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1st Law of Thermodynamics

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The 1st Law of Thermodynamics tells us that energy is neither created nor destroyed, thus the energy of the universe is a constant. However, energy can be transferred from one part of the universe to another. To work out thermodynamic problems we will need to isolate a certain portion of the universe, the system, from the remainder of the universe, the surroundings.

The energy transfer between different systems can be expressed as:

E1 = E2                       (1)


E1 = initial energy

E2 = final energy

The internal energy encompasses:

  • The kinetic energy associated with the motions of the atoms
  • The potential energy stored in the chemical bonds of the molecules
  • The gravitational energy of the system

The first law is the starting point for the science of thermodynamics and for engineering analysis.

Based on the types of exchange that can take place we will define three types of systems:

  • isolated systems: no exchange of matter or energy
  • closed systems: no exchange of matter but some exchange of energy
  • open systems: exchange of both matter and energy

The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines.

Internal Energy

Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.


Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is called heating.


When work is done by a thermodynamic system, it is usually a gas that is doing the work. The work done by a gas at constant pressure is W = p dV, where W is work, p is pressure and dV is change in volume.
For non-constant pressure, the work can be visualized as the area under the pressure-volume curve which represents the process taking place.

Heat Engines

Refrigerators, Heat pumps, Carnot cycle, Otto cycle


The change in internal energy of a system is equal to the heat added to the system minus the work done by the system:

dE = Q - W                          (2)


dE = change in internal energy

Q = heat added to the system

W = work done by the system

1st law does not provide the information of direction of processes and does not determine the final equilibrium state. Intuitively, we know that energy flows from high temperature to low temperature. Thus, the 2nd law is needed to determine the direction of processes.

Enthalpy is the "thermodynamic potential" useful in the chemical thermodynamics of reactions and non-cyclic processes. Enthalpy is defined by

H = U + PV                       (3)


H = enthalpy

U = internal energy

P = pressure

V = volume

Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables.

Go to Thermodyamics key values internationally agreed, Standard state and enthalpy of formation, Gibbs free energy of formation, entropy and heat capacity and Standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity of organic substances for listing of values for a lot of inorganic and organic substances. 

Entropy is used to define the unavailable energy in a system. Entropy defines the relative ability of one system to act to an other. As things moves toward a lower energy level, where one is less able to act upon the surroundings, the entropy is said to increase. Entropy is connected to the Second Law of Thermodynamics.

For the universe as a whole the entropy is increasing.

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Related Topics


Work, heat and energy systems.

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Entropy and disorder.

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Energy Transfer Equation

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The mechanical, thermal, electrostatic, phase or chemical states of equilibrium.

Heat, Work and Energy

Heat vs. work vs. energy.

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The Mechanical Energy Equation compared to the Extended Bernoulli Equation.

Standard State and Enthalpy of Formation, Gibbs Free Energy of Formation, Entropy and Heat Capacity

Definition and explanation of the terms standard state and standard enthalpy of formation, with listing of values for standard enthalpy and Gibbs free energy of formation, as well as standard entropy and molar heat capacity, of 370 inorganic compounds.

Standardized Enthalpies and Entropies

Standardized enthalpies and entropies for some common substances.

Symbols Used to Denote a Chemical Reactions and Process or Condition

Explanation of symbols used as subscripts or superscripts to tell more about the type of chemical reaction, process or condition.

Thermodynamic Key Values Internationally Agreed

Internationally agreed, internally consistent, values for the thermodynamic properties (standard enthalpy of formation, entropy and [H°(298)-H°(0)]) of key chemical substances.

Thermodynamic Terms - Functions and Relations

Common thermodynamic terms and functions - potential energy, kinetic energy, thermal or internal energy, chemical energy, nuclear energy and more.

Third Law of Thermodynamics

The entropy of a substance is zero if the absolute temperature is zero.

WABT - Weighted Average Bed Temperature

Definition and examples of calculation of weighted average bed temperature in adiabatic reactors.

Water-Steam Mollier Diagram

Enthalpy-entropy diagram for water and steam.

Work done by Force

Work done by a force acting on an object.

Zeroth Law of Temperature

The direction of heat flow.

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