C1210 lecture 14

C1210 lecture 14 - Break a bond: pull objects apart that...

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Break a bond: pull objects apart that attract each other— increase P.E. Form a bond: objects that attract each other move closer together— decrease P.E. Break weak bonds, form strong bonds: new molecules are more stable than old ones (new molecules have lower potential energy than old ones). Energy is released from the reaction, releasing heat ( cf . combustion, a flame is hot). Frequently, when reactions occur, bonds are broken and other bonds are formed, so that the total P.E. after the reaction is different from that before the reaction occurs: Burn CH 4 CH 4 ( g ) + 2O 2 ( g ) CO 2 ( g ) + 2H 2 O + heat Exothermic (exoergic) Plants convert CO 2 to O 2 and glucose by photosynthesis CO 2 ( g ) + 6H 2 O + solar energy C 6 H 12 O 2 + 3O 2 ( g ) Endothermic (endoergic) l chlorophyl steps many ⎯→ End 6.4
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The heat absorbed or released during a reaction is called the Heat of Reaction . • Heats of reaction are measured in an insulated vessel called a calorimeter. • These measurements are collectively called calorimetry . • Heats of reaction are measured either at constant volume or constant pressure . area force pressure = 1 atmosphere = 14.7 lb in -2 or 1.0133 bar or 760 torr or 101,325 pascals. We have to specify whether we measured heat, q at constant pressure or constant volume, we use the notation q p and q v .
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Why the difference? When gases (or anything else, for that matter) expand or contract, work is done on or by the system, by or on the surroundings. Therefore, we have to account for this work in the heat of reaction. The difference between q p and q v is largest for gas-phase reactions where expansion or pressure-volume work is largest. Example: we measure q for a gas-phase reaction in a vessel immersed in a precise amount of water, all in an insulated bucket. The vessel has a piston that can be locked in position. We measure initial and final T of the water to measure q v with the piston locked so that the volume of the vessel is fixed. We do it again to measure q p with the piston free so that the pressure in the vessel is constant. The heat capacity of the calorimeter is 8.101 kJ/°C: Runpiston T initial (°C) T final (°C) 1 a (locked) 24.00 28.91 2 b (free) 27.32 31.54
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The temperature of the water increases, so the reaction is exothermic.
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This note was uploaded on 12/10/2009 for the course CHE 1220 taught by Professor Jespersen during the Spring '09 term at St. John's.

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C1210 lecture 14 - Break a bond: pull objects apart that...

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