energy <=

# Heat

Almost everyone knows what is heat, but to describe it in scientific terms is difficult. We know heat flow from a hot place to a cold place, but we have to be able to compare hot and cold. Then we have to know properties of heat (thermodynamics and kinetics).

• First of all, we need an objective way to compare, for which temperature scales has been invented.

• Joseph Black (1728-1799) considered heat an indestructible caloric fluid. He defined heat capacity and determined latent heats of solids.

• Later, the quantity 1.00 cal was defined as the heat required to raise 1.00 g water from 288.5 to 289.5 K.

• Benjamin Thompson (1753-1814) discovered that heat was generated from mechanical work while making cannons. J.P. Joule (1818-1889) determined that 1.0 cal is about 4.2 J, and today, 1.00 cal is defined exactly 4.184 J.

• Heat capacity is the amount of heat required to raise the temperature of a system by 1.00oC or 1.00 K. Generally, the heat capacities for water and ice are 1.0 and 0.5 cal g-1 K-1.

• Heat is energy being transferred via a medium from a high-temperature source to a low-temperature sink.

• Heat within a system consists of rotational, vibrational, and translational kinetic energies of molecules and atoms. In addition, electrons in atoms and molecules can also absorb energy and raise to higher energy levels.

Example 1

How much energy is required to heat 500 mL water from 288 K to 370 K to make two cups of coffee?

Solution

Heat capacity of water is 4.184 J ml-1 K-1 Thus, heat q is

q = 500 * 4.184 * (370 - 288) ml J ml-1 K-1 K
= 171544 J (or 172 kJ)

Example 2

The heat of melting for ice is 80 cal g-1 (6.02 kJ mol-1). How much ice will melt when 172 kJ is consumed?

Solution

The amount of ice melt by 172 kJ is

172 kJ
----------
6.02 kJ mol-1
= 28.5 mol
= 513 g
Energy <=

E-mail: cchieh@uwaterloo.ca