Name: calculating reaction heat with hess's law
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 3 min 33 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Enthalpies of Reaction

对于在标准条件下发生的反应，用一个一般方程式来计算反应的 标准焓变。该方程式的求解是通过求出 产物的标准生成焓之和 与反应物的标准生成焓 之和之间的差值，每一项各乘以其化学计量系数。比如在标准条件下 2 摩尔 的乙炔气与 5 摩尔的氧气发生燃烧反应，形成 4 摩尔的二氧化碳气体 和 2 摩尔的水蒸气。该反应的焓等于 二氧化碳气体生成焓 的四倍和水蒸气生成焓 的两倍之和，减去乙炔气生成焓 的两倍和氧气生成焓的五倍 之和。焓方程式是结合两个概念推导 出来的：标准生成焓和赫斯定律。第一项代表产物的标准生成焓；由碳和氧气生成 二氧化碳—方程式 1，以及由氢气和氧气生成水的过程—方程式 2。二氧化碳和水的已知标准生成焓 分别为负的 393.5 千焦耳和负的 241.8 千焦耳。由于燃烧反应会产生 4 摩尔的二氧化碳，H1 应该为二氧化碳的 标准生成焓乘以 4，即负的 1574 千焦耳。燃烧同时会产生 2 摩尔的水，所以焓变—H2—应该为水的标准生成焓乘以 2，即负的 483.6 千焦耳。这就产生了产物的 净标准生成焓为负的 2058 千焦耳。第二项代表乙炔分解成 碳和氢气的反应—方程式 3。这个反应与反应物的标准 生成焓的反应是相反的，因此它的焓值，227.4 千焦耳，前面有一个负号，这在焓方程中也可以看到。因为反应消耗了 2 摩尔的乙炔，H3 应该是乙炔的标准生成焓 乘以 2 的负值，等于负的 453.4 千焦耳。氧气的标准生成焓为零。因此，反应物的净标准 生成焓为负的 453.4 千焦耳。回想一下赫斯定律，如果一个一步反应是在 多个步骤中进行的，那么 每一步的焓之和，等于净焓变化。将生成焓的值代入 方程式中，得出反应的焓 为负的 2511 千焦耳。

<ol>
	<li>戴维斯，托马斯 W. "对赫斯定律的常见误解。"化学教育杂志 28，第 11 期（1951 年）:584。</li>
	<li>Lee， A. L.， H. L. Feldkirchner， F. C. Schora， 和 J. J. Henry. "氢和煤的反应热。" 工业与工程化学过程设计与开发 7， no. 2 （1968）: 244-249.</li>
</ol>

反应焓

如果可以得到相应的反应物和产物的生成焓，则可以使用赫斯定律来确定任何反应的焓变。主要反应可分为逐步反应：（i）反应物分解成其组成元素，其焓变与反应物形成焓的负数成正比，−ΔHf°（反应物），然后（ii）重新组合元素（在步骤1中获得），得到产物，其焓变与生成的焓成正比。产品 ΔHf°（产物）。因此，整个反应的标准焓变等于：（ii）所有产物形成的标准焓之和加上（i）反应物形成的标准焓的负数之和，由下式给出：以下等式，其中∑代表“之总和” n 代表化学计量系数。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-01.png" style="height: 40px;" />
 
 通常将等式重新排列为以下形式：
 
 <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-02.png" style="height: 40px;" />
 
 以下示例详细显示了此方程为何有效以及如何使用该方程计算反应的标准焓变：
 
 <img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-03.png" style="height: 40px;" />
 
 在此，使用赫斯定律的特殊形式以及反应物和产物的形成热值： 
 ΔHf° (HNO3) = −206.64 kJ/mol; ΔHf° (NO) = +90.2 kJ/mol; ΔHf° (NO2) = +33 kJ/mol; ΔHf° (H2O) = −285.8 kJ/mol.
 
 <img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-04.png" style="height: 40px;" />
 
 <img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-05.png" style="height: 40px;" />
 
 <img alt="Eq6" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-06.png" style="height: 40px;" />
 
 <img alt="Eq7" src="https://www.jove.com/files/core/CDNSource/lessons/802/802_Equations-07.png" style="height: 40px;" />
 
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy"> Openstax，化学2e，第5.3节：焓。</a>

若已知反应物和生成物相应的标准生成焓，便可利用赫斯定律确定任何反应的焓变。

Education

教育

JoVE Core

化学

Thermochemistry

热化学

KEY TERMS AND CONCEPTS

关键术语与概念

calculating reaction heat with hess's law

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 negative of the standard enthalpies of formation of the reactants, written as -ΔHf°(reactants). Next, those elements are recombined to form the products, and this step has an enthalpy change proportional to ΔHf°(products).

The standard enthalpy change of the overall reaction is found by adding the sum of the standard enthalpies of formation of all the products and the sum of the negatives of the standard enthalpies of formation of the reactants. In the equation, Σ means &quot;the sum of&quot; and n is the stoichiometric coefficient. The formula is usually rearranged into a simpler form for calculation.

A worked example shows why the equation works and how to use it to calculate the standard enthalpy change of a reaction. In that example, the heat of formation values used are ΔHf°(HNO3) = -206.64 kJ/mol, ΔHf°(NO) = +90.2 kJ/mol, ΔHf°(NO2) = +33 kJ/mol, and ΔHf°(H2O) = -285.8 kJ/mol. This method follows the special form of Hess's law described in OpenStax Chemistry 2e, Section 5.3: Enthalpy.

Calculating Reaction Heat with Hess's Law

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

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 lets chemists find enthalpy changes without measuring every reaction directly. Some reactions are hard or impossible to test in the lab. Even when a reaction can be measured, it is often easier to calculate the heat change from other known enthalpy values. Hess's law says that if a process can be written as the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the steps. This works...

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

Use Hess's law and standard enthalpies of formation to calculate reaction heat. Learn the stepwise method and follow a worked chemistry example.

Calculating Reaction Heat with Hess's Law | 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

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

CH.7.d

Texas Essential Knowledge and Skills

High School

CHEM.13

CHEM.13.A

Alabama Course of Study Science

Environmental Science

Earth and Human Activity

9-12.EVS.4.6

Georgia Standards of Excellence

SC5.

heat-changes-in-chemical-and-physical-processes

Heat Changes in Chemical and Physical Processes

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

surface-tension-capillary-action-and-viscosity

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded ...

States of Matter and Phase Changes

The internal energy of a substance&#8212;the total kinetic energy of all its molecules and the potential energy of their associated forces&#8212;depends ...

Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

phase-transitions-vaporization-and-condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Vapor Pressure

When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules move randomly about, they will occasionally ...

Clausius-Clapeyron Equation

The equilibrium between a liquid and its vapor depends on the temperature of the system; a rise in temperature causes a corresponding rise in the vapor ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

Use models to explain how changes in energy affect the arrangement and movement of the particles in solids, liquids, and gases, as well as the relative strengths of their intermolecular forces.

PS.Chm.7.1

Energy Basics

Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.

Chemical changes and their accompanying ...

First Law of Thermodynamics

Energy Conservation

Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form ...

Internal Energy

The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E. Suppose a system with ...

Quantifying Heat

Thermal Energy&#160;

Microscopically, thermal energy is the kinetic energy associated with the random motion of atoms and molecules. Temperature is a ...

Quantifying Work

As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the ...

Enthalpy

Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum ...

Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Constant Pressure Calorimetry

Calorimetry is a technique used to measure the amount of heat involved in a chemical or physical process or to measure the heat transferred to or from a ...

Constant Volume Calorimetry

Calorimeters are useful to determine the heat released or absorbed by a chemical reaction. Coffee cup calorimeters are designed to operate at constant ...

using-differential-scanning-calorimetry-to-measure-changes-in-enthalpy

Using Differential Scanning Calorimetry to Measure Changes in Enthalpy

Source: Laboratory of Dr. Terry Tritt &#8212; Clemson University
Differential Scanning Calorimetry (DSC) is a method of thermodynamic analysis based on ...

Hess's Law

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally ...

Standard Enthalpy of Formation

Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state is a commonly ...

Hess&rsquo;s law can be used to determine the enthalpy change of any reaction if the corresponding enthalpies of formation of the reactants and products ...

Use mathematics and computational thinking to execute simple calorimetric calculations based on the Law of Conservation of Energy.

definition-measurement-pressure-atmospheric-pressure-barometer

Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

Gas pressure is caused by force exerted by gas molecules colliding with the surfaces of objects. Although the force of each collision is very small, any ...

gas-laws-boyle-s-gay-lussac-charles-avogadro-s-and-ideal-gas-law

Gas Laws: Boyle's, Gay-Lussac, Charles', Avogadro's, and Ideal Gas Law

Through experiments, scientists established the mathematical relationships between pairs of variables, such as pressure and temperature, pressure and ...

applications-of-the-ideal-gas-law-molar-mass-density-and-volume

Applications of the Ideal Gas Law: Molar Mass, Density, and Volume

The volume occupied by one mole of a substance is its molar volume. The ideal gas law, PV = nRT, &nbsp;suggests that the volume of a given quantity of gas ...

mixtures-of-gases-dalton-s-law-of-partial-pressures-and-mole-fractions

Mixtures of Gases: Dalton's Law of Partial Pressures and Mole Fractions

Unless individual gases chemically react with each other, the individual gases in a mixture of gases do not affect each other&rsquo;s pressure. Each gas ...

chemical-stoichiometry-gases-using-ideal-gas-law-to-determine

Chemical Stoichiometry and Gases: Using Ideal Gas Law to Determine Moles

Chemical stoichiometry describes the quantitative relationships between reactants and products in chemical reactions.

In addition to measuring quantities ...

basic-postulates-kinetic-molecular-theory-particle-size-energy

Basic Postulates of Kinetic Molecular Theory: Particle Size, Energy, and Collision

The ideal-gas equation, which is empirical, describes the behavior of gases by establishing relationships between their macroscopic properties. For ...

kinetic-molecular-theory-gas-laws-explain-properties-gas

Kinetic Molecular Theory and Gas Laws Explain Properties of Gas Molecules

The test of the kinetic molecular theory (KMT) and its postulates is its ability to explain and describe the behavior of a gas. The various gas laws ...

kinetic-molecular-theory-molecular-velocities-temperature-kinetic

Kinetic Molecular Theory: Molecular Velocities, Temperature, and Kinetic Energy

The kinetic molecular theory qualitatively explains the behaviors described by the various gas laws. The postulates of this theory may be applied in a ...

behavior-gas-molecules-molecular-diffusion-mean-free-path

Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many ...

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.

Calculating Reaction Heat with Hess's Law

Description

Transcript

More videos by Standards