PChem Lecture 7

# PChem Lecture 7 - Bond Enthalpies vs Standard Enthalpy of...

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Bond Enthalpies vs. Standard Enthalpy of Formation Compare the change in enthalpy of formation calculated using enthalpies of formation and bond enthalpies for HI(g) ½ H2(g) + ½ I2(s) & HI(g) ΔH=26.5kJ Since formation, you want 1 mole of product H-H I-I H-I + 1/2mol(436kj/mol) + 1/2mol(267kj/mol) - 1/2mol(298kj/mol) = 53.5 kJ H-I is negative because negative when forming bonds and positive when breaking Answers are different

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Click to edit Master subtitle style CHEM 230 W11 Lecture 09 Chapter 7: Entropy 1) Microstates 2) What changes entropy?
What is Entropy? Entropy = “disorder” Entropy of an isolated system increases in spontaneous change (~2nd Law) High entropy High disorder High probability Low entropy High order Low probability Natural progression Picture 1 Picture 12

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Thermodynamic Entropy = Sharing Entropy is a measure of the degree of spreading and sharing of thermal energy within a system This “spreading and sharing ” can be spreading of the thermal energy into a larger volume or its sharing amongst previously inaccessible microstates of the system.
A microstate Imagine that you could take an instantaneous snapshot of the energy of all the individual molecules in a flask. Each molecule's energy is quantized; it is specific energy level. Each accessible energy level could have zero, one, or many molecules on it. The snapshot showing the exact distribution of all of the specific molecules on specific energy levels at one instant in time is called a microstate .

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Energy Quantization Energy is quantized not continuous Recall energy levels of electrons Picture 7
Example: 3 particles in a system with a total energy of 3 Note: Distinguish between particles http://www.chem1.com/acad/webtext/thermeq/TE1.html#SEC3

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## This note was uploaded on 01/31/2011 for the course CHEM 230 taught by Professor Sharp during the Winter '08 term at University of Michigan.

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PChem Lecture 7 - Bond Enthalpies vs Standard Enthalpy of...

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