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What Is Standard Entropy?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 07 November 2016
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Standard entropy, in general, is a measure of the amount of heat energy in a closed system that is not available for work, and is usually considered to be the amount of disorder a system contains. The definition of standard entropy has slightly different meanings depending on the field of science to which it is being applied. In chemistry, standard molar entropy is defined as the entropy of 1 mole, or gram molecule, of matter at a standard atmospheric pressure of 14.7 lbs/inch2 (101.3 kPa) and a given temperature.

Physical systems in nature are assumed to be undergoing standard entropy change. This entails increasing levels of standard entropy as time passes, with the ultimate result that the universe will one day encounter maximum entropy. Known as heat death, it is a state where all energy is equally distributed throughout space and at the same temperature, making it no longer capable of performing any work.

The symbol used to represent standard entropy is S° and is expressed in units of work or energy known as joules, per mole of temperature kelvin, such that expressing standard molar entropy would be Sm°/J mole-1 K-1. This is broken down to a unit-less number in a standard entropy table. The most durable of substances have the lowest intrinsic entropy, where diamond at a standard temperature of 77° Fahrenheit (25° Celsius or 298 Kelvin) has the lowest known entropy of 2.377, with liquid water one of 69.9 and helium one of 126.

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The laws of thermodynamics state that energy is neither created nor destroyed. Calculating standard entropy, therefore, is a method of determining the movement of energy between matter and systems, where the net energy of the entire universe, considered as a closed system, always remains constant. Often statistical mechanics is used to calculate this energy transfer in chemistry and physics, as it can model the movement of molecules in various energy states.

Though entropy is said to be increasing overall throughout space, the illusion in human activity is that it is being reduced. When matter is manufactured into something useful for work, the standard entropy or disorder of the chemical state of the raw matter used is reduced. Far more unrecoverable energy is used in producing the product, however, than it is worth.

This illusion that standard entropy is being reduced on Earth as civilization brings order to chaos is perpetuated by the fact that the Earth is not a closed system. As highly structured chemicals like refined fossil fuels are burned, more net heat energy is lost to space in the same way that the sun radiates most of its heat into space. This heat can never be recovered.

This is why materials like diamond have a lower standard entropy state at 2.377 than graphite at 5.74, though both are composed of the same element, carbon. Much more natural energy and pressure went into producing the diamond than the graphite, giving it a higher level of intrinsic order. Therefore, the higher the order of a system or material, the more standard entropy it has contributed to the universe in its production.

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