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

大部分的化学反应发生在一大气压下。在这个恒压的环境下，与反应相关的热变化—delta Q，等于焓变delta H，也称为 反应中的焓或热。反应的焓是反应物及 生成物焓的变化量。当生成物的焓比 反应物的焓多，delta H 即为正值。这些的反应吸收热能且为吸热的。相反的，假使反应物的焓 比生成物的焓多，delta H即为负的。这些反应放出热能且为放热的。对于所有的化学反应，伴随的焓的大小决定于反 应物及生成 物的化学计量量，即为平衡方程式的系数。一个平衡方程式其 包含相位标签及焓（delta H）被称作热化学方程式。就甲烷的燃烧—主要的燃料成分。燃烧甲烷会释放热至周围环境。反应的放热性质在热 化学方程式中以负的 焓变化量表示。燃烧的反应指出当一 莫耳的甲烷与两莫耳的氧气反应，以产生一莫耳的二氧化碳及 两莫耳的液态水，和释出890.8千焦耳的 热至周遭环境中。反应物或生成物与热的 莫耳比例可以被用为转换的因 子而计算反应中所交换的热。假使一个汽缸装了25.5公斤的甲烷，且汽缸内全部的甲烷都燃烧了，试问多少会产出多少热？基本上，概念上是将质量转为 莫耳再将莫耳转换至反应热。首先，将25.5公斤乘以 1000以得到质量的公克数。接着，将甲烷的质量除以其莫耳质量 每莫耳16.0克，以得到1594莫耳的甲烷。最终，将甲烷的莫耳及反 应热以转换因子转换。1594莫耳的甲烷会释放 14.2乘以十至六千焦耳的热，其便是反应热。最终的答案会是负值，因为热在此反应中是放热反应。

<ol>
	<li>Canagaratna， Sebastian G. "焓计算中的视觉辅助工具。"化学教育杂志 77，第 9 期（2000 年）:1178。</li>
	<li>凯弗，大卫。"焓和热力学第二定律。"化学教育杂志 96，第 7 期（2019 年）:1407-1411。</li>
	<li>Khalil， Mutasim I. "用矩阵法计算反应焓。"化学教育杂志 77，第 2 期（2000 年）:185. 
	 </li>
</ol>

热化学方程

对于在恒定压力下进行的化学反应（系统）只做膨胀或收缩引起的工作＆ndash;反应的焓（也称为反应热，Δ H rxn ）等于与周围环境交换的热（ q p ）。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />
 
 焓的变化是外延性质，它取决于参与反应的反应物的量（或反应物的摩尔数）。焓的变化是特定于反应的，并且反应物和产物种类的物理状态很重要。放热反应的特征是−ΔHrxn 值，而吸热反应具有+ΔHrxn 值。 
 
 由于反应释放或吸收的热量与反应消耗或产生的每种物质的数量相对应，因此使用热化学方程式表示物质和能量的变化很方便。在热化学方程式中，反应的焓变化表示为Δ H rxn ，并且通常根据反应方程式提供。 ΔH H rxn 的大小表示与化学方程式所示的反应相关的热量。如所写，＆lt; H rxn 的符号表示反应是放热的还是吸热的。在以下等式中，1摩尔的氢气和1/2摩尔的氧气（在一定的温度和压力下）反应生成1摩尔的液态水（在相同的温度和压力下）。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />
 
 该方程式表明，向周围环境释放了286 kJ的热量。换句话说，对于消耗的每摩尔氢气或所产生的每摩尔水，释放出286 kJ的热量（反应是放热的）。因此，反应焓是一个转换因子，可用于计算在涉及特定量的反应物和产物的反应过程中释放或吸收的热量。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />
 
 如果化学方程式的系数乘以某个因子（即，如果一种物质的量发生了变化），那么焓变必须乘以相同的因子。
 
 （数量增加了两倍）
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />
 
 （数量减少了两倍）
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />
 
 为说明反应的焓变取决于反应物和产物的物理状态，请考虑形成气态水（或水蒸气）。当1摩尔的氢气和 ½ 摩尔的氧气反应生成1摩尔的气态水，仅释放242 kJ的热量，而液态水形成时释放286 kJ的热量。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

在恒压条件下进行的化学反应（体系），且仅因膨胀或收缩做功时，其……

Education

教育

JoVE Core

化学

Thermochemistry

热化学

KEY TERMS AND CONCEPTS

关键术语与概念

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...

constant volume calorimetry

Bomb Calorimeter Heat Measurement

Bomb calorimetry measures the heat released by reactions that produce a lot of energy, especially combustion reactions. A coffee cup calorimeter works at constant pressure and is useful for reactions in solution. A bomb calorimeter works at constant volume and is used for reactions that make large amounts of heat and gaseous products. The word "bomb" refers to the strong, explosive force some reactions can have. In a bomb calorimeter, the volume does not change, so 94V = 0. That means the...

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 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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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