Heat Capacity, Enthalpy, and Calorimetry

Section 12.2 defines specific heat capacity as the amount of heat required to increase the temperature of 1 g of material by 1 K. That definition is somewhat imprecise, because, in fact, the amount of heat required depends on whether the process is conducted at constant volume or at constant pressure. This section describes precise methods for measuring the amonnt of energy transferred as heat during a process and for relating this amonnt to the thermodynamic properties of the system under investigation. 

The heat capacity, C, is defined as the amonnt of energy that mnst be added to the system to increase its temperatnre by 1 K. The heat capacity is a property of the system as a whole and has nnits of J 

In thermodynamics, the molar heat capacities Cy and Cp (the system heat capacities Cy and Cp divided by the nnmber of moles of snbstance in the system) are particnlarly nsefnl Cy is the amonnt of heat reqnired to increase the temperatnre of 1 mol of snbstance by 1 K at constant volnme, and Cp is the corresponding amonnt reqnired at constant pressnre. If the total heat transferred to n moles at constant volnme is qy, then 

TABLE 12.1 Specific Heat Capacities at Constant Pressure (at 25°C) 

FIGURE 12.8 A Styrofoam cup calorimeter. As the piece of metal cools, it releases heat to the water. The amount of heat released can be determined from the temperature change of the water. The hot metal is the system the water is the surroundings. The Styrofoam cup wall prevents energy exchange with the remainder of the room and is the boundary of the thermodynamic universe for this problem. 

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