Chem 1000: Heat

HEAT

HEAT is the energy transferred between a system and its surroundings as a result of a temperature difference. Energy that passes from a warmer body to a colder body is transferred as heat. The energy content of a system is a quantity called INTERNAL ENERGY. Heat is simply a form in which a quantity of energy may be transferred across a boundary between a system and its surroundings. The quantity of heat, q, required to change the temperature of a system depends on :

• how much temperature is to be changed
• the quantity of substance
• the nature of the substance (type of atoms or molecules)

The quantity of heat required to change the temperature of one gram of water by one degree Celsius is called CALORIE. The SI unit for heat is joule (J).

The quantity of heat required to change the temperature of a system by one degree is called the HEAT CAPACITY of the system.

•  If the system is a mole of substance, the term molar heat capacity is used.
• If the system is one gram of a substance, the applicable term is specific heat capacity or SPECIFIC HEAT

Note that the specific heat of water is : 4.18 J/g.C

Note that when temperature:

• Increases ( Tf > Ti) , the change in temperature is positive. A positive q signifies that heat is absorbed or gained by the system
• Decreases (Tf < Ti), the change in temperature is negative. A negative q signifies that heat is released or lost by the system

LAW OF CONSERVATION OF ENERGY

This law states:

In interactions between a system and its surroundings, the total energy remains constant, energy is neither created or destroyed

This means that:

q_system + q_surroundings = 0

 

Thus, heat gained by a system is lost by its surroundings or vice versa :

q_system= - q_surroundings