Editing Chemistry (section)

===Energy===
{{Main|Energy}}

In the context of chemistry, energy is an attribute of a substance as a consequence of its [[atomic structure|atomic]], [[molecular structure|molecular]] or aggregate [[chemical structure|structure]]. Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it is invariably accompanied by an [[endothermic reaction|increase]] or [[exothermic reaction|decrease]] of [[energy]] of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or [[photochemistry|light]]; thus the products of a reaction may have more or less energy than the reactants.

A reaction is said to be [[exergonic reaction|exergonic]] if the final state is lower on the energy scale than the initial state; in the case of [[endergonic reaction]]s the situation is the reverse. A reaction is said to be [[exothermic reaction|exothermic]] if the reaction releases heat to the surroundings; in the case of [[endothermic reaction]]s, the reaction absorbs heat from the surroundings.

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the [[activation energy]]. The ''speed'' of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor <math>e^{-E/kT} </math> – that is the probability of a molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the [[Arrhenius equation]].
The activation energy necessary for a chemical reaction to occur can be in the form of heat, light, [[electricity]] or mechanical [[force]] in the form of [[ultrasound]].<ref>Reilly, Michael. (2007). [https://www.newscientist.com/article/dn11427-mechanical-force-induces-chemical-reaction.html Mechanical force induces chemical reaction], {{Webarchive|url=https://web.archive.org/web/20140814004108/http://www.newscientist.com/article/dn11427-mechanical-force-induces-chemical-reaction.html#.Uy6ySlendfA|date=14 August 2014}}, NewScientist.com news service.</ref>

A related concept [[thermodynamic free energy|free energy]], which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in [[chemical thermodynamics]]. A reaction is feasible only if the total change in the [[Gibbs free energy]] is negative, <math> \Delta G \le 0 \,</math>; if it is equal to zero the chemical reaction is said to be at [[chemical equilibrium|equilibrium]].

There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of [[quantum mechanics]], which require [[quantization (physics)|quantization]] of energy of a bound system. The atoms/molecules in a higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive; that is, more amenable to chemical reactions.

The phase of a substance is invariably determined by its energy and the energy of its surroundings. When the [[intermolecular force]]s of a substance are such that the energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with water (H<sub>2</sub>O); a liquid at room temperature because its molecules are bound by [[hydrogen bonds]].<ref>[http://www.chem4kids.com/files/matter_changes.html Changing States of Matter], {{Webarchive|url=https://web.archive.org/web/20070428171905/http://www.chem4kids.com/files/matter_changes.html|date=28 April 2007}}, Chemforkids.com.</ref> Whereas [[hydrogen sulfide]] (H<sub>2</sub>S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker [[dipole–dipole interaction]]s.

The transfer of energy from one chemical substance to another depends on the ''size'' of energy [[quantum|quanta]] emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the [[phonons]] responsible for vibrational and rotational energy levels in a substance have much less energy than [[photons]] invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, [[ultraviolet]] electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy.

The existence of characteristic energy levels for different [[chemical substance]]s is useful for their identification by the analysis of [[spectral lines]]. Different kinds of spectra are often used in chemical [[spectroscopy]], e.g. [[infrared spectroscopy|IR]], [[microwave spectroscopy|microwave]], [[NMR]], [[electron spin resonance|ESR]], etc. Spectroscopy is also used to identify the composition of remote objects – like stars and distant galaxies – by analyzing their radiation spectra.
[[File:Emission spectrum-Fe.svg|thumb|upright=2.2|Emission spectrum of [[iron]]]]

The term [[Energy#Chemical energy|chemical energy]] is often used to indicate the potential of a chemical substance to undergo a transformation through a [[chemical reaction]] or to transform other chemical substances.