notes_Lecture_04_100810

notes_Lecture_04_100810 - Lecture 4: Calorimetry & Hess Law...

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Lecture 4: • Reading: Zumdahl 9.4, 9.5 • Outline – Calorimetry (9.4) • Examples of calorimetry calculations – Hess’s Law (9.5) • Motivation for and definition of Hess’s Law • Examples of using Hess’s Law
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What do we know so far? Ideal Monatomic Gas • C V = (3/2)R • C P = C V + R = (5/2)R Polyatomic Gas • C V > (3/2)R • C P > (5/2)R All Ideal Gases Δ E = nC V Δ T = q V Δ H = nC P Δ T = q P Δ E = q + w • w = -P ext Δ V (for now) If Δ T = 0, then Δ E = 0 and q = -w (Summary of what we’ve covered in sections 9.1-9.3) mol K J 31451 . 8 mol K atm L 08206 . 0 R = =
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Heat Capacity Heat Capacity (C) : • the energy required to raise the temp. of a sample of a substance by 1 o C; • the ability of a substance to absorb heat. • C is an extensive property (depends on amount of substance) Recall: thermal energy is associated with the random motions of atoms and molecules Heat capacity gives you a measure of how much more random those motions can become. q C T = ⋅Δ f i T T T Δ = - Units of C: J/ o C Calorimetry (9.4)
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Specific Heat Capacity Specific heat (s) : the energy required to raise the temperature of 1 gram of a substance by 1 o C. You can also have molar heat capacities. Specific heat is an intensive version of heat capacity (like density is an intensive version of mass) (intensive – does NOT depend on amount of substance) If we know the specific heat of a substance, the mass, and the temperature change, we can determine the heat flow. q s m T = ⋅ ⋅Δ { C Units of s: J/ o C·g
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Specific Heat Example • A 25.0-g block of iron is heated to 120.0 o C and placed in 225 mL of water at 30.0 o C. What is the final temperature of the water? (s Fe = 0.45 J/g· o C ; s H2O = 4.184 J/g· o C) T H2O = 30 o C T Fe = 120 o C 2 Fe H O q q - = + q The hot iron block will lose heat energy to the cooler water. Will the iron block lose all of its heat energy? No…it will retain enough so that its avg. atomic KE is the same as the avg. molecular KE of the water . Once the heat transfer is complete, will the iron block have a higher or lower temperature than the water? Neither…it will have the same temperature.
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• A 25.0-g block of iron is heated to 120.0 o C and placed in 225 mL of water at 30.0 o C. What is the final temperature of the water? (s Fe = 0.45 J/g· o C ; s H2O = 4.184 J/g· o C) T H2O = 30 o C T Fe = 120 o C 2 Fe H O q q - = + q 2 2 2 Fe Fe Fe
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This note was uploaded on 09/29/2011 for the course CHM 101 taught by Professor Ma during the Spring '11 term at Beacon FL.

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notes_Lecture_04_100810 - Lecture 4: Calorimetry & Hess Law...

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