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

La mayoría de las reacciones químicas ocurren a presión atmosférica. En condiciones de presión constante, el cambio de calor asociado con la reacción delta Q, es igual al cambio de la entalpía delta H, también llamada la entalpía o el calor de reacción. La entalpía de la reacción es la diferencia entre las entalpías de los productos y los reactivos.Cuando la entalpía de los productos es mayor que la entalpía de los reactivos, el delta H es positivo. Tales reacciones absorben calor y son endotérmicas. Por el contrario, si la entalpía de los reactivos es mayor que la entalpía de los productos, el delta H es negativo.Tales reacciones liberan calor y son exotérmicas. Para cualquier reacción química, la magnitud del cambio de la entalpía que lo acompaña depende de las cantidades estequiométricas de los reactivos y productos, como lo indican los coeficientes de la ecuación balanceada. Una ecuación química balanceada que incluye las etiquetas de fase y la entalpía de reacción, el delta H, se llama ecuación termoquímica.Considere la combustión del metano:una fuente primaria de combustible. La quema de metano libera calor a los alrededores. La naturaleza exotérmica de la reacción está indicado por el cambio de entalpía negativa en la ecuación termoquímica.La ecuación de combustión muestra que cuando un mol de gas metano reacciona con 2 moles de oxígeno gaseoso para producir 1 mol de gas de dióxido de carbono y 2 moles de agua líquida, se liberan 890, 8 kilojulios de calor al entorno. La relación molar entre los reactivos o productos y el calor de reacción se puede utilizar como factores de conversión para calcular el calor intercambiado durante la reacción. Si un cilindro de gas contiene 25, 5 kilogramos de metano, y todo el metano en el cilindro se quema, qué cantidad de calor se producirá?En general, el plan conceptual es convertir la masa en moles y luego los moles al calor de reacción. Para empezar, 25, 5 kilogramos se multiplican por 1.000 para encontrar la masa en gramos. Luego, la masa de metano se divide por su masa molar:16, 0 gramos por mol, para producir 1.594 moles de metano.Finalmente, usando el factor de conversión entre moles de metano y calor de reacción, 1.594 moles de metano liberan 14, 2 veces diez los 6 kilojulios de calor, que es el calor de la reacción. La respuesta es negativa ya que la reacción es exotérmico a medida que el calor se desarrolla en la reacción.

<ol>
	<li>Canagaratna, Sebastian G. "Una ayuda visual en los cálculos de entalpía". Revista de Educación Química 77, no. 9 (2000): 1178.</li>
	<li>Keifer, David. "La entalpía y la segunda ley de la termodinámica". Revista de Educación Química 96, no. 7 (2019): 1407-1411.</li>
	<li>Khalil, Mutasim I. "Cálculo de la entalpía de reacción por un método matricial". Revista de Educación Química 77, no. 2 (2000): 185. 
	 </li>
</ol>

ecuaciones termoquímicas

Para una reacción química (el sistema) realizada a presión constante – con el único trabajo realizado causado por expansión o contracción – la entalpía de la reacción (también llamada el calor de la reacción, ΔHrxn) es igual al calor intercambiado con el entorno (qp).
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />
 
 El cambio en la entalpía es una propiedad extensiva y depende de la cantidad de los reactantes que participan en la reacción (o el número de moles de los reactantes). El cambio en la entalpía es específico de la reacción, y los estados físicos de las especies reactantes y el producto son importantes. Una reacción exotérmica se caracteriza por un valor de −ΔHrxn, mientras que una reacción endotérmica tiene un valor de +ΔHrxn.  
 
 Debido a que la cantidad de calor liberado o absorbido por una reacción corresponde a la cantidad de cada sustancia consumida o producida por la reacción, es conveniente utilizar una ecuación termoquímica para representar los cambios tanto en materia como en energía. En una ecuación termoquímica, el cambio en la entalpía de una reacción se muestra como ΔHrxn, y generalmente es proporcionada a continuación de la ecuación para la reacción. La magnitud de ΔHrxn indica la cantidad de calor asociada a la reacción mostrada en la ecuación química. El signo de ΔHrxn indica si la reacción es exotérmica o endotérmica, tal como se explicó previamente. En la siguiente ecuación, 1 mol de gas hidrógeno y 1/2 mol de gas oxígeno (a cierta temperatura y presión) reaccionan para formar 1 mol de agua líquida (a la misma temperatura y presión).
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />
 
 Esta ecuación indica que 286 kJ de calor se liberan al entorno. En otras palabras, 286 kJ de calor se liberan (la reacción es exotérmica) por cada mol de hidrógeno que se consume o por cada mol de agua que se produce. Por lo tanto, la entalpía de la reacción es un factor de conversión que se puede utilizar para calcular la cantidad de calor que se libera o absorbe durante las reacciones que implican cantidades específicas de reactivos y productos.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />
 
 Si los coeficientes de la ecuación química se multiplican por algún factor (es decir, si se cambia la cantidad de una sustancia), el cambio en la entalpía debe multiplicarse por ese mismo factor.
 
 (aumento de dos veces las cantidades indicadas)
 
 <img alt=""Eq1"" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />
 
 (reducción de dos veces las cantidades indicadas)
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />
 
 Para ilustrar que el cambio de entalpía de una reacción depende de los estados físicos de los reactivos y productos, considere la formación de agua gaseosa (o vapor de agua). Cuando 1 mol de gas hidrógeno y ½ mol de gas oxígeno reaccionan para formar 1 mol de agua gaseosa, sólo se liberan 242 kJ de calor, en contraposición a 286 kJ de calor, que se libera cuando se forma agua líquida.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

Ecuaciones Termoquímicas

Para una reacción química (el sistema) llevada a cabo a presión constante, con el único trabajo realizado causado por expansión o contracción, el ...

Education

Educación

JoVE Core

Química

Thermochemistry

Termoquímica

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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...

constant volume calorimetry

Bomb Calorimeter Heat Measurement

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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

chemistry

JoVE Core Química da vida a los conceptos de química general mediante lecciones de vídeo basadas en animaciones, concisas y fáciles de entender. Además, en los vídeos de científicos en acción, se muestran experimentos de investigación clásicos y originales realizados en laboratorios de todo el mundo.

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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Heat Changes in Chemical Equations

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