Chemistry
Endothermic and Exothermic Processes
Endothermic processes absorb heat from their surroundings, causing a decrease in temperature, while exothermic processes release heat into their surroundings, leading to a temperature increase. In endothermic reactions, the products have more energy than the reactants, while in exothermic reactions, the products have less energy than the reactants.
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9 Key excerpts on "Endothermic and Exothermic Processes"
eBook - PDF- Martinus A.J.S. van Boekel(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
In this respect, it is instructive to consider both enthalpy and entropy changes. If the reaction enthalpy is negative, implying heat transfer from the system to the surroundings, this is called an exothermic reaction. If the reaction enthalpy is positive, this implies a heat fl ow from the surroundings to the system, and then it is called an endothermic reaction. An endothermic reaction is only possible if the change in reaction entropy is positive. So, for a reaction as depicted in Equation 3.1 the possibilities are as shown in Table 3.6. An irreversible adiabatic process (no heat exchange between system and surroundings, see Table 3.4) necessarily leads to an increase in entropy of the system in which the process takes place. The tendency of increasing entropy in a given system can be counteracted by putting energy into that system. The total energy of the system plus surroundings remains, of course, constant ( fi rst law) and the total entropy increases (second law), but the entropy of the system receiving the energy may increase, decrease, or remain constant. If the entropy change of the system is negative, the system must lose heat such that the entropy of the surroundings increases by at least the same amount; in that case the process is necessarily exothermic. If the entropy change of the system is positive the system can absorb heat such that the decrease in entropy of the surroundings matches the increase in the system; the chemical process can then be either exothermic or endothermic. It is thus possible that an endothermic reaction proceeds, if the gain in entropy (dispersal of energy) is suf fi cient. This gain in entropy in the system is able to overcome the loss of entropy in the surroundings brought about by the in fl ux of heat (or other forms of energy, such as electric energy) from the surroundings into the system.
- Barron's Educational Series, Albert S. Tarendash(Authors)
- 2021(Publication Date)
- Barrons Educational Services(Publisher)
Chapter FiveEnergy and chemical reactions
Key Ideas
This chapter focuses on the role energy plays in chemical reactions and the factors that determine whether a chemical process will occur under a given set of conditions.KEY OBJECTIVES
At the conclusion of this chapter you will be able to:- Define the terms system and surroundings as they relate to chemical processes.
- Define the terms internal energy and heat.
- Distinguish between heat and temperature.
- Distinguish between exothermic and endothermic reactions.
- Define the term specific heat, and use specific heats to solve calorimetry problems.
- Relate the first law of thermodynamics to the law of conservation of energy.
- Define the term heat of reaction, and solve problems involving heats of reaction.
- Define the terms standard heat of formation and formation reaction, and use the appropriate reference tables to solve problems related to the standard heat of formation.
- Interpret a potential energy diagram.
- Define the term activation energy.
- Define the term spontaneous reaction, and name and describe the factors that drive spontaneous reactions.
- Define the term entropy, and predict whether a given reaction leads to an increase or a decrease in entropy.
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SECTION I—BASIC (REGENTS-LEVEL) MATERIAL
NYS REGENTS CONCEPTS AND SKILLS
Note: By the time you have finished Section I, you should have mastered the concepts and skills listed below. The Regents chemistry examination will test your knowledge of these items and your ability to apply them.Concepts are the basic ideas that form the body of the Regents chemistry course (what you need to know!).Skills are the activities that demonstrate your mastery of these concepts (how you show that you know them!).Following each concept or skill is a page reference (given in parentheses) to this chapter.5.1 Concepts: - Heat is a transfer of energy (usually thermal energy) from a body of higher temperature to a body of lower temperature. (Page 112
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 ).
No longer available |Learn more- Gustavo Blanco, Antonio Blanco(Authors)
- 2017(Publication Date)
- Academic Press(Publisher)
Chapter 7Elements of Thermodynamics and Biochemical Kinetics
Abstract
Chemical reactions proceed with energy changes. Heat is one of the most common forms of energy and it is easy to measure. At constant pressure and temperature, the change in heat is the change in enthalpy (∆H ). Free energy (G ) is the fraction of released energy available to perform work. The free energy change (∆G ) of a reaction is given by the equation: ∆G = ∆H – T · ∆S . A reaction occurs spontaneously when the change in G (∆G ) is negative. The tendency to increase entropy (S, increased disorder) determines the reaction direction. All processes occur with free energy decrease until equilibrium is reached, in which G is minimal. Reactions with negative ∆G are exergonic, while those with positive ∆G are endergonic. Endergonic reactions are possible when coupled with others sufficiently exergonic. ATP, energy rich compound, participates in coupled reactions. Chemical reactions can be coupled when the product of one of them is a reagent for the next. In this case, the overall process ∆G equals the sum of ∆G of individual reactions. In biochemical transformations, endergonic reactions are possible by coupling them with other sufficiently exergonic, so that the overall ∆G is negative. From a kinetic standpoint, reactions can be classified according to the relationship between the reaction rate and the concentration of reactants. A reaction is zero order when the rate of reaction is constant regardless of the reagent concentration. It is first order when the rate is directly proportional to the reactant concentration. Activation energy is the energy needed by the reactants to reach the transition or activated state, from which the reaction can proceed spontaneously. Catalysts
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
When energy is converted from one form into another, energy is neither created nor destroyed (law of conservation of energy or first law of thermodynamics). Matter has thermal energy due to the KE of its molecules and temperature that corresponds to the average KE of its molecules. Heat is energy that is transferred between objects at different temperatures; it flows from a high to a low temperature. Chemical and physical processes can absorb heat (endothermic) or release heat (exothermic). The SI unit of energy, heat, and work is the joule (J). Specific heat and heat capacity are measures of the energy needed to change the temperature of a substance or object. The amount of heat absorbed or released by a substance depends directly on the type of substance, its mass, and the temperature change it undergoes. 5.2 Calorimetry Calorimetry is used to measure the amount of thermal energy transferred in a chemical or physical process. This requires careful measurement of the temperature change that occurs during the process and the masses of the system and surroundings. These measured quantities are then used to compute the amount of heat produced or consumed in the process using known mathematical relations. Chapter 5 | Thermochemistry 267 Calorimeters are designed to minimize energy exchange between the system being studied and its surroundings. They range from simple coffee cup calorimeters used by introductory chemistry students to sophisticated bomb calorimeters used to determine the energy content of food. 5.3 Enthalpy If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, q for the change is called the enthalpy change with the symbol ΔH, or ΔH 298 ° for reactions occurring under standard state conditions.
eBook - PDF- Young, William Vining, Roberta Day, Beatrice Botch(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
● A change in internal energy is the sum of the work and heat added to or removed from a system (10.1b). ● Sign conventions are used to indicate the direction of heat and work flow between a system and the surroundings (10.1b). ● The first law of thermodynamics states that the total energy for an isolated system is constant (10.1b). 10.2 Enthalpy ● Enthalpy is the sum of the internal energy of a system plus the product of pressure and volume (10.2a). ● A change in enthalpy is equal to the heat exchanged between the system and surroundings at constant pressure (10.2a). ● Heat is transferred from the system to the surroundings in an exothermic process (10.2a). ● Heat is transferred from the surroundings to the system in an endothermic process (10.2a). 10.3 Energy, Temperature Changes, and Changes of State ● Specific heat capacity is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius or one kelvin (10.3a). ● Heat transfer occurs from a hot object to a cooler object until thermal equilibrium is reached (10.3b). ● The heat lost by a hot object is equal to the amount of heat gained by the cooler object (10.3b). ● During a phase change, temperature does not change because heat energy is used to overcome the forces between particles (10.3c). ● Enthalpy of fusion is the energy required to melt a solid and enthalpy of vaporization is the energy required to vaporize a liquid (10.3c). 10.4 Enthalpy Changes and Chemical Reactions ● Enthalpy change for a reaction is the energy released or absorbed during a chemical reaction (10.4a). 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 305 ● Enthalpy is an extensive variable; it is dependent on the amount of substance present (10.4b). ● Bond energy values can be used to calculate the enthalpy change for gas-phase reactions (10.4c).
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 - ePub- Nils O. Petersen(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
6The basics of thermodynamics
6.1 SOME BASIC CONCEPTS
Thermodynamics refers to the field of chemistry which is concerned with understanding the flow of energy within a system or when a system changes from one state to another. One ultimate purpose of thermodynamics is to predict whether a change will occur spontaneously. The field of thermodynamics is based on three fundamental laws which lead to a set of parameters such as the free energy and the chemical potential, which are key to our understanding of spontaneous change.In contrast to the field of quantum mechanics, the field of thermodynamics is, to a first approximation, not concerned with the detailed structures of the materials, and hence the outcomes become very generally applicable.Our first task is to remind ourselves of some basic concepts of systems and how we describe these.A system is simply that region of space or matter that we are interested in understanding. The surroundings of the system is everything else —in principle, everything else in the universe, but in practice everything else that matters, such as the laboratory in which we study the system. For example, the system could be a gas held in a glass container, with the surroundings being the glass container and everything around it. Similarly, the system could be a solution of molecules in a solvent where the system is the molecules in solution and the surroundings include the solvent, the container and everything around it.We characterize a system by a number of functions (or variables) that tell us something unique about the system. Examples would be the temperature of the system, the pressure of the system, the volume of the system, the mass (or the number of molecules) in the system, and the energy of the system. If the state of the system changes, we expect one or more of these functions to change as well, for example, if the number of molecules in the system changes, the volume might change; or if a chemical reaction occurs, the temperature might change. In the context of thermodynamics we are most interested in understanding the change in energy, which is in turn often related to the changes in temperature, pressure, volume, and mass.
eBook - PDFChemistry
An Industry-Based Introduction with CD-ROM
- John Kenkel, Paul B. Kelter, David S. Hage(Authors)
- 2000(Publication Date)
- CRC Press(Publisher)
This is because so much heat energy is exchanged in the chemical reactions. But we are getting ahead of ourselves. Before we can talk about energy exchange, we need to define “energy.” We discussed this previously in Section 3.3.1, but will now take a slightly different approach in its application. When you pick up a very heavy book, do you use energy? When you run fast, are you using energy? When you pull two strong magnets apart, do you require energy? What about when you jump in the air? Is energy required to melt a cake of ice and is more required to boil the water that results? Your experience Chemistry Professionals at Work CPW Box 13.1 W HY S TUDY T HIS T OPIC ? imply stated, every process that occurs in the universe is accompanied by energy changes. This was true at the time of the “Big Bang,” after which energy changes led to the formation of the atoms that are the elements. This is also true now, as our bodies undergo the reactions that allow us to live. We require food that provides the energy for us to continue to live. Plants require energy from the sun to produce the fruits, vegetables, and grain that is our food. We are at a stage now in our social and scientific development in which we can use our understanding of chemistry and energy exchange in the industrial production of goods. We can also use chemicals to generate energy, such as in an automobile, train, or plane. Understanding the nature of energy exchanges, therefore, gives us insight into how and why chemical processes occur. We will use this knowledge, along with our study of kinetics in the next chapter, to learn how we can affect the success of these chemical processes. For Homework: Use the Internet search engine “metacrawler” to find the Web addresses of several major chemical process industries, such as Eastman Chemical Company and Procter & Gamble. Find out about s ome of the products they manufacture. How is energy exchange important in the manufacture of each one? S
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