2. Mr. ShieldsRegents ChemistyU03 L04

3. Heat Let’s now discuss something called HEAT and HEAT CAPACITY Heat is the energy transferred between particles and the surroundings as a result of a temp. difference It ALWAYS FLOWS from Hot to Cold. NEVER from Cold to hot! Ex. You put a cold pack on your leg. Heat flows from your leg to the cold pack. Your leg Loses heat so it “feels” cold. “COLD” DOES NOT FLOW FROM THE ICE PACK 70 deg C30 deg C

4. Heat Energy (Q) Heat energy results from the movement of atoms & molecules in matter. This Energy depends upon how much of the substance is present (an extensive property) Remember the SI Unit for energy is the JOULE (calories are used in Eng. System) 1 calorie = 4.184 Joules and 1 calorie (or 4.184J) can raise the temp of 1g water 1°C

5. Heat Flow Remember …heat always flows from hotter objects to cooler objects. And the Q QQ Quantity of heat transferred depends on: Temperature difference Mass of both substances And the SPECIFIC HEAT of both substances HC Hey! What’s this?

6. Specific Heat Some materials absorb or transfer heat better than other materials. Consider what happens to the temperature of a metal spoon handle vs. a plastic spoon handle immersed in a cup of hot coffee. We use cloth pot holders to pick up hot metal pots. We also use cast iron radiators (not plastic or wood) to heat our homes by running hot water through it.

7. The SPECIFIC HEAT of a substance is the amount of heat required to raise the temperature of 1 g of the substance by 1 degree centigrade. Each substance has it’s own unique specific heat (C p ) Also 1 cal 1 deg (Cp)

8. Specific heats of various Materials MaterialC p Joules/gram deg C Aluminum0.900 Silver0.230 Gold0.129 Graphite0.720 Diamond0.502 Copper0.385 Brass0.377 Iron0.444 Mercury0.139 Water4.184 Ice2.01 Ethanol2.46 Materials with LOW C p are Good heat Conductors Which material Is the best Heat Insulator?   What material would make The best Radiator? Cu, Brass or Iron?

9. Specific Heat Equations HEAT is typically represented by the term “q” Specific Heat is represented by the term “C p ” Recall that SPECIFIC HEAT of a substance is the amount of heat required to raise the temperature of 1 g of the substance by 1 degree C. Therefore …. q = m x Cp x (T f – T i ) m = mass T f = Final Temperature T i = Initial Temperature Change in Temp

10. Specific Heat Calculations Let’s try a problem. How much heat energy in Joules is needed to raise the temperature of 30 grams of water from 20 deg. C to 30 deg. C? Recall that q = m x Cp x (T f – T i ) q = 30 g x 4.18 J/g deg C x (30 deg C – 20 deg C) q = 125.4 x 10 deg C = 1254 Joules

11. Specific Heat Calculations Let’s try another problem. If 1500 joules are added 100 grams of water at 20 deg C what is the final temperature of the water? Heat = mass x specific heat x (final temp – initial temp) q = m x Cp x (T f – T i ) 1500 = 4.18 J/g deg C x 100 g x (final temp – 20 deg C) 1500= 418 x (final temp – 20)final temp -20 = 1500/418 Final temp = 3.59 + 20= 23.59 deg C