03 Enthalpy

# 03 Enthalpy - Problem from Last Time 100 g of milk at 10oC...

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1 1 Problem from Last Time 100 g of milk at 10 o C is added to 250 g of coffee at 90 o C. What is the final temperature of the solution? Assume c milk = 3 J/g deg and c coffee = 4 J/g deg Energy is transferred from one object to another, what is the final temperature of both objects. How confident are you that you can solve such problems? A) very confident B) somewhat confident C) not confident D) haven’t a clue 2 QRT 6.38 Asks about the molar heat capacity of diamond . But a diamond is just one big molecule because all the C atoms are bonded together by covalent bonds. So this term refers to the heat capacity of a mole of C atoms bonded together to form diamond. Similarly the molar heat capacity of NaCl(s), which is an ionic solid, is the heat capacity of a mole of Na + ions and a mole of Cl - ions bonded together by ionic bonds.

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2 3 Insight In using the term molar heat capacity for solids, it has to be made clear what the term mole is referring to. Molar heat capacity of solid sulfur. c s x 32 g/mol Molar heat capacity of solid S 8 or O 2 c s x (8 x 32 g/mol) c s x (2 x 16 g/mol) 4 QRT Homework Chapter 6 23, 43, 45, 47, 80
3 5 Energy and Work Work is done when a force, f, moves an object some distance d. w = f d When work is done on an object, energy is transferred to that object. Here Sam is doing work, pushing a rock up a hill. The potential energy of the rock is increasing. Sam is losing energy. The energy Sam got by eating a Big Mac and a soda is being used to do work and is being converted into the potential energy of the rock. 6 Energy and Heat In addition to doing work, Sam is getting hot. He also is losing energy by transferring thermal energy to his surroundings. E = the change in Sam’s energy, Sam is the system E = q + w (Energy is conserved, First Law) This thermal energy being transferred is called heat, q.

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4 7 In-Class Problem
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