Thermodynamic States & Processes

Thermodynamic States

A thermodynamic state of a system is its condition of a system at a specific time, that is fully identified by values of a suitable set of parameters known as state variables.

State Variables

Pressure (P)

Volume (V)

Temperature (T)

Amount - mole (n)

Why do we study thermodynamic processes?

Thermodynamic processes are likely paths to convert the system from one state to another.

Khan Academy: Macro-states and micro-states

The difference between macro-states and micro-states. Thermodynamic equilibrium.

Khan Academy: Quasi-static and reversible processes

Using theoretically quasi-static and/or reversible processes to stay pretty much at equilibrium.

Fundamental thermodynamic processes

We considered several thermodynamic processes:

(a) An isobaric process, during which the system’s pressure does not change.

(b) An isothermal process, during which the system’s temperature remains constant.

(c) An isochoric process, during which the system’s volume does not change.

(d) An adiabatic process, during which no heat is transferred to or from the system.

Isobaric (constant pressure) process

Isobaric process is a thermodynamic process in which the pressure stays constant: ΔP = 0. The heat transferred to the system does work, but also changes the internal energy of the system.

PV diagrams - part 1: Work and isobaric processes

Heat energy is added to a system (+Q)

The work done by a confined gas in moving a piston a distance d is given by:

Problem

Determine the change of internal energy (ΔU) of the system if 1000 Joules of heat energy were added to the system and the piston does 250 Joules of work.

Isothermal (constant temperature) process

An isothermal process is a change of a system, in which the temperature remains constant: ΔT =0.

PV diagrams - part 2: Isothermal, isometric, adiabatic processes

When a gas is compressed slowly from P1, V1 to P2, V2 the work done on the system is represented by the shaded area under the p-V curve.

Several isotherms of an ideal gas on a p-V diagram

Isochoric (constant volume) process

An isochoric process, also called a constant-volume process, an isovolumetric process, or an isometric process, is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant. An isochoric process is exemplified by the heating or the cooling of the contents of a sealed, inelastic container:

Isochoric (constant volume)

- U = Q + W (is zero) so Q = U
  - Work done (±W): none
  - Heat flow (±Q): Heat added
  - Change in internal energy (±∆U): Increases
  - Q = U

Adiabatic (constant heat) process

A process that does not involve the transfer of heat or matter into or out of a system, so that Q = 0

An adiabatic expansion leads to a lowering of temperature, and an adiabatic compression leads to an increase of temperature.

Adiabatic

- U = -W
  - Heat flow (±Q): No heat is added
  - Work done (±W):
  - Change in internal energy (±∆U):
  - W = - U

PV Diagrams, How To Calculate The Work Done By a Gas

This video explains how to calculate the work done by a gas for an isobaric process, isochoric process, isothermal process, and an adiabatic process. It also explains how to calculate work done for a cyclic process.

The Organic Chemistry Tutor

Thermodynamic Processes

We use the PhET Gas Properties simulation and a (PV) state diagram to demonstrate some standard types of thermodynamic processes.

The paths ABC, AC, and ADC represent three different quasi-static transitions between the equilibrium states A and C.

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