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Unformatted text preview: any function X as:
∆X = Xfinal – Xinitial If X increases then ∆X > 0
If X decreases then ∆X < 0. Energy Profiles
Eproducts < Ereactants
reactions absorb heat
Eproducts > Ereactants ENTHALPY, H
• The enthalpy, H, is the heat content at constant
– H = qp
– Commonly employed energy measure for chemical reactions since most reactions
analyzed occur at constant atmospheric pressure. ENTHALPY of REACTIONS, ∆Hrxn
∆Hrxn is the heat of reaction.
If ∆Hrxn < 0 the reaction is exothermic.
If ∆Hrxn > 0 the reaction is endothermic.
∆Hrxn = Hproducts - Hreactants
∆Hrxn = Hsubstances produced - Hsubstances consumed
∆Hrxn = Hfinal state – Hinitial state 4 1/7/2014 Thermochemical Equations and Enthalpy of
C5 H12( l ) + 8 O 2(g) → 5 CO 2(g) + 6 H 2 O ( l ) + 3523 kJ
1 mole 8 moles 5 moles 6 moles • The stoichiometric coefficients in thermochemical equations must be
interpreted as numbers of moles of each component. • 1 mol of C5H12 reacts with 8 mol of O2 to produce 5 mol of CO2, 6 mol of
H2O, and releasing 3523 kJ of heat per mole of reaction. C5 H12(l ) + 8 O 2(g) → 5 CO2(g) + 6 H 2 O (l ) ∆H o = - 3523 kJ/mol
rxn N2(g) + 2O2(g) 2NO2(g) ∆Horxn = 15.6 kcal/mol N2(g) + 2O2(g) + 15.6 kcal 2NO2(g) Standard Molar Enthalpies of
• Standard molar enthalpy of formation - enthalpy for
the reaction in which one mole of a substance is
formed from its constituent elements.
– ∆Hfo kJ/(mole of substance formed) • The standard molar enthalpy of formation fo...
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