CHM1045 Lecture 17 Outline of Current Lecture I Energy II 2 Types of Processes III Thermodynamics IV State Functions V First law of thermodynamics VI Work Done By the System On the Surroundings VII Enthalpy and the First Law of Thermodynamics VIII Enthalpy IX Thermochemical Equations X A Comparison of H and U Current Lecture Energy Energy is the capacity to do work o Work energy change Radiant Energy ex sun Thermal Energy random motion of atoms Chemical Energy Energy stored with in bonds Nuclear Stored within collection Heat transfer between energy of 2 bodies at different temperatures Temperature measure of thermal energy Thermochemistry Study of heat change in chemical reactions System Specific part of universe we re interested in 2 Types of Processes 1 Exothermic given off heat transfers thermal energy a 2H2 O2 2H2O 2 Endothermic heat is absorbed applied a Energy 2HgO 2Hg O2 b Energy H20 H20 Thermodynamics Def Study of inter conversion of heat and other kind of energy State Functions Properties determined by state of system regardless how you got their getting from point A to B First law of thermodynamics energy can be converted from one form to another but cannot be created or destroyed DUsystem DUsurroundings 0 DUsystem DUsurroundings C3H8 5O2 3CO2 4H2O Exothermic chemical reaction Work Done By the System On the Surroundings w P DV DV 0 PDV 0 w 0 Example 1 A certain gas expands in volume from 2 0 L to 6 0 L at constant temperature Calculate the work done by the gas if it expands a against a vacuum b against a constant pressure of 1 2 atm a P 0 W p V W 0 4 W 0 b p 1 2 atm w 1 2 4 w 4 8L atm 4 8 101 3J 1L atm 4 9 102 J Example 2 The work done when a gas is compressed in a cylinder like that shown in Figure 6 5 is 462 J During this process there is a heat transfer of 128 J from the gas to the surroundings Calculate the energy change for this process U q w w 462 q 128 U 128 462 334J Enthalpy and the First Law of Thermodynamics U q w At constant pressure q H and w P V U H P V H U P V Enthalpy Enthalpy H is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure DH H products H reactants Thermochemical Equations The stoichiometric coefficients always refer to the number of moles of a substance H2O s H2O l H 6 01 kJ mol If you reverse a reaction the sign of H changes H2O l H2O s H 6 01 kJ mol If you multiply both sides of the equation by a factor n then DH must change by the same factor n 2H2O s 2H2O l H 2 x 6 01 12 0 kJ The physical states of all reactants and products must be specified in thermochemical equations H2O s H2O l H 6 01 kJ mol H2O l H2O g H 44 0 kJ mol Example3 Given the thermochemical equations 2SO2 O2 2SO2 H 198 2kj mol Calculate the heat evolved when 87 9g of SO2 is converted to SO387 9gSO2 1molSO2 64 07gSO2 1 372 mol SO2 1 372gSO2 198 2kj 2 mol SO2 136 kj mol A Comparison of H and U 2Na s 2H2O l 2NaOH aq H2 g DH 367 5 kJ mol DU 367 5 kJ mol 2 5 kJ mol 370 0 kJ mol U H
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