Energy Chemistry

in context of chemistry, energy attribute of substance consequence of atomic, molecular or aggregate structure. since chemical transformation accompanied change in 1 or more of these kinds of structures, invariably accompanied increase or decrease of energy of substances involved. energy transferred between surroundings , reactants of reaction in form of heat or light; products of reaction may have more or less energy reactants.

a reaction said exergonic if final state lower on energy scale initial state; in case of endergonic reactions situation reverse. reaction said exothermic if reaction releases heat surroundings; in case of endothermic reactions, reaction absorbs heat surroundings.

chemical reactions invariably not possible unless reactants surmount energy barrier known activation energy. speed of chemical reaction (at given temperature t) related activation energy e, boltzmann s population factor




e

−
e

/

k
t




{\displaystyle e^{-e/kt}}

- probability of molecule have energy greater or equal e @ given temperature t. exponential dependence of reaction rate on temperature known arrhenius equation. activation energy necessary chemical reaction occur can in form of heat, light, electricity or mechanical force in form of ultrasound.

a related concept free energy, incorporates entropy considerations, useful means predicting feasibility of reaction , determining state of equilibrium of chemical reaction, in chemical thermodynamics. reaction feasible if total change in gibbs free energy negative,



Δ
g
≤
0



{\displaystyle \delta g\leq 0\,}

; if equal 0 chemical reaction said @ equilibrium.

there exist limited possible states of energy electrons, atoms , molecules. these determined rules of quantum mechanics, require quantization of energy of bound system. atoms/molecules in higher energy state said excited. molecules/atoms of substance in excited energy state more reactive; is, more amenable chemical reactions.

the phase of substance invariably determined energy , energy of surroundings. when intermolecular forces of substance such energy of surroundings not sufficient overcome them, occurs in more ordered phase liquid or solid case water (h2o); liquid @ room temperature because molecules bound hydrogen bonds. whereas hydrogen sulfide (h2s) gas @ room temperature , standard pressure, molecules bound weaker dipole-dipole interactions.

the transfer of energy 1 chemical substance depends on size of energy quanta emitted 1 substance. however, heat energy transferred more substance because phonons responsible vibrational , rotational energy levels in substance have less energy photons invoked electronic energy transfer. thus, because vibrational , rotational energy levels more closely spaced electronic energy levels, heat more transferred between substances relative light or other forms of electronic energy. example, ultraviolet electromagnetic radiation not transferred efficacy 1 substance thermal or electrical energy.

the existence of characteristic energy levels different chemical substances useful identification analysis of spectral lines. different kinds of spectra used in chemical spectroscopy, e.g. ir, microwave, nmr, esr, etc. spectroscopy used identify composition of remote objects - stars , distant galaxies - analyzing radiation spectra.

emission spectrum of iron

the term chemical energy used indicate potential of chemical substance undergo transformation through chemical reaction or transform other chemical substances.