CH101 Lec9

CH101 Lec9 - While you wait: Q: What is the sign of H o for...

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Unformatted text preview: While you wait: Q: What is the sign of H o for the evaporation of water? A: positive; heat is needed to break the IM forces in the liquid to get gas Q: What is the sign of H o for the cooling of liquid water? A: negative; heat must be given off by water in order for it to cool down Q: A system gives off 600 J of heat while 200 J of work are done on the system. What are E, E surr , and E univ E = q + w = -600 J + 200 J = -400 J E surr = 600 J + (-200 J) = 400 J univ surr While you wait... a) Calculate H from bond dissociation energies for the following reaction. CCl 4 + O 2 CO 2 + 2Cl 2 H = -276 kJ/mol b) What is the sign of G at low and high temperature? G is negative at low temperature and positive at high temperature. Section 9.1 CHAPTER 9: Reaction Energetics Thermodynamics- Kinetics- the study of energy and its transformations ( energy) considers the difference in energy of reactants and products only (does not consider the path) can predict if a reaction is favorable but not how much time it will take to occur the study of the rates and mechanisms of chemical reactions deals with the path of the reaction and how fast or slow can predict if a reaction will occur in a reasonable amount of time Thermodynamics + Kinetics = Energetics H 2 + 1/2 O 2 H 2 O nergy released = -237 kJ Thermodynamics predicts that the reaction is favorable. (negative E) However, no reaction is observed when H 2 and O 2 are mixed. THERMODYNAMICS Once a small amount of energy is added (spark) the reaction proceeds! KINETICS Thermodynamics Universe = system + surroundings what youre studying everything else E universe = E system + E surrondings = 0 E sys = E sur If the system gives off heat, the surroundings gain heat Heat and work are the most common ways to exchange energy between the system and its surroundings Energy system = heat (q) and work (w) E = q + w The sign on q and w indicates the flow of energy from system to surroundings. q > 0 (pos.) heat absorbed by the system- endothermic q < 0 (neg.) heat released by the system- exothermic w > 0 (pos.) work done on the system w < 0 (neg.) work done by the system Systems at Work volume 1 volume 2 1 atm System: hot expanding gas Surroundings: piston V1V2: Gas is expanding therefore doing work and releasing heat, so -q, -w V2V1: Piston is doing work on the system (compressing gas), so w is positive Work requires a change in pressure The heat transferred in a chemical reaction (constant P) is the enthalpy or heat of reaction, H. At constant pressure (i.e., reactions in a lab) there is no change in volume and work = 0 E = q + w E = q (at constant P) q = H (Enthalpy) Enthalpy: Heat of reaction H 2 + 1/2 O 2 H 2 O Heat of reaction ( H) = -237 kJ Energy is released, reaction is exothermic 6CO 2 (g) + 6H 2 O(l) C 6 H 12 O 6 (s) + 6O 2 (g) Heat of reaction ( H) = +2816 kJ Energy is consumed, reaction is endothermic R...
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CH101 Lec9 - While you wait: Q: What is the sign of H o for...

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