# The Three Laws of Thermodynamics

## The laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems.

#### Key Points

• The first law, also known as Law of Conservation of Energy, states that energy can not be created or destroyed; it can only be redistributed or changed from one form to another.

• The second law of thermodynamics says that the entropy of any isolated system not in thermal equilibrium almost always increases.

• The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches zero.

#### Terms

• A thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.

• The process of reaching thermal equilibrium by mutual interaction

#### Figures

1. ##### A Thermodynamic System

A diagram of a generic thermodynamic system

Thermodynamics defines macroscopic variables (such as temperature, internal energy, entropy, and pressure) that describe average properties of material bodies and radiation, explains how they are related and by what laws they change with time. Everything that is not a part of the system constitutes the surroundings. The system and surroundings are separated by a boundary Figure 1. If our system is one mole of a gas in a container, then the boundary is simply the inner wall of the container itself. The single property that the boundary must have is that it be clearly defined, so we can clearly say whether a given part of the world is in our system or in the surroundings. If matter is not able to pass across the boundary, then the system is said to be closed; otherwise, it is open. A closed system may still exchange energy with the surroundings unless the system is an isolated one, in which case neither matter nor energy can pass across the boundary.

Thermodynamics makes no distinction between kinetic and potential energy and it does not assume the existence of atoms and molecules. In the context of chemistry, the internal energy is the sum of the kinetic energy of the molecules, and the potential energy represented by the chemical bonds between the atoms and any other intermolecular forces that may be operative.

The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can be neither be created nor destroyed; it can only be transferred or changed from one form to another. For example: The dissolution of ammonium nitrate in water in a single-use cold pack may appear to destroy energy as the temperature of the cold pack decreases. However, the heat energy is only converted to a different form, chemical energy that is invested in chemical bonds.

A way of expressing the first law of thermodynamics is that any change in the internal energy (∆U) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by the surroundings:

$ΔU = q + w$

This law says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system. Since both heat and work can be measured and quantified, this is the same as saying that any change in the energy of a system must result in a corresponding change in the energy of the world outside the system. In other words, energy cannot be created or destroyed. If heat flows into a system or the surroundings to do work on it, the internal energy increases and the sign of q or w is positive. Conversely, heat flow out of the system or work done by the system will be at the expense of the internal energy, and will therefore be negative.

The second law of thermodynamics says that the entropy of any isolated system not in thermal equilibrium almost always increases. Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system—in a process known as "thermalization".  Equivalently, perpetual motion machines of the second kind are impossible.  More simply put: the entropy of the world only increases and never decreases.

A simple application of the second law of thermodynamics is that a room, if not cleaned and tidied, will invariably become more messy and disorderly with time - regardless of how careful one is to keep it clean. When the room is cleaned, its entropy decreases, but the effort to clean it has resulted in an increase in entropy outside the room that exceeds the entropy lost.

Third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches zero.  The entropy of a system at absolute zero is typically zero, and in all cases is determined only by the number of different ground states it has.  Specifically, the entropy of a pure crystalline substance at absolute zero temperature is zero.  This statement holds true if the perfect crystal has only one state with minimum energy.

#### Key Term Glossary

absolute zero
The coldest possible temperature, zero on the Kelvin scale, or approximately −273.15 °C, −459.67 °F; total absence of heat; temperature at which motion of all molecules would cease.
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Absolute zero
The theoretical lowest possible temperature. By international agreement, absolute zero is defined as 0K on the Kelvin scale and as −273.15° on the Celsius scale.
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atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
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bond
a link or force between neighboring atoms in a molecule
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chemical bond
Any of several attractive forces that serve to bind atoms together to form molecules.
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chemical energy
the potential of a substance to undergo a transformation through a reaction, or to transform other substances
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chemistry
The branch of natural science that deals with the composition and constitution of substances and the changes that they undergo as a consequence of alterations in the constitution of their molecules.
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closed system
A system that can exchange heat and work, but not matter, with its surroundings.
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constant
Consistently recurring over time; persistent
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crystal
A solid composed of an array of atoms or molecules possessing long-range order and arranged in a pattern which is periodic in three dimensions.
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crystalline
having a regular three-dimensional molecular structure
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dissolution
Dissolving, or going into solution.
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energy
a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent
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entropy
A thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.
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equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
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gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
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ground state
the stationary state of lowest energy of a particle or system of particles.
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heat
Heat is defined as the energy transferred from one system to another by thermal interaction.
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intermolecular
from one molecule to another; between molecules
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intermolecular force
Any of the attractive or repulsive interactions that occur between atoms or molecules and become significant at separations of about 1 nanometer or less
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intermolecular forces
attractive and repulsive forces between molecules
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internal energy
A property characteristic of the state of a thermodynamic system, the change in which is equal to the heat absorbed minus the work done by the system.
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isolated system
A system that does not interact with its surroundings; that is, its total energy and mass stay constant.
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kinetic
of or relating to motion
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kinetic energy
The energy possessed by an object because of its motion, equal to one half the mass of the body times the square of its velocity.
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kinetics
The branch of chemistry that is concerned with the rates of chemical reactions.
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matter
The basic structural component of the universe. Matter usually has mass and volume.
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mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
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molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
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potential energy
the energy possessed by an object because of its position (in a gravitational or electric field), or its condition (as a stretched or compressed spring, as a chemical reactant, or by having rest mass)
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Pressure
the amount of force that is applied over a given area divided by the size of this area
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state
The physical property of matter as solid, liquid, gas or plasma
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substance
Physical matter; material.
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surroundings
All parts of the universe that are not within the thermodynamic system of interest.
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system
the part of the universe being studied, arbitrarily defined to any size desired
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temperature
A measure of cold or heat, often measurable with a thermometer.
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thermal
pertaining to heat or temperature.
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thermalization
The process of reaching thermal equilibrium by mutual interaction
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thermodynamics
The science of the conversions between heat and other forms of energy.
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work
In thermodynamics, work performed by a closed system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system.