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Unformatted text preview: L ECTURE 21. T HERMODYNAMICS L ET S G ET Q UANTITATIVE Today we will examine the quantitative side of thermodynamics, while actually coming up with numerical values for H, S, etc. Up to this point we have been more concerned with the signs of thermo data and what they mean. These are summarized below and should be as much a part of you as your own name: (-) Exothermic, Heat Released, Hot H (+) Endothermic, Heat Absorbed, Cold (-) Spontaneous G (+) Non Spontaneous (+) Disordered S (-) Ordered (-) A Bomb, Work Done on Surroundings w (+) Arming a Bomb Work Done on System But now we will be adding numbers: C 3 H 8 + 5O 2 3C0 2 + 4H 2 O Is NOT just G = (-) It IS G = -2070kJ which means it is really spontaneous. (Still, while we learn to do these calculations, dont forget the importance of simply looking at a reaction and being able to assign the signs! It is knowing the signs that proves you understand thermo-- anyone can put numbers in a calculator but since half the questions I put on tests dont require a calculator..) How to complete the Thermodynamic Reaction Worksheet Predict the values for H, n gas, w, S, and G. At the least provide a sign, but if you can offer a guess of the magnitude without using a calculator, that is even better. First the signs. Can you explain how you know them? Reaction H n gas w S G CH 4(g) + O 2(g) CO 2(g) + 2H 2 O (g) (-) 0 0 (-) (-) 2H 2(g) + O 2(g) 2H 2 O (g) (-) (-) + (-) (-) And now the calculations....
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- Spring '07