Heat Capacity

Heat is a form of energy that can be transferred from one form to another. In this topic, we shall discuss the topics related to energy exhibited by matter or a substance which may be heat capacity, specific heat capacity, or molar heat capacity. There are several factors affecting Heat capacity. These might be the temperature, mass and pressure of the system.

Heat capacity:

It is defined as the amount of heat required to raise the temperature of a substance by one degree Celsius. It is given by: C=(q/∆T)

Specific Heat Capacity:

The specific heat capacity of a system may be defined as the amount of heat required to raise the temperature of the unit mass of a substance through one degree. If an amount of AQ of heat is given to a mass m of the substance and its temperature rises by AT, the specific heat capacity s is given by the following equation. S= ∆ Q/ m ∆ T The specific heat capacity of a system, particularly in a gaseous system, determined at constant volume is different from that determined at constant pressure. Thus, to define the specific heat capacity of gas the process should also be specified. Thermal capacity can be related to specific heat in the following manner. Thermal capacity= m x s Therefore, Specific heat capacity is the amount of heat required to raise the temperature of a substance (1g) through one degree Celsius. q = (C x ∆t x m) q= m. C. ∆t

Molar heat Capacity:

It is the amount of heat required to raise the temperature of 1 mole of substance through one degree Celsius.
Molar heat capacity = c_m = heat capacity for 1mole.
c_m = (c/n)
Where cm is for one mole, n=total number of moles and c is heat capacity

Calorimetry

Calorimetry is concerned with the measurement of heat, the basic apparatus for this purpose being called the calorimeter. When two bodies at different temperatures are ‘mixed’, heat ‘flows’ from the body at a higher temperature to the one at a lower temperature, until a common ‘equilibrium’ temperature is reached. Assuming this ‘heat exchange’ to be confined to the two bodies alone (i.e, neglecting any heat loss to the surroundings) we have, from the law of energy conservation: Heat gained by one body = heat lost by the other.

Joule-Thomson Effect:

• According to this effect, the change of temperature occurs when a gas is made to expand adiabatically from a region of high pressure to a region of extremely low pressure through a small jet.

• Joule-Thomson effect is zero for an ideal gas i.e. when an ideal gas expands in a vacuum, there is neither absorption nor evolution of heat i.e., q = 0.

• For an adiabatic reversible expansion of a gas, PV = constant.

• Work done in an adiabatic reversible expansion of a gas i.e.  w= n R(T₂-T₁)

Heat Transfer

Heat can be transferred from one place to another by three different methods, namely, conduction, convection, and radiation. Conduction usually takes place in solids, convection in liquids and gases, and no medium is required for radiation.
(i) Conduction: It is the flow of heat through an unequally heated body from places of higher temperature to those of lower temperature. The rate of heat transfer is given by where K is called Thermal Conductivity and A is an area of cross-section.
(ii) Convection: It is the flow of heat by the motion of the hot body itself carrying its heat with it.
(iii) Radiation: It is the mode of heat transfer in which heat travels directly from one place to another without the agency of any intervening medium.

Thermal conductivity:

It is defined as heat energy transferred in unit time from the unit area having a unit difference in temperature over unit length. It is expressed in Js^-1 m^-1 °C^-1 or W^-1 K^-1

Conclusion:

Heat capacity is, therefore, the amount of heat required to raise the temperature of a unit mass of a substance by one unit degree change. It is different from Thermal capacity and Molar thermal Capacity. Henceforth in this chapter we dealt with heat, heat transfer types, Heat Capacity, specific heat capacity, and molar heat capacity.