Specific Heat Capacity Specific Heat Capacity Heat capacity (usually denoted by a capital C, often with subscripts), or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of joule(s) (J) per kelvin (K). Derived quantities that specify heat capacity as an intensive property, i.e., independent of the size of a sample, are the molar heat capacity, which is the heat capacity per mole of a pure substance, and the specific heat capacity, often simply called specific heat, which is the heat capacity per unit mass of a material. Occasionally, in engineering contexts, a volumetric heat capacity is used. Because heat capacities of materials tend to mirror the number of atoms or particles they contain, when intensive heat capacities of various substances are expressed directly or indirectly per particle number, they tend to vary within a much more narrow range. Here in this page we are going to discuss about specific heat capacity and calculating specific heat capacity concepts.
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<<--- Calorimetry is the science of determining the heats of the reaction or physical changes and specific heat capacity of substances. <<--- Calorimeter is the device used to perform these measurements. <<--- Various types of calorimeters are available for the different type of investigations but all work on a similar principle. <<--- The system under study is coupled to water which acts as a sink. <<--- The heat changes in the system under study are transferred to water and the calorimeter and all temperature measurements are taken from the water system. <<--- Water is an ideal sink material as it is economical and due to its high specific heat large changes in thermal energy can be measured. <<--- q or ΔHrxn = - (qwater + qcal), where, q/ΔHr is the heat of the system/enthalpy of reaction under study, qwater and qcal are the heat content of the water and the calorimeter. <<--- qwater = m.Cwater ΔT, where, m= mass of water, Cwater is specific heat of water = 4.184 J/g-ºC and ΔT is the change in temperature. <<--- qcal = Ccal ΔT, where qcal is the heat, Ccal is the calorimeter constant (specific heat of calorimeter) and ΔT is the temperature change. Solved Problems :Problem 1: 1.0g sample of octane (C8H18) is combusted in a bomb calorimeter containing 1500 g of water at an initial temperature of 22.00ºC. The final temperature of the water is 28.56ºC. The heat capacity of the calorimeter is 1046 J/ºC. Calculate the heat of combustion of octane.
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Solution :Heat released or q = - (qwater + qcal) Calculate the heat absorbed by water qwater = m C ΔT = (1500 g) x (4.184 J/g-ºC)x (28.56ºC-22.00ºC) = (1500 g) x (4.184 J/g-ºC)x (6.56ºC) = 41170.56 kJ = 41.2 kJ Calculate the heat absorbed by the calorimeter (qcal) qcal = Ccal ΔT = (1046 J/ºC) (6.56ºC) = 6861.76 J = 6.86 kJ Heat released or qrxn = - (qwater + qcal) = -(41.2 kJ + 6.86 kJ) = -48.06 kJ This is the heat released on burning of 1g of octane ΔHc for octane is -48.06 kJ/g Molar mass of octane is 114g/mol So, ΔHc in kJ/mol = (-48.06 kJ/g) (114g/mol) = -5483 kJ/mol
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