|Thermodynamics : Temperature, Heat, and Work|
Thermodynamics - The study of the relationship between heat, work, and other forms of energy.
Thermochemistry - A branch of thermodynamics which focuses on the study of heat given off or absorbed in a chemical reaction.
Temperature - An intensive property of matter; a quantitative measurement of the degree to which an object is either "hot" or "cold".
- There are 3 scales for measuring temperature
- Fahrenheit - relative
- 32F is the normal freezing point temperature of water; 212F is the normal boiling point temperature of water.
- Celsius (centigrade) - relative
- 0C is the normal freezing point temperature of water; 100C is the normal boiling point temperature of water.
- Kelvin - absolute
- 0 K is the temperature at which the volume and pressure of an ideal gas extrapolate to zero.
Conversion Factors for Temperature
- A form of energy associated with the random motion of the elementary particles in matter.
Heat capacity - The amount of heat needed to raise the temperature of a defined amount of a pure substance by one degree.
- Specific heat - The amount of heat needed to raise the temperature of one gram of a substance by 1C (or 1 K)
- SI unit for specific heat is joules per gram-1 Kelvin-1 (J/g-K)
- Calorie - The specific heat of water = 4.184 J/g-K
- Molar heat capacity - The amount of heat required to raise the temperature of one mole of a substance by 1C (or 1 K)
- SI unit for molar heat capacity is joules per mole-1 Kelvin-1 (J/mol-K)
- Btu (British thermal unit) - The amount of heat needed to raise the temperature of 1 lb water by 1F.
NOTE: The specific heat of water (4.184 J/g-K) is very large relative to other substances. The oceans (which cover over 70% of the earth) act as a giant "heat sink," moderating drastic changes in temperature. Our body temperatures are also controlled by water and its high specific heat. Perspiration is a form of evaporative cooling which keeps our body temperatures from getting too high.
Latent Heat versus Sensible Heat
Sensible heat - Heat that can be detected by a change in the temperature of a system.
Latent heat - Heat that cannot be detected because there is no change in temperature of the system.
- e.g. The heat that is used to melt ice or to evaporate water is latent heat.
There are two forms of latent heat:
- Heat of fusion - The heat that must be absorbed to melt a mole of a solid.
- e.g. melting ice to liquid water
- Heat of vaporization - The heat that must be absorbed to boil a mole of a liquid.
- e.g. boiling liquid water to steam
Caloric Theory of Heat
- Served as the basis of thermodynamics.
- Is now known to be obsolete
- Based on the following assumptions
- Heat is a fluid that flows from hot to cold substances.
- Heat has a strong attraction to matter which can hold a lot of heat.
- Heat is conserved.
- Sensible heat causes an increase in the temperature of an object when it flows into the object.
- Latent heat combines with particles in matter (causing substances to melt or boil)
- Heat is weightless.
- The only valid part of the caloric theory is that heat is weightless.
- Heat is NOT a fluid, at it is NOT conserved.
1798 - Sir Benjamin Thompson (Count Rumford)
Kinetic Theory of Heat
- Divides the universe into two parts:
- System - The small portion of the universe in which we are interested.
- Surroundings - Everything not included in the system, i.e. the rest of the universe.
- A BOUNDARY separates the system and the surroundings from each other and can be tangible or imaginary.
- Heat is something that is transferred back and forth across boundary between a system and its surroundings
- Heat is NOT conserved.
- The kinetic theory of heat is based upon the last postulate in the kinetic molecular theory which states that the average kinetic energy of a collection of gas particles is dependent only upon the temperature of the gas. (See Kinetic Molecular Theory notes)
where R is the ideal gas constant (0.0821 L-atm/mol-K) and T is temperature (Kelvin)
|When heat enters a system, it causes
an increase in the speed
at which the particles in the system move.
- Defined as mechanical energy equal to the product of the force (F) applied to an object and the distance (d) that the object is moved:
Heat and Work
- Thompson's canon-boring experiment showed how work (boring the canon) could produce heat.
1838 - James Prescott Joule
Next: "First Law of Thermodynamics"