Lecture 37 - Topics for the day Administrative stuff Where...

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Topics for the day Administrative stuff Where were we? Standard states Hess’s law Combining formation enthalpies Bond enthalpy Calorimetry Constant pressure Administrative stuff We probably won’t get to the last two questions on your Quest homework until Friday. Luckily it isn’t due till Sunday. ALEKS objective is due on Monday Questions about CH302 ALEKS is available, even if the instructor doesn’t use it for homework I don’t teach CH302. Take Research Methods! Where were we? DeFnitions: Open / Closed / Isolated systems, Potential / Kinetic energy Universe = system + surroundings State functions (e.g., P, V, T, H, E) are properties of a system that are independent of pathway. Thermodynamics looks at the change in state functions: Δ X = X Fnal – X initial Gains to the system are positive and losses from the system are negative. The Frst law of thermodynamics is the conservation of energy law: Δ E system = – Δ E surroundings Heat is the ±ow of thermal energy from a higher temperature to lower temperature. Exothermic / Endothermic The heat capacity of a material is how much energy is needed to raise the temperature of that material. Enthalpy change Δ H is the ±ow of thermal energy under constant P. 1 2 3
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A. Large piece of metal B. Small piece of metal C. It wouldn’t matter because they would be the same temperature. iClicker Time Thought experiment: Aluminum + blow torch. Which of these two pieces of metal would you be more willing to touch? s(Al) = 0.89 J ˚C –1 g –1 Same q, same s. Smaller m means larger Δ T q s m Δ T = Using the units to deduce the equation, s = Δ Tm q q = sm Δ T Standard State The standard state of any chemical is its pure form under the following conditions: Solids: Pure solid Liquid: Pure liquid Gases: 1 bar pressure* Solutions: 1 M at 1 bar pressure Elements: the most stable form at 1 bar pressure and 25˚C The standard value of a property X is denoted with a ˚, like this: X˚ * Most tables of data are still for the old deFnition of standard state (1 atm). The small change in standard pressure makes a negligible difference to most numbers. Unless your application is critical, it is usually safe to use data for 1 atm instead of 1 bar. A. H (g) B. H 2 (g) C. H 2 (l) D. H (s) iClicker Time What is the standard state of hydrogen? Remember you need to know which elements are diatomics in their standard state: H 2 , O 2 , N 2 , Cl 2 , ±l 2 , Br 2 , I 2 Possible mnemonics include: BrINClHO± (brinklehoff) HONCl±IBr (honkleFbber) Have No ±ear Of Ice Cold Beer I Have No Bright Or Clever ±riends Standard state of an element = most stable form at 1 atm and 25˚C 4 5 6
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Recall Δ H depends on the physical state of each reactant and product. The
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This note was uploaded on 08/29/2010 for the course CH 52375 taught by Professor Ruth during the Spring '09 term at University of Texas at Austin.

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Lecture 37 - Topics for the day Administrative stuff Where...

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