a change occurs.

Gas Expansion

The process where a gas increases in volume.

Gas Compression

The process where a gas decreases in volume.

Heat Absorption

When a system gains heat energy (q > 0).

Heat Release

When a system loses heat energy (q < 0).

ΔU=q+w

The equation representing the change in internal energy as the sum of heat and work.

w=−PΔV

The formula for calculating work done during a gas expansion or compression.

ΔH=ΔU+PΔV

The equation used to calculate enthalpy change at constant pressure.

Constant Volume Process

A thermodynamic process where the volume remains constant, resulting in no work done (w = 0).

Constant Pressure Process

A process where the pressure remains constant.

Sign Convention for Heat

If heat is absorbed by the system, q is positive; if heat is released, q is negative.

Sign Convention for Work

If work is done on the system, w is positive; if work is done by the system, w is negative.

Heat Capacity

The amount of heat required to change a substance's temperature by one degree.

Specific Heat

The amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius.

Thermodynamic System

A quantity of matter or a region in space chosen for study.

Surroundings

Everything outside the thermodynamic system.

Isolated System

A system that does not exchange matter or energy with its surroundings.

Closed System

A system that can exchange energy but not matter with its surroundings.

Open System

A system that can exchange both energy and matter with its surroundings.

Phase Change

A transition of a substance from one state of matter to another.

Thermal Equilibrium

The state at which two bodies in thermal contact cease to exchange energy.

Combustion

A chemical reaction that occurs between a fuel and an oxidant, producing heat and light.

Sublimation

The process of a solid turning directly into a gas without passing through a liquid phase.

Heat Transfer

The movement of thermal energy from one object to another.

Thermodynamic Cycle

A series of processes that return a system to its initial state.

Work Done on a Gas

The energy transferred to the gas when external pressure is applied (typically seen in compression).

Work Done by a Gas

The energy transferred by the gas when it expands against external pressure.

Molar Enthalpy of Vaporization

The amount of energy needed to vaporize one mole of a liquid at constant temperature and pressure.

Thermal Energy

The total energy of all the particles in a substance.

Heat Flow

The movement of thermal energy from a high-temperature area to a low-temperature area.

Energy Conversion

The process of changing one form of energy into another.

Ideal Gas Law

An equation of state for an ideal gas, represented as PV=nRT.

Real Gas Behavior

Deviations from the ideal gas laws observed at high pressures and low temperatures.

Gibbs Free Energy

A thermodynamic potential used to predict the spontaneity of a process at constant temperature and pressure.

Calorimetry

The measurement of heat transfer in a chemical reaction or physical change.

Latent Heat

The heat absorbed or released during a phase change without changing the temperature.

Thermodynamic Equilibrium

A state in which there are no net macroscopic flows of matter or energy.