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06 Entropy

06 Entropy - QRT Homework Chapter 18 19 22 25 31 35 39 4...

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1 4 QRT Homework Chapter 18 19, 22, 25, 31, 35, 39 5 Where does chemical energy come from? The energy of the products is less than the energy of the reactants, so energy is transferred to the surroundings. Differences in covalent (bonding) and noncovalent (intermolecular) interactions in going from reactants to products. Often the latter can be neglected, and differences in bond energies can be used alone to estimate H reaction .

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2 6 Example ½O 2 (g) + ½ N 2 (g) NO(g) H = ? Bond energies = energy required to break the bond O 2 498 kJ/mol N 2 946 NO 632 Energy needed to break O-O and N-N bonds – energy released when NO bond forms = reaction enthalpy ½(498) + ½(946) – 632 = 90 kJ/mol NO Works well for diatomics, works ok for polyatomics, may not work so well in solids and liquids where noncovalent interactions may be important. 7 Spontaneous Processes Every process has a preferred direction, which is called the spontaneous direction. A spontaneous process occurs on its own accord unless restrained in some way. Spontaneous does not mean fast. Examples of spontaneous processes. Water runs down hill. Unless restrained by a dam. A pump is needed to move water up a hill. Snow melts on a warm day. Leaves turn brown in the Fall.
3 8 Some chemical reactions are spontaneous, others are not! Baking powder + vinegar carbon dioxide + water Spontaneous, try this at home! Hydrogen + oxygen water Explodes spontaneously if started by tiny spark Don’t try this at home! Nitrogen + oxygen nitrogen oxides Won’t explode spontaneously, even if started by a bolt of lightening! 9 Why are some reactions spontaneous and others are not? Maybe only exothermic reactions are spontaneous? Energy is transferred out to the system so the energy of the system goes down. Just like a car rolls down a hill spontaneously because the potential energy decreases.

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4 10 Hot Packs and Cold Packs Hot packs use an exothermic chemical reaction to to spontaneously produce energy (heat).
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06 Entropy - QRT Homework Chapter 18 19 22 25 31 35 39 4...

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