Name: using standard enthalpy values in chemistry
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Standard Enthalpy of Formation

De enthalpie-verandering van een reactie kan worden gemeten met een calorimeter, of het kan worden gevonden door het verschil in enthalpie tussen de reactanten en de producten te berekenen. De absolute enthalpie van de reactanten en producten kunnen echter niet rechtstreeks worden gemeten;daarom gebruiken chemici over het algemeen de verandering in enthalpie, of delta H, ten opzichte van een referentiestof in een overeengekomen standaardtoestand. De standaardtoestand wordt bepaald door een specifieke reeks voorwaarden.Dit omvat een ingestelde temperatuur, meestal 25 graden Celsius of 298 Kelvin, en een constante druk van 1 bar voor gassen. Voor oplossingen is de standaardtoestand een 1 molaire concentratie van een zuivere opgeloste stof in een oplosmiddel. De standaardtoestand voor een stof omvat ook de fysische toestand van de stof waarin de stof onder deze omstandigheden voorkomt.Bijvoorbeeld natriumchloride als vaste stof, kwik als vloeistof of helium als gas. Als een element onder deze omstandigheden in meer dan één vorm voorkomt, wordt de meest stabiele vorm van het element gedefinieerd als de standaardtoestand. Koolstof kan bijvoorbeeld bestaan als grafietkristallen of als diamant, maar grafiet is de meest stabiele vorm en daarom de standaardtoestand van koolstof.Wanneer elementen in hun standaardtoestanden combineren om 1 mol van een zuivere verbinding te vormen, wordt de enthalpie van de reactie standaard enthalpie of standaard vormingswarmte genoemd. Dit wordt aangeduid met delta H nul, subscript f, waarbij nul de standaardtoestanden van de samenstellende elementen aangeeft, terwijl f de vorming aangeeft. De standaard vormingsenthalpie voor zuivere elementen onder standaardtoestandsomstandigheden is altijd nul omdat er geen reactie is, en dus geen verandering in enthalpie, wanneer het element zich al in de standaardtoestand bevindt.De waarden van de standaard vormingsenthalpie in kilojoules per mol voor een verbinding zijn te vinden in referentietabellen. Deze stoffen omvatten elementen in niet-standaardtoestanden, zoals gasvormig natrium en verbindingen zoals natriumchloride. De standaard enthalpie-verandering van een reactie kan worden berekend uit het verschil tussen producten en reactanten, die op hun beurt kunnen worden berekend met behulp van de enthalpie-waarden uit de referentietabel.

standaardformatie-enthalpie

Enthalpieveranderingen worden doorgaans in tabelvorm weergegeven voor reacties waarbij zowel de reactanten als de producten zich onder dezelfde omstandigheden bevinden. Een standaardtoestand is een algemeen aanvaarde reeks voorwaarden die als referentiepunt wordt gebruikt voor het bepalen van eigenschappen onder andere verschillende omstandigheden. Voor scheikundigen verwijst de IUPAC-standaardtoestand naar materialen onder een druk van 1 bar en oplossingen bij 1 M en specificeert geen temperatuur. Veel thermochemische tabellen vermelden waarden met een standaardtoestand van 1 atm. Omdat de ΔH van een reactie heel weinig verandert bij zulke kleine drukveranderingen (1 bar = 0,987 atm), zijn de ΔH-waarden (behalve de meest nauwkeurig gemeten waarden) in wezen hetzelfde onder beide sets standaardomstandigheden. Een superscript “o” in het enthalpieveranderingssymbool geeft de standaardstatus aan. Aangezien de gebruikelijke (maar technisch gezien niet standaard) temperatuur 298,15 K is, wordt van deze temperatuur uitgegaan, tenzij een andere temperatuur wordt gespecificeerd. Het symbool (ΔH°) wordt dus gebruikt om een enthalpieverandering aan te duiden voor een proces dat onder deze omstandigheden plaatsvindt. (Het symbool ΔH wordt gebruikt om een enthalpieverandering aan te duiden voor een reactie die plaatsvindt onder niet-standaard omstandigheden.) De enthalpieveranderingen voor veel soorten chemische en fysische processen zijn beschikbaar in de referentieliteratuur, inclusief die voor verbrandingsreacties, faseovergangen en vormingsreacties. Omdat de enthalpieverandering voor een bepaalde reactie evenredig is met de hoeveelheden betrokken stoffen, kan deze op die basis worden gerapporteerd (dat wil zeggen als de ΔH voor specifieke hoeveelheden reactanten). We vinden het echter vaak nuttiger om de ene uitgebreide eigenschap (ΔH) te delen door een andere (hoeveelheid stof), en een intensieve waarde van ΔH per hoeveelheid te rapporteren, vaak “genormaliseerd” naar een per mol-basis. <h4>Standaardformatie-enthalpie</h4> De standaardvormingsenthalpie ΔHf° is een enthalpieverandering voor een reactie waarbij precies 1 mol van een zuivere stof wordt gevormd uit vrije elementen in hun meest stabiele toestand onder standaardomstandigheden. Deze waarden zijn vooral handig voor het berekenen of voorspellen van enthalpieveranderingen voor chemische reacties die onpraktisch of gevaarlijk zijn om uit te voeren, of voor processen waarvoor het moeilijk is om metingen uit te voeren. Met behulp van bekende waarden van standaardvormingsenthalpieën kan de enthalpieverandering voor elke reactie worden bepaald. De standaardvormingsenthalpie van CO2 (g) is −393,5 kJ/mol. Dit is de enthalpieverandering voor de exotherme reactie: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;"> beginnend met de reactanten bij een druk van 1 atm en 25 °C (waarbij de koolstof aanwezig is als grafiet, de meest stabiele vorm van koolstof onder deze omstandigheden) en eindigend met één mol CO2, eveneens bij 1 atm en 25 °C. Voor stikstofdioxide is NO2 (g), ΔHf° 33,2 kJ/mol. Dit is de enthalpieverandering voor de endotherme reactie: <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;"> Een reactievergelijking met 1/2 mol N2 en 1 mol O2 is in dit geval correct omdat de standaardvormingsenthalpie altijd betrekking heeft op 1 mol product: NO2 (g). Per definitie is de standaardvormingsenthalpie van een element in zijn meest stabiele vorm onder standaardomstandigheden gelijk aan nul. De standaardenthalpieën voor de vorming van koolstof (grafeen), diatomisch zuurstofgas, diatomisch stikstofgas, natriummetaal en vloeibaar kwik zijn bijvoorbeeld nul onder standaardomstandigheden. Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Sectie 5.3: Enthalpie.</a>

Standaardformatie-enthalpie

Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state is a commonly ...

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using standard enthalpy values in chemistry

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature.

Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in the enthalpy symbol shows that the value is for standard state. The usual temperature is 298.15 K, so ΔH° is often used for a process under these conditions. ΔH is used for nonstandard conditions.

Standard enthalpy values are available for many chemical and physical processes. These include combustion reactions, phase changes, and formation reactions. Enthalpy change depends on the amount of substance involved, so it can be reported for a specific reaction amount. It is often more useful to divide by moles and give a per-mole value.

The standard enthalpy of formation, ΔHf°, is the enthalpy change when 1 mole of a pure substance forms from its elements in their most stable states under standard conditions. This value is useful when a reaction is hard to measure directly or would be unsafe to run. Known formation enthalpies can also be used to calculate the enthalpy change of other reactions.

For example, the standard enthalpy of formation of CO2(g) is −393.5 kJ/mol. That value matches the exothermic formation of one mole of carbon dioxide from graphite, the most stable form of carbon under these conditions, with all substances at 1 atm and 25 °C. For NO2(g), ΔHf° is 33.2 kJ/mol. Here the balanced formation reaction uses 1/2 mole of N2 and 1 mole of O2 because the value must refer to 1 mole of NO2 product.

By definition, an element in its most stable form has a standard enthalpy of formation of zero. Examples include carbon as graphene, diatomic oxygen gas, diatomic nitrogen gas, sodium metal, and liquid mercury. These zero values help make thermochemical calculations consistent.

Using Standard Enthalpy Values in Chemistry

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standard enthalpy of formation

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature. Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in...

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Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

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Learn how standard enthalpy values are reported, why ΔH° uses reference conditions, and how standard enthalpies of formation help calculate reaction energy.

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Using Standard Enthalpy Values in Chemistry

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