Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

La plupart des réactions chimiques se produisent à la pression atmosphérique. Dans des conditions de pression constante, le changement de chaleur associé à la réaction-delta Q, est égal au changement d'enthalpie-delta H, également appelé enthalpie ou chaleur de réaction. L'enthalpie de la réaction est la différence entre les enthalpies des produits et des réactifs.Lorsque l'enthalpie des produits est plus grand que l'enthalpie des réactifs, delta H est positif. De telles réactions absorbent la chaleur et sont endothermiques. Au contraire, si l'enthalpie des réactifs est plus grande que l'enthalpie des produits, delta H est négatif.De telles réactions libèrent de la chaleur et sont exothermiques. Pour toute réaction chimique, l'amplitude du changement d'enthalpie qui l'accompagne dépend des quantités stœchiométriques de réactifs et de produits, comme indiqué par les coefficients de l'équation équilibrée. Une équation chimique équilibrée qui comprend des étiquettes de phase et l'enthalpie de la réaction, delta H, est appelée une équation thermochimique.Considérons la combustion du méthane-une source primaire de carburant. La combustion du méthane libère de la chaleur dans les environs. La nature exothermique de la réaction est indiquée par le changement d'enthalpie négatif dans l'équation thermochimique.L'équation pour la combustion montre que lorsqu'une mole de méthane réagit avec 2 moles d'oxygène gazeux pour produire 1 mole de dioxyde de carbone gazeux et 2 moles d'eau liquide, 890, 8 kilojoules de chaleur sont libérés dans l'environnement. Le rapport molaire entre les réactifs ou les produits et la chaleur de réaction peut être utilisé comme facteurs de conversion pour calculer la chaleur échangée pendant la réaction. Si une bouteille de gaz contient 25, 5 kilogrammes de méthane et que tout le méthane contenu dans la bouteille subit une combustion, quelle quantité de chaleur sera produite?En général, le plan conceptuel est de convertir la masse en moles, puis les moles en chaleur de réaction. Pour commencer, 25, 5 kilogrammes sont multipliés par 1000 pour trouver la masse en grammes. Ensuite, la masse de méthane est divisée par sa masse molaire 16, 0 grammes par mol, pour donner 1594 moles de méthane.Enfin, en utilisant le facteur de conversion entre les moles de méthane et la chaleur de réaction, 1594 moles de méthane libère 14, 2 fois dix pour les 6 kilojoules de chaleur-qui est la chaleur de la réaction. La réponse est négative car la réaction est exothermique de la chaleur évolue dans la réaction.

<ol>
	<li>Canagaratna, Sebastian G. « Une aide visuelle dans les calculs d’enthalpie. » Journal de l’éducation chimique 77, n° 9 (2000) : 1178.</li>
	<li>Keifer, David. « L’enthalpie et la deuxième loi de la thermodynamique. » Journal de l’éducation chimique 96, n° 7 (2019) : 1407-1411.</li>
	<li>Khalil, Mutasim I. « Calcul de l’enthalpie de la réaction par une méthode matricielle. » Journal of Chemical Education 77, no. 2 (2000) : 185. 
	 </li>
</ol>

Équations thermochimiques

Pour une réaction chimique (le système) réalisée à pression constante – avec le seul travail effectué dû à une dilatation ou contraction – l'enthalpie de réaction (également appelée chaleur de réaction, ΔHR) est égale à la chaleur échangée avec l'environnement extérieur (qp). 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />
 
 La variation d'enthalpie est une propriété extensive, qui dépend de la quantité de réactifs participant à la réaction (ou du nombre de moles de réactifs). La variation d'enthalpie est spécifique à la réaction, et les états physiques des espèces des réactifs et des produits sont importants. Une réaction exothermique est caractérisée par une valeur −ΔHR, tandis qu'une réaction endothermique a une valeur +ΔHR.  
 
 Comme la quantité de chaleur libérée ou absorbée par une réaction correspond à la quantité de chaque substance consommée ou produite par la réaction, il est pratique d'utiliser une équation thermochimique pour représenter les changements dans la matière et l'énergie. Dans une équation thermochimique, la variation de l'enthalpie d'une réaction est indiquée par ΔHR, et elle est généralement fournie après l'équation de la réaction. L'intensité de ΔHR indique la quantité de chaleur associée à la réaction indiquée dans l'équation chimique. Le signe de ΔHR indique si la réaction est exothermique ou endothermique, tel qu'elle est écrite. Dans l'équation suivante, 1 mole d'hydrogène gazeux et 1/2 mole d'oxygène gazeux (à une certaine température et à une certaine pression) réagissent pour former 1 mole d'eau liquide (à la même température et à la même pression). 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />
 
 Cette équation indique que 286 kJ de chaleur sont libérés dans l'environnement. En d'autres termes, 286 kJ de chaleur sont libérés (la réaction est exothermique) pour chaque mole d'hydrogène consommée ou pour chaque mole d'eau produite. Par conséquent, l'enthalpie de la réaction est un facteur de conversion qui peut être utilisé pour calculer la quantité de chaleur libérée ou absorbée lors de réactions impliquant des quantités spécifiques de réactifs et de produits. 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />
 
 Si les coefficients de l'équation chimique sont multipliés par un certain facteur (c.-à-d. si la quantité d'une substance est modifiée), la variation de l'enthalpie doit être multipliée par ce même facteur. 
 
 (deux fois plus en quantités) 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />
 
 (deux fois moins en quantités) 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />
 
 Pour illustrer que la variation d'enthalpie d'une réaction dépend de l'état physique des réactifs et des produits, il faut considérer la formation de l'eau gazeuse (ou de la vapeur d'eau). Lorsque 1 mole d'hydrogène gazeux et ½ mole d'oxygène gazeux réagissent pour former 1 mole d'eau gazeuse, seulement 242 kJ de chaleur sont libérés, par opposition à 286 kJ de chaleur, qui est libérée lorsque l'eau liquide se forme. 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

Pour une réaction chimique (le système) effectuée à pression constante - le seul travail effectué étant causé par l’expansion ou la contraction - le ...

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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...

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quantifying work

Pressure-Volume Work in Chemistry

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enthalpy

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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

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constant volume calorimetry

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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

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...

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

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Heat Changes in Chemical Equations

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