Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 55 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

ほとんどの化学反応は 大気圧下で起こります 一定の圧力下では 反応に伴う熱変化ΔQは エンタルピーの変化ΔHに等しく エンタルピーまたは 反応熱とも呼ばれます 反応のエンタルピーは 生成物と 反応物の エンタルピーの差です 生成物のエンタルピーが 反応物のエンタルピーよりも 大きい場合 デルタHは正の値になります このような反応は熱を吸収する 吸熱反応です 逆に 反応物のエンタルピーが 生成物のエンタルピーよりも 大きい場合 ΔHは負です このような反応は熱を放出する 発熱性です どのような化学反応においても それに伴うエンタルピーの 変化の大きさは 平衡式の 係数によって示されるように 反応物と生成物の 化学量論的量に依存します 相ラベルと反応の エンタルピーΔHを含む バランスのとれた 化学方程式は 熱化学方程式と呼ばれています 主な燃料源であるメタンの 燃焼を考えてみましょう メタンを燃焼させると 周囲に熱が放出されます 反応の発熱性は 熱化学式の 負のエンタルピーの 変化によって示されます 燃焼の式は 1モルのメタンガスが 2モルの酸素ガスと反応して 1モルの炭酸ガスと 2モルの水を生成するとき 890.8キロジュールの熱が 周囲に放出されることを 示しています 反応物または生成物と 反応熱のモル比は 反応中に交換された 熱量を計算するための 換算係数として 使用することができます ガスボンベに25.5キログラム メタンが入っていて ボンベ内のメタンが すべて燃焼した場合 どのくらいの熱が 発生するでしょうか？一般的には質量をモル数に換算し モル数を反応熱に 換算するという 計算方法がとられます まず 25.5キログラムに 1000をかけて グラム単位の 質量を求めます 次に メタンの質量をモル質量の 16.0グラム/モルで割ると メタンが1594モルで あることがわかります 最後に メタンのモル数と 反応熱の換算係数を使うと 1594モルのメタンは 14.2x10の6乗 キロジュールの熱を 反応熱として 放出することになります 反応の中で熱が発生して 発熱するので 反応は発熱的です

<ol>
	<li>Canagaratna、Sebastian G.「エンタルピー計算の視覚的補助」 Journal of Chemical Education 77、no. 9(2000):1178。</li>
	<li>「エンタルピーと熱力学の第二法則」 Journal of Chemical Education 96、no. 7 (2019): 1407-1411.</li>
	<li>Khalil, Mutasim I. "マトリックス法による反応エンタルピーの計算" Journal of Chemical Education 77, no. 2 (2000): 185. 
	 </li>
</ol>

熱化学方程式

化学反応（システム）が一定の圧力で実行され、膨張または収縮によってのみ実行される場合、反応のエンタルピー（反応熱とも呼ばれる Δ Hrxn ）は周囲と交換される熱（ qp）と等しくなります。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />
 
 エンタルピーの変化は、反応に関与する反応物質の量（または反応物質のモル数）に応じて、広範囲に及ぶ特性です。 エンタルピーの変化は反応に固有であり、反応物と生成物の物理的状態が重要です。 発熱反応は− Δ Hrxn 値で特徴付けられ、熱内反応は + Δ Hrxn 値です。  
 
 反応によって放出または吸収される熱の量は、反応によって消費または生成される各物質の量に対応するため、熱化学方程式を使用して物質とエネルギーの両方の変化を表すと便利です。 熱化学方程式では、反応のエンタルピー変化は Δ Hrxn と表示され、一般的には反応の方程式に従って表示されます。 Δ Hrxn の大きさは、化学方程式に示されている反応に関連する熱の量を表す。 Δ Hrxn の符号は、反応が発熱または吸熱のどちらであるかを示しています。 次の式では、 1 モルの気体水素と 1/2 モルの気体酸素（ある温度と圧力下において）が、 1 モルの液体の水（同じ温度と圧力下で）を形成する反応です。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />
 
 この方程式は、周囲に 286 kJ の熱が放出されることを示しています。 つまり、消費される水素のモルごと、または生成される水のモルごとに、 286 kJ の熱が放出されます（発熱反応）。 したがって、反応のエンタルピーは、特定の量の反応物と生成物を含む反応の際に放出または吸収される熱量を計算するために使用できる換算係数です。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />
 
 化学方程式の係数に何らかの係数を掛けた場合（つまり、化学物質の量が変更された場合）、エンタルピーの変化に同じ係数を掛けなければなりません。 
 
 （ 2 倍増加） 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />
 
 （ 2 倍減少） 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />
 
 反応のエンタルピー変化が反応物質や生成物の物理的な状態に依存することを示すために、気体の水（水蒸気）の形成を検討します。 1 モルの水素ガスと½ モルの酸素ガス が気体の水 1 モルを形成するように反応すると、242 kJ の熱のみが放出されます。これに対し、液体の水が形成されると放出される熱の 286 kJ は発生しません。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

一定の圧力で行われる化学反応(システム)の場合、膨張または収縮によって引き起こされる唯一の作業が行われます...

Education

JoVE Core

化学

Thermochemistry

熱化学

KEY TERMS AND CONCEPTS

主要用語と概念

heat changes in chemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property.

The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a positive one. For that reason, a thermochemical equation is useful because it shows both the chemical change and the energy change together.

In a thermochemical equation, ΔHrxn is written after the chemical equation. Its magnitude tells how much heat is linked to the reaction as written. Its sign shows whether the reaction is exothermic or endothermic in that form.

For example, 1 mole of hydrogen gas and 1/2 mole of oxygen gas can react to form 1 mole of liquid water. This reaction releases 286 kJ of heat to the surroundings. That means 286 kJ is released for every mole of hydrogen used or every mole of water made, so ΔHrxn can act like a conversion factor.

If the coefficients in the chemical equation are multiplied, the enthalpy change must be multiplied by the same factor. A two-fold increase in the amounts gives a two-fold increase in ΔHrxn, and a two-fold decrease gives a two-fold decrease. The physical state matters too. When the same reaction makes gaseous water instead of liquid water, only 242 kJ of heat are released.

Heat Changes in Chemical Equations

energy basics

How Energy Changes in Chemical Reactions

Chemical reactions change matter and energy at the same time. A match lighting is a simple example. These energy changes are part of everyday life and appear in food, fuels, and industry. Food provides energy through metabolic reactions. The macronutrients in food break down and help keep the body functioning. Fuels such as gasoline, natural gas, and coal are burned to produce energy for transportation, heating, and electricity generation. Industrial chemical reactions also use large amounts...

first law of thermodynamics

Energy Conservation and Internal Energy

Energy conservation describes how energy changes form during chemical and physical changes. Energy can be converted from one form into another, but the energy present before a change still exists in some form after the change is complete. This is the law of conservation of energy. It says energy can neither be created nor destroyed. The law of conservation of matter also applies during chemical change. In most chemical reactions, the total amount of matter does not show a detectable change.

internal energy

Internal Energy, Heat, and Work

Internal energy is the total of all possible kinds of energy in a substance. It is often written as U, and sometimes as E. For a system that changes energy, the important value is the change in internal energy, or ΔU. ΔU is the difference between the final internal energy and the initial internal energy. The exact values of U final and U initial are usually not known, but the first law of thermodynamics only requires ΔU. A positive ΔU means the system gained energy from the surroundings. A...

quantifying heat

Heat Transfer and Specific Heat

Heat transfer and specific heat explain how thermal energy moves and how materials respond to it. Thermal energy is the kinetic energy linked to the random motion of atoms and molecules. Temperature is a measure of how hot or cold a substance is, and it depends on how much thermal energy those particles have. When atoms and molecules move or vibrate faster, their average kinetic energy is higher. That means the object has more thermal energy and a higher temperature. When they move more...

quantifying work

Pressure-Volume Work in Chemistry

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work. Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work. Work can be defined as a force acting through a distance. Pressure-volume work, also...

enthalpy

Enthalpy and Heat at Constant Pressure

Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself. For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV...

thermochemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property. The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a...

constant pressure calorimetry

Coffee Cup Calorimeter Heat Changes

Coffee cup calorimetry measures heat changes in reactions and physical changes using an insulated calorimeter. The calorimeter is calibrated and designed to limit heat exchange with the outside environment. That makes it possible to measure the heat taken in or released by a substance more accurately, including the energy content of foods. In calorimetry, the system is the substance or substances undergoing change. The surroundings are everything else inside the calorimeter, including the...

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hess's law

Calculating Reaction Heat with Hess's Law

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standard enthalpy of formation

Using Standard Enthalpy Values in Chemistry

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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

Learn how ΔHrxn shows heat released or absorbed in reactions, why equation coefficients matter, and how physical state changes the enthalpy value.

Heat Changes in Chemical Equations | JoVE K12

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

Matter and its Interactions

PS.Chm.4.1

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Tennessee Academic Standards

CHEM1.PS3.3

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

CH.7.c

CHEM.13

CHEM.13.A

CHEM.13.C

Florida Department of Education

SC.912.P.10

SC.912.P.10.7

Georgia Standards of Excellence

SC3.

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For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Constant Pressure Calorimetry

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Constant Volume Calorimetry

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Heat Changes in Chemical Equations

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