Thermal energy is kinetic energy associated with the random motion of atoms and molecules. When thermal energy is transferred into an object, its atoms and molecules move faster on average (higher KEaverage), the object’s temperature increases, and we say that the object is “hotter”. When thermal energy is transferred out of an object, its atoms and molecules move more slowly on average (lower KEaverage), the object’s temperature decreases, and we say that the object is “colder”.
Heating (or heat), represented by q, is the transfer of thermal energy between two bodies at different temperatures .
The heat capacity (C) of an object is the heating required to raise an object’s temperature by 1 °C. (Because 1 K is the same size as 1 °C, ΔT has the same numeric value whether expressed in K or °C.) It typically has units of J/°C. Heat capacity depends on both the type and the quantity of substance, and therefore is an extensive property—its value is proportional to the quantity of the substance.
The specific heat capacity (c) of a substance is the heating required to raise the temperature of 1 g of a substance by 1 °C. It typically has units of J/g·°C. Specific heat capacity depends only on the type of substance and therefore is an intensive property. The molar heat capacity, also an intensive property, is the heat capacity per mole of a particular substance and typically has units of J/mol·°C.
Specific heat capacities of some common substances are listed in the table below.
|Substance||Symbol (state)||Specific Heat Capacity (J/g·°C)|
|ice||H2O(s)||2.093 (at −10 °C)|
|Table: Specific Heat Capacities of Common Substances at 25 °C and 1 bar|
If we know the mass, m, of a sample and its specific heat capacity, c, we can calculate the heat transferred to or from the sample by measuring the temperature change during heating or cooling:
The sign of ΔT tells us whether the substance is being heated (positive value for q) or cooled (negative q ).