Name: calculating reaction heat with hess's law
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 3 min 33 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Enthalpies of Reaction

標準的な条件で 発生する反応では 反応の標準エンタルピー変化を 計算するために 一般的な式が使用されます この式は 生成物の形成の 標準エンタルピーの和と 反応物の形成の 標準エンタルピーの和の 間の差を求め それぞれに 化学量論的係数を 乗算することによって 解くことができます 2モルのアセチレンガスと 5モルの酸素ガスが 標準条件で燃焼して 4モルの炭酸ガスと 2モルの水蒸気を 生成することを 考えてみましょう この反応のエンタルピーは 炭酸ガスの 生成エンタルピーの4倍と 水蒸気の生成エンタルピーの 2倍の和から アセチレンガスの 生成エンタルピーの2倍と 酸素ガスの 生成エンタルピーの5倍の 和を引いたものに 等しくなります エンタルピー方程式は 2つの概念 すなわち 標準生成エンタルピーとヘスの 法則を組み合わせです 最初の項は 生成物の形成の 標準エンタルピーを表しており 式は 炭素と酸素からの 二酸化炭素の形成-式1 および水素と 酸素からの水の形成-式2 です 二酸化炭素と水の既知の 標準生成エンタルピーは それぞれ 負の393.5キロジュールと 負の241.8キロジュールです 燃焼により4モルの 二酸化炭素が生成されるので デルタH1は二酸化炭素の 標準形成エンタルピーの 4倍であり これは負の 1574キロジュールになります また 燃焼により 2モルの水が生成されるので エンタルピーの変化（ΔH2）は 水の形成の 標準エンタルピーの2倍となり 負の483.6キロジュールと なります これは 生成物の形成の 正味の標準エンタルピーとして 負の2058キロジュールが 得られます 2番目の項は アセチレンの 炭素と水素への分解を 表しています-式3 これは 反応物の形成の標準 エンタルピーの反応の逆であり したがって そのエンタルピー値 227.4キロジュールは 負の符号が先行しており これはまた エンタルピーの式で 見ることができます この反応では2モルの アセチレンが消費されるので デルタH3は アセチレンの標準 形成エンタルピーの負の2倍 つまり負の 453.4キロジュールに なります 酸素の標準形成エンタルピーは ゼロです したがって 反応物の 正味の標準生成エンタルピーは 負の453.4キロジュールです ヘスの法則から 1つのステップの反応が 複数のステップで行われる場合 各ステップのエンタルピー合計は 正味のエンタルピー変化に 等しいことを思い出してください 形成エンタルピーの値を 式に代入すると 反応のエンタルピーは 負の2511キロジュールと なります

<ol>
	<li>デイビス、トーマスW.「ヘスの法則の一般的な誤解」 Journal of Chemical Education 28、no.11(1951):584。</li>
	<li>Lee, A. L., H. L. Feldkirchner, F. C. Schora, and J. J. Henry. "Heat of Reaction of Hydrogen and Coal." Industrial & Engineering Chemistry Process Design and Development 7, no. 2 (1968): 244-249.</li>
</ol>

反応エンタルピー

ヘスの法則は、反応物と生成物の対応する生成エンタルピーが得られれば、あらゆる反応のエンタルピー変化を決定するのに使用できます。 主な反応は段階的な反応に分けることができます。（ i ）反応物質の成分元素への分解。エンタルピー変化は反応物質の生成エンタルピーの負に比例します。−ΔHf°（反応物質）、（ ii ）生成物質を得るための（ステップ 1 で得られた）元素の再結合。エンタルピー変化は生成物の生成エンタルピーに比例します。 Δ Hf ° ( 生成物質 ) 。 したがって、反応全体の標準的なエンタルピー変化は次のようになります。 (ii) すべての生成物の標準エンタルピーの合計に加えて (i) 次の方程式で示されるように、反応物質の標準生成エンタルピーの負の合計。ここで、∑は「合計」を表し、 n は化学量論的係数を表します。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-01.png" style="height: 40px;" />
 
 通常、数式は次のように少し並べ替えられます。  
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-02.png" style="height: 40px;" />
 
 次の例では、この方程式が有効である理由と、この方程式を使用して標準反応エンタルピー変化を計算する方法を詳細に示します。 
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-03.png" style="height: 40px;" />
 
 ここでは、特別な形のヘスの法則と、反応物および生成物の生成熱の値が使用されます。 Δ Hf° （ HNO3 ） = -206.64 kJ/mol ； Δ Hf ° （ NO ） = +90.2 kJ/mol ； Δ Hf ° （ NO2 ） = +33 kJ/mol ； Δ Hf ° （ H2O） = -285.8kJ/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;" />
 
 
 このテキストは <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">、 Openstax, Chemistry 2e, Section 5.3: Endalpy から引用しています。</a>

ヘスの法則は、反応物および生成物の形成の対応するエンタルピーの場合、任意の反応のエンタルピー変化を決定するために使用することができます...

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KEY TERMS AND CONCEPTS

主要用語と概念

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

energy basics

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enthalpies of reaction

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...

key terms and concepts

Use Hess's law and standard enthalpies of formation to calculate reaction heat. Learn the stepwise method and follow a worked chemistry example.

Calculating Reaction Heat with Hess's Law | JoVE K12

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Calculating Reaction Heat with Hess's Law

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