110c-lecture3 - Enthalpy (H) vs Energy (U) (pp 779-785) At...

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At constant volume (dV = 0), = = + = Δ 2 1 V V V q PdV q w q U Δ U = q V U measured as constant volume heat (q V ) by calorimetry. What if we measure heat (q p ) at constant pressure (dP = 0)? First, let’s define a new State Function, enthalpy (H): PV U H + = P V V P U H and VdP PdV dU dH Δ + Δ + Δ = Δ + + = At constant pressure (dP = 0): 0 + + = + + = PdV dU VdP PdV dU dH P P P dq PdV PdV q PdV w q PdV dU dH = + = + + = + = δ P P q H = Δ V P U H P Δ + Δ = Δ H measured as constant pressure heat (q P ) Enthalpy useful for describing chemical reactions open to atmosphere: Measure heat (q P ) released or absorbed during reaction at 1 atm. q V = C app (T)* T Bomb Calorimeter Enthalpy (H) vs Energy (U) (pp 779-785)
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Δ r H = change in Enthalpy of reaction. Δ r = standard molar Enthalpy of reaction. Δ c H = change in Enthalpy of combustion. Δ fus H = change in Enthalpy (heat) of fusion (solid liquid). Δ vap H = change in Enthalpy (heat) of vaporization (liquid vapor). Δ f = standard molar Enthalpy of formation (e.g. H 2 (g) + O 2 (g) H 2 O). Enthalpy Notation and Symbols in Textbook
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Relationship of Δ H and Δ U for Ideal Gases Δ H = U + RT n gas The heat of combustion of methane at constant volume (q V ) is measured to be -400 kJ using a bomb calorimeter. Calculate Δ c H at const T and P? CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) Δ c U= ±q V = C app * Δ T = -400 kJ Δ n gas = (# mol gaseous products) – (# mol gas reactants) () RT n U nRT U PV U H gas Δ + Δ Δ + Δ = Δ + Δ = Δ ) ( PV = nRT H = (-400 kJ) + (8.314 J K -1 mol -1 )(298 K)(1 mol gas product – 3 mol gas reactants) Δ c H = -404,955 J vs. Δ c U = -400,000 J Numerical difference between Δ H and Δ U is usually small for most reactions.
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Heat Capacity (pp. 783-785) DEFN: Heat Capacity —the amount of heat required to raise the temperature of a Substance by one Kelvin. Heat Capacity also sometimes called Specific Heat. Heat capacity = C(T) = T q δ is a path function that depends on how the sample is heated. Constant pressure ( Δ H = q P ), P P dq PdV PdV q PdV w q PdV dU dH = + = + + = + = P P dq dH = ) ( T C T q T H P P P = = T q T H T H C P P P Δ = Δ Δ = (valid only for small Δ T) Constant volume ( Δ U = q V ), V V dq dU = V V V C T q T U = = (T) T q T U T U C V V V Δ = Δ Δ = (for small Δ T)
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Electrical power difference supplied to sample vs. reference measures the heat flow (dq/dt) in the sample. dq/dT is calculated and plotted vs. Temp.
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This note was uploaded on 06/22/2010 for the course CHEM 21360 taught by Professor Ame during the Spring '09 term at East Los Angeles College.

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110c-lecture3 - Enthalpy (H) vs Energy (U) (pp 779-785) At...

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