Name: enthalpy and heat at constant pressure
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 59 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

Enthalpy

Some chemical reactions give off tremendous heat and perform work on the surroundings, such as the combustion of rocket fuel causing a space shuttle to lift off from the ground.

The sum of heat &#8220;q,&#8221; and work &#8220;w,&#8221; is the change in internal energy, &#8220;&#916;E,&#8221; as given by the first law of thermodynamics.

For chemical reactions involving gases that occur at atmospheric pressure, the work done is the mechanical work associated with volume changes &#8212; either expansion or contraction. Work is therefore equal to the negative value of the pressure times the change in volume.

Substituting for w in the first law of thermodynamics and rearranging the terms in the equation shows that q = &#916;E + P&#916;V, giving the expression for heat flow under constant pressure.

In other chemical reactions, such as the burning of wood to cook food, it is more relevant to quantify the heat given off to facilitate cooking, than to measure the amount of expansion work done on the surrounding.

Since internal energy accounts for both heat and work, &#916;E is not used for constant pressure conditions. To exclusively discuss the energy flow in the form of heat, a new thermodynamic function &#8211; enthalpy &#8211; is defined.

Enthalpy, H, equals the sum of internal energy, E, and pressure-volume work, P-V. Because energy, pressure, and volume are state functions, enthalpy is also a state function. 

Absolute enthalpy values for specific substances cannot be measured. Only the change in enthalpy can be determined. Enthalpy change, &#916;H, equals the change in internal energy, &#916;E + P&#916;V.

Recalling that the change in energy is the sum of heat and pressure-volume work the equations can be combined to show that under constant pressure conditions &#916;H equals the heat, q, gained, or lost by the system.

If the system loses energy to the surroundings in the form of heat &#8212; as in the burning of wood &#8212; the temperature of the surroundings rises. This is described by a negative sign convention for q. Consequently, &#916;H becomes negative, and the process is described as exothermic.

On the contrary, if the system gains energy from the surroundings in the form of heat &#8212; such as the reaction occurring in a chemical cold pack &#8212; the temperature of the surroundings falls. The heat, in this case, is described by a positive sign convention. This makes &#916;H positive, and the process is called endothermic.

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 of a system&#8217;s internal energy (E) and the mathematical product of its pressure (P) and volume (V):

<img alt="Thermodynamic equation H=E+PV; internal energy, pressure-volume relationship." src="/files/core/CDNSource/lessons/796/796_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Enthalpy is a state function. Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined. For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change (&#916;H) is:

<img alt="&#916;H=&#916;E+P&#916;V, enthalpy change equation, thermodynamics formula illustration." src="/files/core/CDNSource/lessons/796/796_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

The mathematical product P&#916;V represents work (w), namely, expansion or pressure-volume work. By their definitions, the arithmetic signs of &#916;V and w will always be opposite:

<img alt="Thermodynamics equation, PV work formula diagram, illustrating pressure-volume change." src="/files/core/CDNSource/lessons/796/796_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

Substituting this equation and the definition of internal energy at constant pressure (&#916;E = qp + w) into the enthalpy-change equation yields:

<img alt="Thermodynamics equation &#916;H=&#916;E+P&#916;V; enthalpy change formula; chemistry/physics education." src="/files/core/CDNSource/lessons/796/796_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

where qp is the heat of reaction under conditions of constant pressure. 

And so, if a chemical or physical process is carried out at constant pressure with the only work done caused by expansion or contraction (P-V work), then the heat flow (qp) and enthalpy change (&#916;H) for the process are equal.

The heat given off while operating a Bunsen burner is equal to the enthalpy change of the methane combustion reaction that takes place since it occurs at the essentially constant pressure of the atmosphere. Chemists usually perform experiments under normal atmospheric conditions, at constant external pressure with qp = &#916;H, which makes enthalpy the most convenient choice for determining heat changes for chemical reactions.

A negative value of an enthalpy change, &#916;H &#60; 0, indicates an exothermic reaction (heat given off to the surroundings); a positive value, &#916;H &#62; 0, indicates an endothermic reaction (heat absorbed from the surroundings). If the direction of a chemical equation is reversed, the arithmetic sign of its &#916;H is changed (a process that is endothermic in one direction is exothermic in the opposite direction).

Conceptually, &#916;E (a measure of heat and work) and &#916;H (a measure of heat at constant pressure) both represent changes in a state function for the system. In processes where the volume change, &#916;V, is small (melting of ice), and &#916;E and &#916;H are identical. However, if the volume change is significant (evaporation of water), the amount of energy transferred as work will be significant; thus, &#916;E and &#916;H have significantly different values.

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy</a>.

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

Education

JoVE Core

Thermochemistry

KEY TERMS AND CONCEPTS

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 represents expansion work, also called pressure-volume work. The signs of ΔV and w are always opposite, because expansion and work use opposite sign conventions.

At constant pressure, the internal energy change is ΔE = qp + w, where qp is the heat of reaction. When the only work done is due to expansion or contraction, qp and ΔH are equal. This is why enthalpy is a useful choice for measuring heat changes in many lab reactions and physical changes. A Bunsen burner is one example, since methane combustion occurs at about atmospheric pressure.

A negative ΔH means the process is exothermic, which means heat is given off to the surroundings. A positive ΔH means the process is endothermic, which means heat is absorbed from the surroundings. If a chemical equation is reversed, the sign of ΔH also changes. A process that is endothermic in one direction becomes exothermic in the other.

Enthalpy change and internal energy change both describe changes in a state function for the system. When the volume change, ΔV, is small, such as during melting of ice, ΔE and ΔH are essentially the same. When the volume change is larger, such as during evaporation of water, more energy is transferred as work. In that case, ΔE and ΔH can differ by a significant amount.

Enthalpy and Heat at Constant Pressure

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 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

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

Learn how enthalpy connects heat, work, and constant-pressure reactions. Explore exothermic and endothermic changes with clear chemistry examples for high school students.

Enthalpy and Heat at Constant Pressure | JoVE K12

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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.c

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

High School

Physical Science

Matter and Its Interactions

HS-PS1-4

Arizona Science Standards

HS+Phy.P4

HS+Phy.P4U1.6

Texas Essential Knowledge and Skills

CHEM.13

CHEM.13.A

CHEM.13.C

Next Generation Science Standards

HS-PS3-1

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Energy

HS-PS3-1.

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 ...

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 ...

Enthalpies of Reaction

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.

Enthalpy and Heat at Constant Pressure

Description

Transcript

Tags

More videos by Standards