Lecture 2 Enthalpy, Heat Capacity and Calorimetry

Lecture 2 Enthalpy, Heat Capacity and Calorimetry -...

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Summary from Last Time Internal Energy (E) is transferred as heat ( ) when a system and surroundings are at different temperatures or as work ( ) when an object is moved by a force. Signs reflect the system’s point of view: E < 0 means the system loses energy (as heat or work); E > 0 means the system gains energy (also from heat or work). 1 st law of thermodynamics: energy of the universe is conserved. Pressure volume Work: 1 ∆ࡱ ࢛࢔࢏࢜ࢋ࢙࢘ࢋ =∆ࡱ ࢙࢙࢚࢟ࢋ࢓ + ∆ࡱ ࢙࢛࢘࢘࢕࢛࢔ࢊ࢏࢔ࢍ࢙ = 0 =−ࡼ∆ࢂ ∆ࡱ = +ࢗ
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Example: Energy change, work and heat This compression reaction has E = –186 kJ. (2) What is q ? (1) Is the sign of V positive or negative? 50 L 40 L
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State Functions State function: A function or property whose value depends only on the present state (condition) of the system. Stated another way… the path does not matter Stated another way… the change is zero if the final state is the same as the original
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State Function Example: traveling between Toronto and Montreal: Position is a state function (dest. Montreal, regardless of path) • Jet fuel and ticket prices are not state functions (1) Toronto – Montreal, (2) Toronto – Saskatoon – Montreal
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State Functions We have met (and used) several state functions already: E, V, P The change ( ) in the value of state functions can be calculated by simply subtracting the initial from the final state: In Chemistry and Physics, state functions are represented with CAPITAL LETTERS. CHM139 F2011 Jockusch , Thermochemistry L2 5 ∆ܸ = ܸ ௙௜௡௔௟ −ܸ ௜௡௜௧௜௔௟ ∆ܧ = ܧ ௙௜௡௔௟ −ܧ ௜௡௜௧௜௔௟
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Two different paths for the energy change of a system. Even though q and w for the two paths are different, the total E is the same for both. q
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Lecture 2 Enthalpy, Heat Capacity and Calorimetry -...

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