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

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

A maioria das reações químicas ocorre na pressão atmosférica. Em condições de pressão constante, a alteração de calor associada à reação delta Q, é igual à alteração na entalpia Delta H, também chamada a entalpia ou o calor da reação. A entalpia da reação é a diferença entre as entalpias dos produtos e os reagentes.Quando a entalpia dos produtos é maior do que a entalpia dos reagentes, o delta H é positivo. Tais reações absorvem calor e são endotérmicas. Pelo contrário, se a entalpia dos reagentes é maior do que a entalpia dos produtos, o delta H é negativo.Tais reações libertam calor e são exotérmicas. Para qualquer reação química, a magnitude da alteração da entalpia que a acompanha depende das quantidades estequiométricas dos reagentes e produtos, como indicado pelos coeficientes da equação equilibrada. Uma equação química equilibrada que inclui etiquetas de fase e entalpia de reação, delta H, é chamada de equação termoquímica.Consideremos a combustão do metano uma fonte primária de combustível. A queima de metano liberta calor para o meio ambiente. A natureza exotérmica da reação é indicada pela alteração da entalpia negativa na equação termoquímica.A equação da combustão mostra que quando um mol de gás metano reage com 2 mols de oxigénio gasoso para produzir 1 mol de gás de dióxido de carbono e 2 mols de água líquida, 890, 8 kilojoules de calor são libertados para o meio envolvente. A proporção molar entre os reagentes ou produtos e o calor da reação podem ser utilizados como fatores de conversão para calcular o calor trocado durante a reação. Se um cilindro de gás contiver 25, 5 quilos de metano, e todo o metano do cilindro é submetido a combustão, que quantidade de calor será produzida.Em geral, o plano conceitual consiste em converter a massa em mols e depois mols em calor da reação. Para começar, 25, 5 quilos é multiplicado por 1000 para encontrar a massa em gramas. Depois, a massa de metano é dividida pela sua massa molar.16, 0 gramas por mol, para produzir 1594 mols de metano. Finalmente, utilizando o fator de conversão entre os mols de metano e o calor de reação, 1594 mols libertam de metano 14, 2 vezes dez a 6 kilojoules de calor que é o calor da reação. A resposta é negativa uma vez que a reação é exotérmica à medida que o calor evolui na reação.

<ol>
	<li>Canagaratna, Sebastian G. "Um auxílio visual em cálculos de entalpia." Jornal de Educação Química 77, nº 9 (2000): 1178.</li>
	<li>"Entalpia e a Segunda Lei da Termodinâmica." Jornal de Educação Química 96, nº 7 (2019): 1407-1411.</li>
	<li>Khalil, Mutasim I. "Calculando a entalpia da reação por um método de matriz." Journal of Chemical Education 77, nº 2 (2000): 185. 
	 </li>
</ol>

equações termoquímicas

Para uma reação química (o sistema) realizada a uma pressão constante – com o único trabalho feito por expansão ou contração – a entalpia da reação (também chamada de calor da reação, ΔHrxn) é igual ao calor trocado com as imediações (qp).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />

A alteração na entalpia é uma propriedade extensiva, e depende das quantidades dos reagentes que participam da reação (ou do número de moles dos reagentes). A alteração na entalpia é específica da reação, e os estados físicos das espécies de reagentes e produtos são importantes. Uma reação exotérmica é caracterizada por um valor de −ΔHrxn, enquanto que uma reação endotérmica tem um valor de +ΔHrxn. 

Uma vez que a quantidade de calor libertada ou absorvida por uma reação corresponde à quantidade de cada substância consumida ou produzida pela reação, é conveniente utilizar uma equação termoquímica para representar as alterações na matéria e na energia. Em uma equação termoquímica, a alteração na entalpia de uma reação é mostrada como ΔHrxn, e é geralmente fornecida a seguir à equação para a reação. A magnitude de ΔHrxn indica a quantidade de calor associada à reação mostrada na equação química. O sinal de ΔHrxn indica se a reação é exotérmica ou endotérmica, conforme escrito. Na equação seguinte, 1 mole do gás hidrogénio e 1/2 mole do gás oxigénio (a alguma temperatura e pressão) reagem para formar 1 mole de água líquida (à mesma temperatura e pressão).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />

Esta equação indica que 286 kJ de calor são libertados para as imediações. Por outras palavras, 286 kJ de calor são  libertados (a reação é exotérmica) por cada mole de hidrogénio que é consumido ou por cada mole de água que é produzida. Portanto, a entalpia da reação é um factor de conversão que pode ser usado para calcular a quantidade de calor que é libertada ou absorvida durante reações que envolvem quantidades específicas de reagentes e produtos.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />

Se os coeficientes da equação química forem multiplicados por algum factor (ou seja, se a quantidade de uma substância for alterada), a alteração da entalpia deve ser multiplicada por esse mesmo factor.

(aumento em duas vezes nas quantidades)

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />

(diminuição em duas vezes nas quantidades) 

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />

Para ilustrar que a alteração da entalpia de uma reação depende dos estados físicos dos reagentes e produtos, considere a formação de água gasosa (ou vapor de água). Quando 1 mole de gás hidrogénio e ½ mole de gás oxigénio reagem para formar 1 mole de água gasosa, apenas são libertados 242 kJ de calor, em oposição a 286 kJ de calor, que é libertado quando se forma água líquida.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

Equações Termoquímicas

Para uma reação química (o sistema) realizada a pressão constante - com o único trabalho realizado causado por expansão ou contração - o ...

Education

JoVE Core

Química

Thermochemistry

Termoquímica

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

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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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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...

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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

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hess's law

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standard enthalpy of formation

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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 dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o 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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