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Chemistry (181)
CHM 2045 (98)
chu (15)
Lecture

# 4____

3 Pages
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Department
Chemistry
Course Code
CHM 2045
Professor
chu

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19  o 5. Gibbs o 6. Specific Heat o 7. Internal Energy o 8. Energy  2. Intensive • A. Definition: independent of the size of the system • B. If you combined two identical systems, an intensive property would not change • C. Examples o 1. Pressure, P o 2. Temperature, T o 3. Reduction Potentials, E° o 4. Molar heat capacity o 5. Density • Path Functions o A. Definition: Properties that do not describe the state of a system and are dependent upon the pathway used to achieve (think nonconservative forces) o B. Examples of Path Function:  1. Work  2. Heat • Heat: Energy Transfer o 1. Only two ways to transfer energy between systems  1. Heat, q: the natural transfer of energy from warmer to cooler body; types: conduction, convection and radiation  2. Work, w: any energy transfer that is not heat • Heat: Conduction o A. Definition: thermal energy transfer via molecular collisions, i.e. higher energy molecules collide with lower energy molecules o B. Requires: direct physical contact o C. Thermal Conductivity, k: object’s ability to conduct heat: Q/t = kA[hT –cT )/L]  1. A = face area  2. L = length  3. k = thermal conductivity  4. Q = heat  5. Th= hot body  6. Tc= cold body  7. t = time  8. ΔT = change in temperature o E. Resistance to heat flow, R: R = L/kA o D. Rate of Heat Flow, or Heat current, I: I = Q/t o F. Heat Flow: ΔT = IR  1. Think: Ohm’s Law, rate of ideal fluid flow, electric current flow through resistors in series  2. In a steady state system, rate of heat flow is constant across any number of slabs between two heat reservoirs  3. That is, if a series of slabs of different lengths, thicknesses and thermal conductivities, or even order of slabs, were lined up between hot and could sources, the rate, I, would be the same.  4. Why? Got to be. If one was too cool, it would not continue transferring heat to warmer bodies  5. Note: Higher conductivity results in lower temperature difference between any slab of a given length • Heat: Convection o A. Definition: thermal energy transfer via fluid movements o B. Differences in pressure or density drive warm fluid in the direction of cooler fluid, like hot air above the beach rising faster than cooler air above the ocean   19    20  • Heat: Radiation o A. Definition: thermal energy transfer via electromagnetic waves; it is the only type of heat that transfers through a vacuum o B. When metal is hot, it goes red  yellow  white  blue…it’s radiating visible electromagnetic waves o C. All objects above 0K radiate heat 4 o D. Power (rate of electromagnetic radiation), P = σεAT  1. A = surface area  2. T = temperature in K  3. ε = emissivity of the object’s surface, value of 0 to 1  4. σ = Stefan-Boltzmann constant: 5.67 x 10 W/m K2 4 • a. Black body radiator: emissivity of 1; only in theory • b. Dark colors: radiate and absorb more, reflect less • c. Light colors: reflect more, radiate and absorb less o E. Emissivity example: better to paint your house white: reflects more heat in the summer (your house is cooler than the environment), radiates less heat in winter (house is warmer than environment) o
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