Name: using standard enthalpy values in chemistry
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PS.Chm.7.2

Standard Enthalpy of Formation

A alteração da entalpia de uma reação pode ser medida utilizando um calorímetro, ou pode ser encontrada calculando a diferença da entalpia entre os reagentes e os produtos. No entanto, as entalpias absolutas dos reagentes e produtos não pode ser medida diretamente;portanto, os químicos utilizam geralmente a alteração da entalpia, ou delta H, relativa a uma substância de referência num estado padrão acordado. O estado padrão é definido por um conjunto específico de condições.Isto inclui uma temperatura definida, normalmente 25 graus Celsius ou 298 Kelvin, e uma pressão constante de 1 bar para os gases. Para as soluções, o estado padrão é uma concentração molar de 1 mol de um soluto puro num solvente. O estado padrão para uma substância inclui também o estado físico da matéria em que a substância existe sob estas condições.Por exemplo, o cloreto de sódio como sólido, o mercúrio como líquido ou o hélio como gás. Se um elemento existe em mais do que uma forma sob estas condições, a forma mais estável do elemento é definida como o estado padrão. Por exemplo, o carbono pode existir como cristais de grafite ou como um diamante, mas o grafite é a forma mais estável e, portanto, o estado padrão do carbono.Quando os elementos nos seus estados padrão se combinam para formar 1 mol de um composto puro, a entalpia da reação é chamada de entalpia padrão, ou calor padrão, de formação. Isto é indicado por delta H zero, subscrito f, onde zero indica os estados padrão dos elementos constituintes, e f indica a formação. A entalpia padrão da formação para os elementos puros em condições de estado padrão é sempre zero porque não há reação, e, portanto, nenhuma alteração na entalpia, quando o elemento já está no seu estado padrão.Os valores da entalpia padrão de formação em kilojoules por mol para um composto podem ser encontrados em tabelas de referência. Estas substâncias incluem elementos em estados não padronizados, como o sódio gasoso, e compostos como o cloreto de sódio. A alteração da entalpia padrão de uma reação pode ser calculada a partir da diferença de produtos e reagentes, que, por sua vez, podem ser calculados utilizando os valores da entalpia da tabela de referência.

<ol>
	<li>Falcão, Eric Leigh. "O cálculo de entalpias padrão de formação de alcanos: Ilustrando a mecânica molecular e programas de planilhas." Jornal de educação química 76, nº 2 (1999): 278.</li>
	<li>Mazzuca, James W., Alexis R. Downing e Christopher Potter. "Cálculos de estrutura eletrônica empiricamente corrigidos aplicados à entalpia do laboratório de físico-química de combustão." Jornal de Educação Química 96, nº 6 (2019): 1165-1170.</li>
	<li>Jansen, Michael P. "O custo de converter um veículo movido a gasolina em propano. Um Problema de Revisão Prática para o Ensino Médio ou Química Introdutória." Journal of Chemical Education 77, nº 12 (2000): 1578. 
	 </li>
</ol>

entalpia de formação padrão

As mudanças de entalpia são tipicamente tabuladas para reações nas quais tanto os reagentes como os produtos estão nas mesmas condições. Um estado padrão é um conjunto de condições comumente aceite, usado como ponto de referência para a determinação de propriedades sob outras condições diferentes. Para os químicos, o estado padrão IUPAC refere-se a materiais sob uma pressão de 1 bar e soluções a 1 M e não especifica uma temperatura. Muitas tabelas termoquímicas listam valores com um estado padrão de 1 atm. Como a ΔH de uma reação muda muito pouco com alterações tão pequenas na pressão (1 bar = 0,987 atm), os valores de ΔH (excepto os valores medidos com mais precisão) são essencialmente os mesmos em ambos os conjuntos de condições padrão. Um “o” em superscrito no símbolo de alteração da entalpia designa o estado padrão. Uma vez que a temperatura normal (mas não tecnicamente padrão) é de 298,15 K, esta temperatura será assumida a menos que seja especificada outra temperatura. Assim, o símbolo (ΔH°) é usado para indicar uma alteração da entalpia para um processo que ocorre nessas condições. (O símbolo ΔH é utilizado para indicar uma alteração de entalpia para uma reação que ocorre em condições não padrão.)

As alterações da entalpia para muitos tipos de processos químicos e físicos estão disponíveis na literatura de referência, inclusivamente para reações de combustão, transições de fase, e reações de formação. Uma vez que a alteração da entalpia para uma determinada reação é proporcional às quantidades de substâncias envolvidas, pode ser relatada com base nisso (isto é, como ΔH para quantidades específicas de reagentes). No entanto, nós consideramos muitas vezes que é mais útil dividir uma propriedade extensiva (ΔH) por outra (quantidade de substância), e relatar um valor intensivo de ΔH por quantidade, muitas vezes “normalizado” para uma base por mole. 

<h4>Entalpia Padrão de Formação</h4>

A entalpia padrão de formação ΔHf° é uma alteração da entalpia para uma reação em que se forma exactamente 1 mole de uma substância pura a partir de elementos livres nos seus estados mais estáveis, em condições de estado normais. Estes valores são especialmente úteis para a computação ou previsão de alterações de entalpia para reações químicas impraticáveis ou perigosas, ou para processos para os quais é difícil realizar medições. Usando valores conhecidos de entalpia padrão de formação, a alteração da entalpia para qualquer reação pode ser determinada.

A entalpia padrão de formação de CO2 (g) é −393,5 kJ/mol. Esta é a alteração da entalpia para a reação exotérmica: 

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;" />

começando com os reagentes a uma pressão de 1 atm e 25 °C (com o carbono presente como grafite, a forma mais estável de carbono nestas condições) e terminando com um mole de CO2, também a 1 atm e 25 °C . Para o dióxido de azoto, NO2 (g), a ΔHf° é de 33,2 kJ/mol. Esta é a alteração da entalpia para a reação endotérmica:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;" />

Uma equação de reação com 1/2 moles de N2 e 1 mole de O2 está certa neste caso porque a entalpia padrão de formação refere-se sempre a 1 mole do produto: NO2 (g).

Por definição, a entalpia padrão de formação de um elemento na sua forma mais estável é igual a zero em condições normais. Por exemplo, as entalpias padrão de formação de carbono (grafeno), gás de oxigénio diatómico, gás de azoto diatómico, metal de sódio, e mercúrio líquido são zero em condições normais.

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy.</a>

Entalpia de Formação Padrão

As mudanças de entalpia são normalmente tabuladas para reações nas quais os reagentes e os produtos estão nas mesmas condições. Um estado padrão é um comumente ...

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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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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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