Name: calculating reaction heat with hess's law
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 3 min 33 s
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PS.Chm.7.2

Enthalpies of Reaction

Standart koşullar altında meydana gelen bir reaksiyonda, reaksiyonun standart entalpi değişimini hesaplamak için genel bir denklem kullanılır. Bu denklem, ürünlerin standart oluşum entalpilerinin toplamı ile reaktanların standart oluşum entalpilerinin toplamı arasındaki fark hesaplanarak çözülür, her biri stokiyometrik katsayısı ile çarpılır. Standart koşullar altında 4 mol karbondioksit gazı ve 2 mol su buharı oluşturmak üzere 2 mol asetilen gazının 5 mol oksijen gazı ile yanmasını düşünelim.Reaksiyon entalpisi, karbondioksit gazı oluşum entalpisinin dört katı ve su buharı oluşum entalpisinin iki katı toplamı, eksi asetilen gazı oluşum entalpisinin iki katı ve oksijen gazı oluşum entalpisinin beş katının toplamına eşittir. Entalpi denklemi, iki kavramın birleştirilmesinden elde edilir:standart oluşum entalpisi ve Hess yasası. İlk terim, ürünlerin standart oluşum entalpilerini temsil eder;karbon ve oksijenden karbondioksit oluşumu denklem 1 ve hidrojen ve oksijenden su oluşumu denklem 2.Karbondioksit ve su için bilinen standart oluşum entalpileri sırasıyla negatif 393, 5 kilojul ve negatif 241, 8 kilojul'dür. Yanma 4 mol karbondioksit ürettiğinden, delta H1, karbon dioksitin standart oluşum entalpisi çarpı 4, yani eksi 1574 kilojul olacaktır. Yanma ayrıca 2 mol su üretir, dolayısıyla entalpi değişimi delta H2 su standart oluşum entalpisi çarpı 2, yani eksi 483, 6 kilojul'dür.Bu, ürünlerin net standart oluşum entalpisi olarak eksi 2058 kilojül değerini verir. İkinci terim, asetilenin karbon ve hidrojene ayrışmasını temsil eder denklem 3. Bu, reaktanın standart oluşum entalpisi reaksiyonunun tersidir ve dolayısıyla 227, 4 kilojul entalpi değerinden önce entalpi denkleminde de görülebilen negatif bir işaret gelir.Reaksiyon 2 mol asetilen tükettiğinden, delta H3, standart asetilen oluşum entalpisi çarpı 2'nin negatifi eşittir eksi 453, 4 kilojul olacaktır. Oksijenin standart oluşum entalpisi sıfırdır. Bu nedenle, reaktanların net standart oluşum entalpisi eksi 453, 4 kilojul olacaktır.Hess Yasasından hatırladığımız üzere, eğer tek adımlı bir reaksiyon birden fazla adımda gerçekleştirilirse, her adımın entalpilerinin toplamı net entalpi değişimine eşittir. Oluşum entalpilerinin değerlerinin denklemde yer alması reaksiyonun entalpisini eksi 2511 kilojul olarak verir.

<ol>
	<li>Davis, Thomas W. "Hess yasasının yaygın bir yanlış anlaşılması." Kimya Eğitimi Dergisi 28, no. 11 (1951): 584.</li>
	<li>Lee, A. L., H. L. Feldkirchner, F. C. Schora, ve J. J. Henry. "Hidrojen ve Kömürün Reaksiyon Isısı." Endüstri ve Mühendislik Kimyası Proses Tasarımı ve Geliştirme 7, no. 2 (1968): 244-249.</li>
</ol>

reaksiyon entalpileri

Hess yasası, reaktanlara ve ürünlere karşılık gelen oluşum entalpileri mevcutsa, herhangi bir reaksiyonun entalpi değişimini belirlemek için kullanılabilir. Ana reaksiyon aşamalı reaksiyonlara bölünebilir : (i) reaktanların, reaktanların oluşum entalpilerinin negatifi ile orantılı olduğu entalpi değişikliklerinin bileşen elementlerine ayrışması, −ΔHf°(reaktanlar), ardından (ii) ürünlerin oluşum entalpileri ile orantılı entalpi değişiklikleri ile ürünlere vermek için elementlerin yeniden kombinasyonları (1. adımda elde edilmiştir),  ΔHf° (ürünler). Bu nedenle, genel reaksiyonun standart entalpi değişimi aşağıdakilere eşittir: (ii) tüm ürünlerin oluşumunun standart entalpilerinin toplamı artı (i) aşağıdaki denklem tarafından verilen reaktanların oluşumunun standart entalpilerinin negatiflerinin toplamı, burada ∑ “toplamı” temsil eder ve n stokiyometrik katsayıları ifade eder.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-01.png" style="height: 40px;" />
 
 Denklem genellikle aşağıdaki gibi yazılmak üzere yeniden düzenlenir: 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-02.png" style="height: 40px;" />
 
 Aşağıdaki örnek, bu denklemin neden geçerli olduğunu ve bir reaksiyon için standart entalpi değişimini hesaplamak için nasıl kullanılacağını ayrıntılı olarak gösterir:
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-03.png" style="height: 40px;" />
 
 Burada, Hess yasasının özel formu ve reaktanlar ve ürünler için oluşum ısısı değerleri kullanılır: ΔHf° (HNO3) = −206,64 kJ/mol; ΔHf° (NO) = +90,2 kJ/mol; ΔHf° (NO2) = +33 kJ/mol; ΔHf° (H2O) = −285,8 kJ/mol.
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-04.png" style="height: 40px;" />
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-05.png" style="height: 40px;" />
 
 <img alt="Eq6" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-06.png" style="height: 40px;" />
 
 <img alt="Eq7" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-07.png" style="height: 40px;" />
 
 
 Bu metin bu kaynaktan uyarlanmıştır: <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy.</a>

Reaksiyon Entalpileri

Hess yasası, reaktanların ve ürünlerin karşılık gelen oluşum entalpileri ise, herhangi bir reaksiyonun entalpi değişimini belirlemek için kullanılabilir ...

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calculating reaction heat with hess's law

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 negative of the standard enthalpies of formation of the reactants, written as -ΔHf°(reactants). Next, those elements are recombined to form the products, and this step has an enthalpy change proportional to ΔHf°(products).

The standard enthalpy change of the overall reaction is found by adding the sum of the standard enthalpies of formation of all the products and the sum of the negatives of the standard enthalpies of formation of the reactants. In the equation, Σ means &quot;the sum of&quot; and n is the stoichiometric coefficient. The formula is usually rearranged into a simpler form for calculation.

A worked example shows why the equation works and how to use it to calculate the standard enthalpy change of a reaction. In that example, the heat of formation values used are ΔHf°(HNO3) = -206.64 kJ/mol, ΔHf°(NO) = +90.2 kJ/mol, ΔHf°(NO2) = +33 kJ/mol, and ΔHf°(H2O) = -285.8 kJ/mol. This method follows the special form of Hess's law described in OpenStax Chemistry 2e, Section 5.3: Enthalpy.

Calculating Reaction Heat with Hess's Law

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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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Calculating Reaction Heat with Hess's Law | JoVE K12

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Calculating Reaction Heat with Hess's Law

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