Work work done by the by system when a gas expands

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Unformatted text preview: tem may perform Pressure-Volume (P-V) Pressure(PWork. Work Work done by the by system When a gas expands against a expands W>0 external constant external pressure Pex W = F ×d = Pex × A × d = Pex × ∆V W = - Pex ⋅ ∆V No volume change, no Pressure-Volume (P-V) Work Pressure(P- Work done on the on system W <0 Energy Units Units 1. 1. SI Unit for energy is the joule, J: Ek = mv = 1 2 2 1 2 ( 2 kg ) (1 m/s ) 2 = 1 kg m / s = 1 J 2 2 Traditional: calorie (cal): 1 cal = 4.184 J (exactly) cal Nutrition/Food = Calorie (big Cal) 1 Cal (big Cal) = 1000 cal = 1 kcal Question: What is the change in internal energy when a system internal does 135 J work and absorbs 45.0 cal of heat? ∆Esystem = q + w w = -135 J q = + 45.0 cal × 4.184 J/cal = +188 J ∆E = w + q = -135 + 188 = +53 J Enthalpy •H = enthalpy or “Total Heat Content” Enthalpy (H) : used to quantify the heat flow into or out of a used heat into out system in a process that occurs at constant pressure (qp). constant H: a state function. ( Do not need to find H (absolute value) itself, Only difference in H (∆H) is necessary) ∆H = H products - H reactants = q p H products < H reactants ∆H < 0 H products > H reactants ∆H > 0 Thermochemical Equations 1. Enthalpy is an extensive property extensive * Magnitude of H is directly proportional to the amount proportional of substance H2O (s) H2O (l) 2H2O (s) H2O (l) H2O (s) 2H2O (l) ∆H = 6.01 kJ If you reverse a reverse sign reaction, the sign of H changes ∆H = - 6.01 kJ ∆H = 2 x 6.01 = 12.0 kJ Stoichiometric Coefficients CH 4 (g)+2O 2 (g) CO 2 (g)+2H 2 O(g) ∆H = -802 kJ → 2CH 4 (g)+4O 2 (g) 2CO 2 (g)+4H 2 O(g) ∆H = -1604 kJ → Thermochemical Thermochemical Equations 2. Change in enthalpy ( H) depends on physical state: H) physical H2O (l) H2O (g) 2H2O(g) → 2H2O(l) 44.0 ∆H = 44.0 kJ ∆H = ?88 kJ - Question Question: Given the changes of enthalpy of reaction (1) & (2). Calculate how much heat is involved in the 3rd reaction? (1) 2 H 2 ( g ) + O2 ( g ) = 2H 2...
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This document was uploaded on 02/26/2014.

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