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

대부분의 화학 반응은 대기압 조건에서 일어납니다. 일정한 압력 조건에서 반응과 관련된 열 변화 델타 Q는 엔탈피 또는 반응 열이라고도 하는 엔탈피의 변화 델타 H와 같습니다. 반응의 엔탈피는 생성물과 반응물의 엔탈피 사이의 차이입니다.

생성물의 엔탈피가 반응물의 엔탈피보다 크면 델타 H는 양수입니다. 이러한 반응은 열을 흡수하는 흡열성 반응입니다. 반대로 반응물의 엔탈피가 생성물의 엔탈피보다 크면 델타 H는 음수입니다.

그러한 반응은 열을 방출하는 발열성 반응입니다. 화학 반응에서 수반되는 엔탈피 변화의 크기는 균형 방정식의 계수로 표시된 반응물 및 생성물의 화학양론적 양에 따라 달라집니다. 상의 표시와 반응의 엔탈피, 델타 H를 포함하는 균형 화학 방정식을 열화학 방정식이라고 합니다.

연료의 일차 공급원인 메탄의 연소를 생각해 봅시다. 메탄을 태우면 주위로 열이 방출됩니다. 반응의 발열성은 열화학 방정식에서 음의 엔탈피 변화로 나타납니다.

연소 방정식을 통해 1 몰의 메탄가스가 2 몰의 산소 기체와 반응하여 1 몰의 이산화탄소 기체와 2 몰의 액체 물을 생성할 때 890.8 킬로 줄의 열이 주위로 방출된다는 것을 알 수 있습니다. 반응물 또는 생성물 간의 몰비와 반응 열을 변환 계수로 사용하여 반응 중에 교환되는 열을 계산할 수 있습니다. 만약 가스 실린더에 25.5 킬로그램의 메탄가스가 들어 있고 실린더의 모든 메탄가스가 다 연소된다면 얼마만한 양의 열이 생성될까요?

일반적으로 개념 전개에서는 질량을 몰로 변환한 다음 몰을 반응열로 변환합니다. 먼저 25.5 킬로그램에 1000을 곱하여 그램 단위의 질량을 구합니다. 그런 다음 메탄의 질량을 몰 질량, 즉 몰 당 16.0 그램으로 나누면 1594 몰의 메탄이 얻어집니다.

마지막으로 메탄의 몰과 반응 열 사이의 변환 계수를 사용하면 1594 몰의 메탄은 열 14.2 곱하기 10의 6제곱 키로 줄을 방출합니다. 이것이 반응에서 발생한 열입니다. 반응에서 열이 발생하므로 반응은 발열성 반응이고 기호는 음이 됩니다.

<ol>
	<li>Canagaratna, Sebastian G. "엔탈피 계산의 시각적 보조 도구." 화학 교육 저널 77, no. 9 (2000) : 1178.</li>
	<li>케이퍼, 데이비드. "엔탈피와 열역학 제2법칙." Journal of Chemical Education 96, no. 7 (2019): 1407-1411.</li>
	<li>Khalil, Mutasim I. "매트릭스 방법에 의한 반응 엔탈피 계산." Journal of Chemical Education 77, no. 2 (2000): 185. 
	 </li>
</ol>

열화학 방정식

팽창이나 수축으로 인한 유일한 일과 함께 일정한 압력에서 수행되는 화학 반응(시스템)의 경우, 반응 엔탈피(반응열이라고도 함, ΔH_rxn)는 주변과 교환되는 열(q_p)과 같습니다. ).



<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />



엔탈피의 변화는 광범위한 특성이며 반응에 참여하는 반응물의 양(또는 반응물의 몰수)에 따라 달라집니다. 엔탈피의 변화는 반응에 따라 다르며, 반응물과 생성물 종의 물리적 상태가 중요합니다. 발열 반응의 특징은 -ΔH_rxn 값인 반면, 흡열 반응의 특징은 +ΔH_rxn 값입니다.

반응에 의해 방출되거나 흡수되는 열의 양은 반응에 의해 소비되거나 생성되는 각 물질의 양에 해당하기 때문에 물질과 에너지의 변화를 나타내기 위해 열화학 방정식을 사용하는 것이 편리합니다. 열화학 방정식에서 반응 엔탈피의 변화는 ΔH_rxn으로 표시되며 일반적으로 반응 방정식에 따라 제공됩니다. ΔH_rxn의 크기는 화학 반응식에 표시된 반응과 관련된 열의 양을 나타냅니다. ΔH_rxn의 표시는 표시된 대로 반응이 발열 반응인지 흡열 반응인지를 나타냅니다. 다음 방정식에서 1몰의 수소 가스와 1/2몰의 산소 가스(특정 온도 및 압력에서)가 반응하여 1몰의 액체 물(동일한 온도 및 압력에서)을 생성합니다.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />

이 방정식은 286kJ의 열이 주변으로 방출됨을 나타냅니다. 즉, 소비되는 수소 1몰 또는 생성되는 물 1몰당 286kJ의 열이 방출됩니다(발열반응). 따라서 반응 엔탈피는 특정 양의 반응물 및 생성물이 포함된 반응 중에 방출되거나 흡수되는 열의 양을 계산하는 데 사용할 수 있는 변환 계수입니다.



<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />



화학 반응식의 계수에 어떤 인자를 곱하면(즉, 물질의 양이 변하는 경우) 엔탈피의 변화도 같은 인자를 곱해야 합니다.

(양이 2배 증가)

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />

(양이 2배 감소)

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />

반응의 엔탈피 변화가 반응물과 생성물의 물리적 상태에 따라 달라짐을 설명하기 위해 기체 수(또는 수증기)의 형성을 고려하십시오. 1몰의 수소 기체와 1/2몰의 산소 기체가 반응하여 1몰의 기체수를 형성할 때, 액체 물이 형성될 때 방출되는 열은 286kJ인 반면, 242kJ의 열만 방출됩니다.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

일정한 압력에서 수행되는 화학 반응 (시스템)의 경우 – 팽창 또는 수축으로 인한 유일한 작업 – ...

Education

교육

JoVE Core

화학

하나

Thermochemistry

열화학

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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

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 화학은 간결하고 이해하기 쉬운 애니메이션을 통해 일반 화학 개념을 설명합니다. 또한, 과학자가 직접 고전실험과 연구실험을 진행하는 실사 영상을 제공합니다.

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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Vapor Pressure

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PS.Chm.7.1

Energy Basics

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Energy Conservation

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Constant Volume Calorimetry

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Source: Laboratory of Dr. Terry Tritt &#8212; Clemson University
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Hess's Law

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Standard Enthalpy of Formation

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Enthalpies of Reaction

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real-gases-effects-intermolecular-forces-molecular-volume-deriving

Real Gases: Effects of Intermolecular Forces and Molecular Volume Deriving Van der Waals Equation

Thus far, the ideal gas law, PV = nRT, has been applied to a variety of different types of problems, ranging from reaction stoichiometry and empirical and ...

Ideal Gas Law

Source: Laboratory of Dr. Andreas Z&#252;ttel - Swiss Federal Laboratories for Materials Science and Technology
The ideal gas law describes the behavior ...

Derivation of the Ideal Gas Law
Gases are a fundamental state of matter. A gas is a collection of molecules that have a significant distance between their ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation for 10 students ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Determining the Mass of the Unknown LiquidExpand
	According to the ideal gas law for a given ...

Use mathematics and computational thinking to explain the relationships among pressure, temperature, volume, and quantity of gas, both qualitatively and quantitatively.

PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Heat Changes in Chemical Equations

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