CH101 Lec9

# CH101 Lec9 - While you wait Q What is the sign of ΔH o for...

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Unformatted text preview: While you wait: Q: What is the sign of ΔH 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 ΔH 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 ΔE, ΔE surr , and ΔE univ ΔE = q + w = -600 J + 200 J = -400 J ΔE 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 Ε nergy released = -237 kJ Thermodynamics predicts that the reaction is favorable. (negative E) Δ 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 you’re 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 V1→V2: Gas is expanding therefore doing work and releasing heat, so… -q, -w V2→V1: 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 ΔE = 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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## This note was uploaded on 03/29/2008 for the course CH 101 taught by Professor Bigham during the Fall '08 term at N.C. State.

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

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