CHM1045 Lecture 18 Outline of Last 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 Outline of Current Lecture I Heat II Constant Volume Calorimetry III Constant Pressure Calorimetry Current Lecture Example 1 Calculate the change in internal energy when at 1 atm and 25 C 2 moles of CO are converted to 2 moles of CO 2 n 2 3 n 1 U H RT n 566 0kj mol 8 314jk mol 1kj 1000J 298K 1 563 5 kl mol Heat specific heat s a substance is the amount of heat q required to raise the temperature of one gram of the substance by one degree Celsius heat capacity C a substance is the amount of heat q required to raise the temperature of a given quantity m of the substance by one degree Celsius Heat q absorbed or released q m x s x Dt q C x t Dt tfinal tinitial Example 2 A 466 g sample of water is heated from 8 50 C to 74 60 C Calculate the amount of heat absorbed in kilojoules by the water Q ms t 466g 4 184j g C 74 60C 8 50C 1 29 10 5 1Kj 1000j 129jk Constant Volume Calorimetry qsys qwater qbomb qrxn qsys 0 qrxn qwater qbomb qwater m x s x t qbomb Cbomb x t Reaction at Constant V H qrxn H qrxn Example 3 A quantity of 1 435 g of naphthalene C10H8 a pungent smelling substance used in moth repellents was burned in a constant volume bomb calorimeter Consequently the temperature of the water rose from 20 28 C to 25 95 C If the heat capacity of the bomb plus water was 10 17 kJ C calculate the heat of combustion of naphthalene on a molar basis that is find the molar heat of combustion Qcal Ccal t 10 17kj c 25 95C 20 28C 57 666kJ 57 666kJ 128 2g C10H8 1 435C10H8 1 mol C10H8 Constant Pressure Calorimetry qsys qwater qcal qrxn qsys 0 qrxn qwater qcal qwater m x s x t qcal Ccal x t 5 51 10 3 kJ mol Reaction at Constant P H qrxn Example 4 A lead Pb pellet having a mass of 26 47 g at 89 98 C was placed in a constant pressure calorimeter of negligible heat capacity containing 100 0 mL of water The water temperature rose from 22 50 C to 23 17 C What is the specific heat of the lead pellet Qh20 100 0g 4 184kJ C 23 17 22 5 280 3J qpb ms t 280 3J 26 47 s 23 17C 89 98 s 158 J g C
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