Endothermic process

This article is about the physical effect. For self-maintained thermal homeostasis, seeEndotherm.

Anendothermic processis a chemical or physical process that absorbs heat from its surroundings.In terms ofthermodynamicsandthermochemistry,it is athermodynamic processwith an increase in theenthalpyH(orinternal energyU) of the system.In an endothermic process, the heat that a system absorbs isthermal energytransfer into the system. Thus, an endothermic reaction generally leads to an increase in thetemperatureof the system and a decrease in that of the surroundings.

The term was coined by 19th-century French chemistMarcellin Berthelot.The termendothermiccomes from theGreekἔνδον (endon) meaning 'within' and θερμ- (therm) meaning 'hot' or 'warm'.

An endothermic process may be a chemical process, such as dissolvingammonium nitrate(NH₄NO₃) inwater(H₂O), or a physical process, such as the melting oficecubes.

The opposite of an endothermic process is anexothermic process,one that releases or "gives out" energy, usually in the form of heat and sometimes aselectrical energy.Thus,endoin endothermic refers to energy or heat going in, andexoin exothermic refers to energy or heat going out. In each term (endothermic and exothermic) the prefix refers to where heat (or electrical energy) goes as the process occurs.

In chemistry

The formation ofbarium thiocyanatefromammonium thiocyanateandbarium hydroxideis so endothermic that it can freeze abeakerto wetstyrofoam

Due to bonds breaking and forming during various processes (changes in state, chemical reactions), there is usually a change in energy. If the energy of the forming bonds is greater than the energy of the breaking bonds, then energy is released. This is known as an exothermic reaction. However, if more energy is needed to break the bonds than the energy being released, energy is taken up. Therefore, it is anendothermic reaction.

Details

Whether a process can occur spontaneously depends not only on theenthalpychange but also on theentropychange (∆S) andabsolute temperatureT.If a process is aspontaneous processat a certain temperature, the products have a lowerGibbs free energyG=H–TSthan the reactants (anexergonic process),even if the enthalpy of the products is higher. Thus, an endothermic process usually requires afavorable entropy increase(∆S> 0) in the system that overcomes the unfavorable increase in enthalpy so that still∆G< 0.While endothermicphase transitionsinto more disordered states of higher entropy, e.g. melting and vaporization, are common, spontaneous chemical processes at moderate temperatures are rarely endothermic.The enthalpy increase∆H≫ 0in a hypothetical strongly endothermic process usually results in∆G= ∆H–T∆S> 0,which means that the process will not occur (unless driven by electrical or photon energy). An example of an endothermic and exergonic process is

${\ce {C6H12O6 + 6 H2O -> 12 H2 + 6 CO2}}$

$\Delta _{r}H^{\circ }=+627\ {\text{kJ/mol}},\quad \Delta _{r}G^{\circ }=-31\ {\text{kJ/mol}}$.

Examples

• Evaporation
• Sublimation
• Crackingofalkanes
• Thermal decomposition
• Hydrolysis
• Nucleosynthesisof elements heavier thannickelin stellar cores
• High-energy neutrons can producetritiumfromlithium-7in an endothermic process, consuming 2.466MeV.This was discovered when the 1954Castle Bravonuclear test produced an unexpectedly high yield.
• Nuclear fusionof elements heavier thanironinsupernovae
• Dissolving togetherbarium hydroxideandammonium chloride
• Dissolving togethercitric acidandbaking soda

Distinction between endothermic and endotherm

The terms "endothermic" and "endotherm" are both derived fromGreekἔνδονendon"within" andθέρμηthermē"heat", but depending on context, they can have very different meanings.

In physics,thermodynamicsapplies to processes involving a system and its surroundings, and the term "endothermic" is used to describe a reaction where energy is taken "(with)in" by the system (vs. an "exothermic" reaction, which releases energy "outwards" ).

In biology,thermoregulationis the ability of anorganismto maintain its body temperature, and the term "endotherm"refers to an organism that can do so from" within "by using the heat released by its internal bodily functions (vs. an"ectotherm",which relies on external, environmental heat sources) to maintain an adequate temperature.

External links

• Exothermic and Endothermic– MSDS Hyper-Glossary at Interactive Learning Paradigms, Incorporated