Chemistry
Enthalpy Changes
Enthalpy changes refer to the heat energy exchanged during a chemical reaction at constant pressure. They are represented by the symbol ΔH and can be either exothermic (release heat) or endothermic (absorb heat). Enthalpy changes are crucial in understanding the energy transformations that occur in chemical reactions.
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12 Key excerpts on "Enthalpy Changes"
eBook - ePub- Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
p .ΔE = qp − PΔVΔE = ΔH − PΔV; or solving for ΔH: ΔH = ΔE + PΔVEnthalpy is the change in potential energy (ΔPE) of a chemical process measured as heat transfer under conditions of constant pressure; work energy change (expansion or contraction of volume) is not included. Enthalpy, however, is a useful measure of energy change for a wide variety of chemical processes and is often a close approximation of total energy change. For many chemical reactions, such as solubility reactions, reactions in solution, or reactions involving only solids and liquids, volume expansion at constant pressure is very small (ΔV ≅ 0). In these cases, enthalpy and total energy change are approximately equal: ΔE ≅ ΔH.Volume expansion or contraction can be significant whenever gases are formed or consumed in a reaction; the number of moles of gas then changes from reactants to products. Even in many of these cases, however, the total energy contribution from the work term (−PΔV) can often be small as compared to the enthalpy term (ΔH).Example:C 2H 8N 2( I )+ 2N 2O→ 34 ( g )N2( g )+ 2 CO2( g )+ 4H 2O( g )2 moles of gas → 9 moles of gasAt constant pressure, the work of gas expansion (w = −PΔV) equals −22 kJ/mole. (Properties of gases and energy are described in Chapter 20
eBook - PDF- Young, William Vining, Roberta Day, Beatrice Botch(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Vasilyev/Shutterstock.com Thermochemistry Unit Outline 10.1 Energy 10.2 Enthalpy 10.3 Energy, Temperature Changes, and Changes of State 10.4 Enthalpy Changes and Chemical Reactions 10.5 Hess’s Law 10.6 Standard Heats of Reaction In This Unit… This unit begins an exploration of thermochemistry, the study of the role that energy in the form of heat plays in chemical processes. We inves-tigate the energy changes that take place during phase changes and the chemical reactions you have studied previously and learn why some chemical reactions occur while others do not. In Electromagnetic Radiation and the Electronic Structure of the Atom (Unit 3), you stud-ied energy changes at the molecular level and the consequences those energy changes have on the properties of atoms and elements. 10 Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 Unit 10 Thermochemistry 272 10.1 Energy 10.1a Energy and Energy Units Chemical reactions involve reactants undergoing chemical change to form new substances, products. reactants S products What is not apparent in the preceding equation is the role of energy in a reaction. For many reactions, energy, often in the form of heat, is absorbed—that is, it acts somewhat like a reactant. You might write an equation for those reactions that looks like this: energy 1 reactants S products In other reactions, energy is produced—that is, it acts like a product: reactants S products 1 energy In Chemistry, Matter on the Atomic Scale (Unit 1), we defined energy (the ability to do work) and work (the force involved in moving an object some distance). From a chemist’s point of view, energy is best viewed as the ability to cause change, and thermochemistry is the study of how energy in the form of heat is involved in chemical change.
eBook - ePub- Vladimir Danek †(Author)
- 2006(Publication Date)
- Elsevier Science(Publisher)
Chapter 4Enthalpy
Publisher Summary
Enthalpy is the function of state and its value depends only on the starting and final state of the system. This chapter focuses on the application and determination of enthalpy. The heat measurement of various reactions is the first step in entering the realm of thermodynamics. The measurement of enthalpy, or heat, of different chemical processes is the objective of the first thermodynamic principle, the law of energy conservation. Every chemical process is connected with a certain amount of enthalpy, which the system receives from the surroundings. The reactions proceed at constant pressure so it is convenient to characterize the reaction by enthalpy change. The absolute values of the enthalpies or the internal energies, depends on the standard state, which are compared with the given quantity. The standard states mostly used are gases, liquids, and solids. There are different kinds of reaction enthalpy that is observed in molten salt chemistry, such as bonding energy, enthalpy of mixing, enthalpy of dissolution, and enthalpy of polymorphic transformation. The reliability of estimation of the enthalpy of fusion of the binary compounds and eutectic mixtures depends on the volume of input information, the choice of simplifying conditions, the difference between the melting points of the components and binary compounds, and the eutectic temperatures.The heat measurement of various reactions is the Þrst step in entering the realm of thermodynamics. Every study of any chemical process starts with the laboratory work connected with the Þrst law of thermodynamics. On the other hand, when the thermodynamic considerations do not lead to a reasonable result, it is necessary to go back to the laboratory work.The basic unit of heat is joule (J). The older unit is calorie (cal). The relation between them is 1 J = 4.314 cal.
eBook - ePub- Atef Korchef(Author)
- 2022(Publication Date)
- CRC Press(Publisher)
- How is the heat absorbed or released during a phase change calculated?
- How is the enthalpy change of a chemical reaction calculated?
- What is the bond enthalpy?
- How is the lattice enthalpy of an ionic compound calculated?
- How can you tell whether a chemical reaction or a physical process is endothermic or exothermic?
Summary of Chapter 5
Energy is the capacity to do work or transfer energy. Energy can only be transformed from one form to another. The energy of the universe is constant.The system is a portion of space which includes the particles of interest that we study. The surroundings are everything else. The entire system and surroundings constitute the universe. The system is bounded by a real or fictitious surface through which the exchanges of energy and matter are made with the surroundings.Energy can transfer as work (w) or as heat (q). Heat is the transfer of thermal energy (defined as the kinetic energy related to the random motion of atoms or molecules) between two matters at different temperatures. Work is the energy used to cause an amount of matter to move. At constant pressure, w = −PΔV.When the heat is absorbed by the system, the process is endothermic (q > 0). When the heat is released by the system, the process is exothermic (q < 0). The work done on the system by the surroundings is considered to be a positive quantity (w > 0) and the work done by the system on the surroundings is considered to be a negative quantity (w < 0).A state function depends only on the initial and final states of the system. The internal energy does not depend on either the path by which the system achieved these states, or on how it is used.The internal energy
- Charles G. Hill, Thatcher W. Root(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
Because chemical reactions involve the formation, destruction, or rearrangement of chemical bonds, they are invariably accompanied by changes in the enthalpy and Gibbs free energy of the system. The enthalpy change on reaction provides information that is necessary for any engineering analysis of the system in terms of the first law of thermodynamics. Standard Enthalpy Changes are also useful in determining the effect of temperature on the equilibrium constant for the reaction and thus on the reaction yield. Gibbs free energy changes are useful in determining whether or not chemical equilibrium exists in the system being studied and in determining how changes in process variables can influence the yield of the reaction.In chemical kinetics there are two types of processes for which one is typically interested in changes in these energy functions:- A chemical process whereby stoichiometric quantities of reactants, each in its standard state, are completely converted to stoichiometric amounts of products, each in its standard state, under conditions such that the initial temperature of the reactants is equal to the final temperature of the products.
- An actual chemical process as it might occur under either equilibrium or nonequilibrium conditions in a chemical reactor.
One must be very careful not to confuse actual energy effects with those that are associated with the process whose initial and final states are the standard states of the reactants and products, respectively.To have a consistent basis for comparing different reactions and to permit the tabulation of thermochemical data for various reaction systems, it is convenient to define enthalpy and Gibbs free energy changes for standard reaction conditions. These conditions involve the use of stoichiometric amounts of the various reactants (each in its standard state at some temperature ). The reaction proceeds by some unspecified path to end up with complete conversion of reactants to the various products (each in its standard state at the same temperature
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Water has an unusually high specific heat. We can compute a heat flow when we know the mass and specific heat of an object using the equation q = ms ∆t. The heat, q, is given a positive sign when it flows into a system and a negative sign when it flows out. Describe the energy changes in exothermic and endothermic reactions. Bond breaking increases potential energy (chemical energy); bond formation decreases potential energy (chemical energy). In an exo- thermic reaction, chemical energy is changed to molecular kinetic energy. If the system is adiabatic (no heat leaves it), the internal temperature increases. Otherwise, the heat has a tendency to leave the system. In endothermic reactions, molecular kinetic energy of the reactants is converted into potential energy of the products. This 300 CHAPTER 6 Energy and Chemical Change tends to lower the system’s temperature and lead to a flow of heat into the system. State the first law of thermodynamics and explain how it applies to chemistry. The change in chemical potential energy in a reaction is the heat of reaction, q, which can be measured at constant volume or constant pressure. Pressure is the ratio of force to the area over which the force is applied. Atmospheric pressure is the pressure exerted by the mix- ture of gases in our atmosphere. When the volume change, ∆V, occurs at constant opposing pressure, P, the associated pressure– volume work (expansion work) is given by w = −P ∆V. The energy expended in doing this pressure–volume work causes heats of reaction meas- ured at constant volume (q v ) to differ numerically from heats meas- ured at constant pressure (q p ). The first law of thermodynamics, a specific example of the law of conservation of energy, says that no matter how the change in energy accompanying a reaction may be allocated between q and w, their sum, ∆E, is the same: ∆E = q + w. The algebraic sign for q and w is negative when the system gives off heat to or does work on the surroundings.
eBook - PDFChemistry
Principles and Reactions
- William Masterton, Cecile Hurley(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
187 8 ▼ Thermochemistry Chapter Outline 8-1 Principles of Heat Flow 8-2 Measurement of Heat Flow; Calorimetry 8-3 Enthalpy 8-4 Thermochemical Equations 8-5 Enthalpies of Formation 8-6 Bond Enthalpy 8-7 The First Law of Thermodynamics ▼ T his chapter deals with energy and heat, two terms used widely by both the gen-eral public and scientists. Energy, in the vernacular, is equated with pep and vitality. Heat conjures images of blast furnaces and sweltering summer days. Scientifically, these terms have quite different meanings. Energy can be defined as the capacity to do work. Heat is a particular form of energy that is transferred from a body at a high temperature to one at a lower temperature when they are brought into contact with each other. Two centuries ago, heat was believed to be a material fluid (caloric); we still use the phrase “heat flow” to refer to heat transfer or to heat effects in general. Thermochemistry refers to the study of the heat flow that accompanies chemical reactions. Our discussion of this subject will focus on ■ the basic principles of heat flow (Section 8-1). ■ the experimental measurement of the magnitude and direction of heat flow, known as calorimetry (Section 8-2). ■ the concept of enthalpy, H (heat content) and enthalpy change , D H (Section 8-3). ■ the calculation of D H for reactions, using thermochemical equations (Section 8-4) and enthalpies of formation (Section 8-5). ■ heat effects in the breaking and formation of covalent bonds (Section 8-6). ■ the relation between heat and other forms of energy, as expressed by the first law of thermodynamics (Section 8-7). Scala/Art Resource, NY The candle flame gives off heat, melting the candle wax. Wax melting is a phase change from solid to liquid and an endothermic reaction. Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. —ROBERT FROST Fire and Ice Copyright 2016 Cengage Learning.
eBook - PDFPhysical Chemistry
Thermodynamics
- Horia Metiu(Author)
- 2006(Publication Date)
- Taylor & Francis(Publisher)
12 ENTHALPY AND ENERGY CHANGE DURING A THERMODYNAMIC TRANSFORMATION Introduction §1. Why Bother? Why would anyone want to calculate how the enthalpy or the energy changes during a thermodynamic transformation? One reason is heat. As you will see later, the heat exchanged between a system and the environment, during a thermodynamic transformation performed at constant pressure, is equal to the change of enthalpy during that transformation. This is true for any iso-baric transformation, including chemical reactions, electrochemical processes in batteries, etc. The easiest way to calculate the heat exchanged in an isobaric trans-formation is to calculate the change of enthalpy. This will be particularly useful in the next chapter, in which we study heats of reaction. The heat exchanged between a system and the environment during a trans-formation performed at constant volume is equal to the change in energy. 219 220 Change of Enthalpy and Energy Calculating the change of energy is the easiest way of calculating the amount of heat produced in a thermodynamic process. Since reactions are carried out at constant volume less frequently than at constant pressure, I will explain how to calculate energy changes in Supplement 12.1. Knowing that material is useful, but is not required for understanding the rest of the book. We need to fuss over heat because heating and cooling are expensive. When design-ing or running a chemical factory we must know how much heat needs to be produced or removed. Even the simple task of compressing a gas produces heat. If the temperature becomes too high it will ruin the equipment. To prevent this we have to cool it. To determine how much coolant is required, the amount of heat to be removed must be calculated. Industrial equipment is often heated by condensing steam. We need to know how much heat the steam produces when it condenses at given temperature and pres-sure. The steam used in one place in a factory is produced somewhere else.
eBook - PDFChemistry
The Molecular Nature of Matter
- James E. Brady, Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
The heat capacity for a pure substance can be computed from its mass, m, using the equation C = ms, where s is the specific heat of the material (the heat needed to change the temperature of 1 g of a substance by 1 °C). Water has an unusually high specific heat. We can compute a heat flow when we know the mass and specific heat of an object using the equation q = ms ∆t. The heat, q, is given a positive sign when it flows into a system and a negative sign when it flows out. Describe the energy changes in exothermic and endothermic reactions Bond breaking increases potential energy (chemical energy); bond formation decreases potential energy (chemical energy). In an exothermic reaction, chemical energy is changed to molecu- lar kinetic energy. If the system is adiabatic (no heat leaves it), the internal temperature increases. Otherwise, the heat has a ten- dency to leave the system. In endothermic reactions, molecular kinetic energy of the reactants is converted into potential energy of the products. This tends to lower the system’s temperature and lead to a flow of heat into the system. State the first law of thermodynamics and explain how it applies to chemistry The change in chemical potential energy in a reaction is the heat of reaction, q, which can be measured at constant volume or constant pressure. Pressure is the ratio of force to the area over which the force is applied. Atmospheric pressure is the pressure exerted by the mixture of gases in our atmosphere. When the volume change, ∆V, occurs at constant opposing pressure, P, the associated pressure–volume work (expansion work) is given by w = -P ∆V. The energy expended in doing this pressure– volume work causes heats of reaction measured at constant vol- ume (q v ) to differ numerically from heats measured at constant pressure (q p ).
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
INTRODUCTION CHAPTER 5 Thermochemistry 5.1 Energy Basics 5.2 Calorimetry 5.3 Enthalpy Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter. Societies at all levels of development could not function without the energy released by chemical reactions. In 2012, about 85% of US energy consumption came from the combustion of petroleum products, coal, wood, and garbage. We use this energy to produce electricity (38%); to transport food, raw materials, manufactured goods, and people (27%); for industrial production (21%); and to heat and power our homes and businesses (10%). 1 While these combustion reactions help us meet our essential energy needs, they are also recognized by the majority of the scientific community as a major contributor to global climate change. Useful forms of energy are also available from a variety of chemical reactions other than combustion. For example, the energy produced by the batteries in a cell phone, car, or flashlight results from chemical reactions. This chapter introduces many of the basic ideas necessary to explore the relationships between chemical changes and energy, with a focus on thermal energy. Figure 5.1 Sliding a match head along a rough surface initiates a combustion reaction that produces energy in the form of heat and light. (credit: modification of work by Laszlo Ilyes) CHAPTER OUTLINE 1 US Energy Information Administration, Primary Energy Consumption by Source and Sector, 2012, http://www.eia.gov/totalenergy/ data/monthly/pdf/flow/css_2012_energy.pdf. Data derived from US Energy Information Administration, Monthly Energy Review (January 2014).
eBook - PDF- R. Prasad(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
It is important to note that in thermodynamic analysis of chemical reactions all gases and vapors are treated as ideal gas, unless specified otherwise. Let us consider an ideal gas that undergoes a reversible compression so that its volume and temperature change from initial values T i , V i to final values T f , V f . According to the first law, dQ r = dU + PdV 9.18 Here dQ r is the heat evolved in the reversible process. Also, dU = C V dT and for an ideal gas P = RT V . With these substitutions Eq. 9.18 becomes dQ r = C dT V + RT dV V 382 Classical and Quantum Thermal Physics Dividing the above equation by T throughout, one gets dQ T r = C dT T R dV V V + But dQ T dS r ∫ Hence, S S V T T i f i f dS C dT T Ú = Ú + R dV V V V i f Ú Or S f – S i = DS = C T T V f i ln + NR V V f i ln 9.19 Though Eq. 9.19 gives the change in the entropy when a system goes from initial equilibrium state (T i , V i ) to the final equilibrium state (T f , V f ) by a reversible process, but the same expression may be used for calculating change in entropy if the system reaches the same final state by an irreversible process as entropy is a state function. Further, it may be shown that change in entropy may also be written as, S f – S i = DS = C T T R P P P f i f i ln ln - 9.20 Equations 9.19 and 9.20 may be used to calculate the change in entropy of a given system. 9.14 Spontaneity of a Chemical Reaction According the second law, the sum of the changes in entropies of a system and its surrounding must increase in an irreversible process. Since all spontaneous processes in nature are irreversible, a chemical reaction will be spontaneous if the sum of the changes in the entropy of the system and its surroundings is positive. The change in the entropy of the system (the reactants and the reaction products) may be determined using Eqs. 9.19 and 9.20. To determine the change in the entropy of the surroundings one may use the following relation, ( DS) surroundings = DH T rea T ( ) 9.21 In Eq.
eBook - PDF- H. Donald Brooke Jenkins(Author)
- 2008(Publication Date)
- Wiley-Blackwell(Publisher)
At constant volume, q V , the heat absorbed which raises the temperature of the material (equation 10.2) equates to d U and hence: C v = ∂ U ∂ T V (10.13) Changing partial derivatives to ordinary derivatives – which we can do if we are keeping P constant – and rearranging equation (10.12) d H = C p . d T (10.14) and then integrating between limits of i and f we have: H = H f ∫ H i d H = T f ∫ T i C P . d T (10.15) where H is the enthalpy change when the temperature of a substance (a heat capacity, C p ) is raised from T i to T f . 10.3 Methods of Calculating Δ H There are several possible approaches which can be used to calculate H and these are given in Figure (10.1). Calculation of H , the enthalpy change involved when the temperature of a substance with heat capacity, C p , is raised from T i to T f . It should be noted that these calculations (equations (10.18) and (10.19) in the figure) lead to a value of H ( = m H ) in units of J mol – 1 and that this must then be divided by 10 3 (i.e. multiplied by kJ/(1000 J)) in order to give H ( = m H ) in its more usual units of kJ mol – 1 (10 3 J = 1 kJ). This fact is important and should be remembered in respect of heat capacity calculations. 10.4 Use of Heat Capacity for the Calculation of Enthalpy Change, Δ H Suppose we wish to calculate the enthalpy change, H , to raise the temperature of the products ( H prod ) and reactants ( H react ) of a chemical reaction from T 1 to T 2 . Considering the two reactions, at temperatures, T 1 and T 2 : aAB + bCD → cEF at temperature T 2 H react ↑ ↑ H prod aAB + bCD → cEF at temperature T 1 then using equation (10.18) (assuming that C p , m (AB), C p , m (CD) and C p , m (EF), the molar heat capacities of the reactants AB and CD and the products, EF, are independent of temperature change between T 1 and T 2 ), then: H react = [a C p , m (AB) + b C p , m (CD)]( T 2 − T 1 ) (10.16) and H prod = [c C p , m (EF)]( T 2 − T 1 ) (10.17) 31
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