CHM1045 Exam 3 CHM 1045 General Chemistry Chapters 6 7 8 9 Exam 3 Chapter 6 The Nature of Energy and Types of Energy Energy is usually defined as the capacity to do work o Work force x distance o Chemists define work as directed energy change resulting from o Kinetic thermal energy the energy produced by a moving a process object Thermal energy is the energy associated with the random motion of atoms and molecules KE m v 2 CT thermal energy o Radiant energy or solar energy comes from the sun and is Earth s primary energy sources Transfer of energy in the form of heat Ex Fireplace can be transferred between molecules o Photon energy can be transferred by waves of light particles o Chemical energy is stored within the structural units of o Potential energy is energy available by virtue of an object s chemical substances position gravitational chemical electrostatic In chemistry electrostatic forces regulate potential energy If you separate a negative and positive molecule potential energy increases potential energy is based on the arrangement of forces Conservation of Energy o Law of conservation of energy the total quality of energy in the universe is assumed constant Energy may be converted from one form to another U total energy KE PE Energy Changes in Chemical Reactions CHM1045 Exam 3 Almost all chemical reactions absorb or produce release energy generally in the form of heat o Heat is the transfer of thermal energy between two bodies that are different temperatures Thermochemistry is the study of heat change in chemical reactions System the specific part of the universe that is of interest to us o Surrounding the rest of the universe outside of the system o Open system can exchange mass and energy usually in the o Closed system allows the transfer of energy heat but not o Isolate system does not allow the transfer of either mass or form of heat with its surroundings mass energy Energy may be transferred from the system to the surroundings or vice versa via heat and work U q w Endothermic heat has to be supplied to the system energy 2HgO s 2Hg l O2 g Exothermic any process that gives off heat that is transfers thermal energy to the surroundings Ex Ex 2Hg2 g O2 g 2H2O l energy Introduction to Thermodynamics Thermochemistry the study of heat change in chemical reactions is a part of a broader subject called thermodynamics which is the scientific study of the interconversion of heat and other kinds of energy o In thermodynamics we study changes in the state of a system which is defined by the values of all relevant macroscopic properties for example composition energy temperature pressure and volume Energy pressure volume and temperature are said to be state functions properties that are determined by the state of the system regardless of how that condition was achieved CHM1045 Exam 3 When the state of a system changes the magnitude of change in any state function depends only on the initial and final stages of the system and not on how the change is accomplished The state of a given amount of gas is specified by its volume pressure and temperature The First Law of Thermodynamics o The first law of thermodynamics which is based on the law of conservation of energy states that energy can be converted from one form to another but cannot be created or destroyed o Change in internal energy U is given by U Uf Ui o The internal energy of a system has two components kinetic energy and potential energy The kinetic energy component consists of various types of molecular motion and the movement of electrons within molecules Potential energy is determined by the attractive interaction between electrons and nuclei and by repulsive interactions between electrons and between nuclei in individual molecules as well as by interaction between molecules We can accurately measure the change in energy content U given by o U U product U reactants Ex S s O2 g SO2 g U energy content of 1 mol SO2 g energy content of 1 mol S s 1 mol O2 g We find that this reaction gives off heat Therefore the energy of the product is less than that of the reactants and U is negative Interpreting the release of heat in this reaction to mean that some of the chemical energy contained in the molecules has been converted to thermal energy we conclude that the transfer of energy from the system to the surroundings does not change the total energy of the universe That is the sum of the energy changes must be zero CHM1045 Exam 3 Usys Usurr 0 Usys Usurr o In chemistry we are normally interested in the energy changes associated with the system which may be a flask containing reactants and products not with its surroundings Therefore a more useful form of the first law is U q w We drop the subscript sys for simplicity The above equation says that the change in the internal energy U of a system is the sum of the heat exchange q between the system and the surroundings and the work done w on or by the system The sign conventions for q and w are as follows q is positive for an endothermic process and negative for a exothermic process and w is positive for work done on the system by the surroundings and negative for work done by the system on the surroundings Ex 30g H2O 280K A 50 g H2O 330K B Tf Assume no heat loss q A q B q m x s x t q A q B m A x s A x Tf 280 m B x s B x Tf 330 Tf 311K Work and Heat o Work Work can be defined as force F multiplied by distance d w F x d In thermodynamics work has a broader meaning that includes mechanical work for example a crane lifting a steel beam electrical work a battery supplying electrons to light the bulb of a flashlight and surface work blowing up a soap bubble In this section we will concentrate on mechanical work Table 6 1 Sign Conventions for Work and Heat Process Sign CHM1045 Exam 3 Work done by the system on the surroundings Work done on the system by the surroundings Heat absorbed by the system from the surroundings endothermic process Heat absorbed by the surroundings form the system exothermic process w and q are path functions Work done by gas on its surroundings is w P V where V the change in volume is given by Vf Vi The minus sign takes care of the sign convention for w For gas expansion work done by the system V 0 so P V is a negative quantity For gas compression work done on the system V 0 and P V is a positive quantity The above equation derives from the fact that pressure x volume can be expressed as force area x volume that is o P x V F d 2 x d 3 F x d w pressure volume o
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