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

Standard Enthalpy of Formation

反応のエンタルピー変化は 熱量計を使って 測定することもできますし 反応物と生成物の エンタルピーの差を 計算することで 算出することもできます しかし 反応物と 生成物の絶対エンタルピーを 直接測定することはできません そのため 化学者は一般的に 合意された標準状態の 基準物質に対する エンタルピー変化（ΔH）を 使用します 標準状態は 特定の条件によって定義されます これには 設定された温度 通常25℃または 298ケルビンと 気体の1バールの圧力が 含まれています 溶液の場合 標準状態は 溶媒中の純粋な 溶質の1モル濃度です また 物質の標準状態には その物質が このような状態で存在する 物質の物理的な 状態も含まれます 例えば 固体としての 塩化ナトリウム 液体としての水銀 気体としてのヘリウムなどです ある元素がこれらの条件で 複数の形態で存在する場合 その元素の最も安定な形態を 標準状態と定義します 例えば 炭素は ダイヤモンドとしても存在し 黒鉛の結晶としても存在しますが 黒鉛が最も安定した形であるため 黒鉛が炭素の標準状態となります 標準状態の元素が結合して 1モルの純粋な化合物を 形成するとき反応エンタルピーは 標準エンタルピー 標準形成熱）と 呼ばれます これをΔH naught 添え字fで表し naughtは構成元素の 標準状態を fは生成を表します 純粋な元素の 標準状態での生成の 標準エンタルピーは 標準状態にあるときには 反応が起こらず エンタルピーが変化しないため 常にゼロになります 化合物の1モルあたりの キロジュール単位での 標準生成エンタルピーの値は 参照表に記載されています これらの物質には 気体ナトリウムのような 非標準状態の元素や 塩化ナトリウムのような 化合物が含まれます 反応の標準エンタルピー変化量は 生成物と反応物の 差から計算することができ これを参考表の エンタルピー値を用いて 計算することができます

<ol>
	<li>「アルカンの形成の標準エンタルピーの計算:分子力学とスプレッドシートプログラムの図解」 Journal of chemical education 76, no. 2 (1999): 278.</li>
	<li>「燃焼物理化学実験室のエンタルピーに適用される経験的に修正された電子構造計算」 Journal of Chemical Education 96、no. 6 (2019): 1165-1170.</li>
	<li>ヤンセン、マイケルP.「ガソリン車をプロパンに改造するコスト。A Practical Review Problem for Senior High School or Introductory Chemistry." Journal of Chemical Education 77, no. 12 (2000): 1578. 
	 </li>
</ol>

標準生成エンタルピー

エンタルピー変化は通常、反応物と生成物が同じ条件にある反応について表されます。 標準状態とは、他の異なる条件で特性を決定するための基準となる、一般的に受け入れられる一連の条件のことです。 化学者にとって、 IUPAC 標準状態は 1バールの圧力下にある物質と 1 M の溶液を指し、温度を指定しません。 多くの熱化学表では、標準の状態が 1 atm の値が記載されています。 このようなわずかな圧力変化（ 1 bar = 0.987 atm ）では反応の Δ H はほとんど変化しないため、 Δ H 値（最も正確に測定された値を除いて）は、どちらのの標準条件で基本的に同じです。 エンタルピー変化記号の上付き "o" は、標準状態を示します。 通常の（技術的には標準ではない）温度は 298.15 K であるため、他の温度が指定されていない限り、この温度が想定されます。 したがって、この記号（ Δ H ° ）は、これらの条件下で発生する過程のエンタルピー変化を示すために使用されます。 （記号 Δ H は、非標準条件で発生した反応のエンタルピー変化を示すために使用される）。 
 
 燃焼反応、相転移、形成反応など、さまざまな種類の化学的・物理プロセスのエンタルピー変化が参考文献に掲載されています。 特定の反応に対するエンタルピー変化は、関与する物質の量に比例するため、それに基づいて報告されることがあります（すなわち、特定の量の反応物質の Δ Hとして報告される）。 ただし、 1 つの示量性（ Δ H ）を別の（物質の量）で除算し、量ごとの示強性 Δ H を報告すると、多くの場合、モル単位で「平均化」されます。  
 
 <h4>標準生成エンタルピー </h4>
 
 標準生成エンタルピー Δ Hf ° は、標準状態で最も安定した状態にある自由元素から純粋な物質 1 モルが形成される反応のエンタルピー変化です。 これらの値は、実行するのが現実的でない、または危険な化学反応や測定が困難な過程のエンタルピー変化を計算または予測する場合に特に便利です。 標準エンタルピーの値を使って、どの反応に対してもエンタルピーの変化を調べることができます。 
 
 CO2 （ g ）生成の標準エンタルピーは -393.5 kJ/mol です。 これは発熱反応に対するエンタルピー変化です。  
 
 <img alt=""Eq1"" src=""https://www.jovecore.com/CDNSource/lessons/801/801_Equations-01.png"" style=""height: 40px;"" />
 
 1 atm と 25 ° C で反応を開始し（炭素はグラファイトで、これらの条件下で最も安定した炭素の形態）、同じく1 ATM と 25 ° C において 1モルの CO2 で反応を終了します。二酸化窒素NO2 （ g ）の場合、 、 Δ Hf ° は 33.2 kJ/mol です。 これは吸熱反応に対するエンタルピー変化です。 
 
 <img alt=""Eq2"" src=""https://www.jovecore.com/CDNSource/lessons/801/801_Equations-02.png"" style=""height: 40px;"" />
 
 この場合、標準生成エンタルピーは常に 1 モルの生成物 NO2 (g) を参照するため、 1/2 モルのN2 と 1 モルのCとの反応方程式は正しくなります。 
 
 定義上、最も安定した形で元素を形成する標準エンタルピーは、標準条件ではゼロになります。 例えば、炭素（グラフェン）、二原子酸素ガス、二原子窒素ガス、ナトリウム金属、液体水銀の標準エンタルピーは、標準条件下ではゼロです。 
 
 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>

エンタルピー変化は、通常、反応物と生成物の両方が同じ条件にある反応について表にされます。標準状態は一般的に...

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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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Using Standard Enthalpy Values in Chemistry

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