Understanding Enthalpy as a State Function and its Application in Chemical Reactions, Study notes of Chemistry

Download Understanding Enthalpy as a State Function and its Application in Chemical Reactions and more Study notes Chemistry in PDF only on Docsity! Thursday, Lecture #4 Enthalpy Really IS a State Function: Hess’s Law • Office Hours TAs posted… • Bill Reinhardt OH: Th, Mon 12:30-1:30 in CHB 404B….. • Read Zumdhal, pp 355-371 • How was the QUIZ #1????? • Chem 152 is graded on a curve! • 90% of WebAssign credit, if only error is “sig figs”….so please don’t worry! Review, Energy, Enthalpy • H = E + PV….E,P,V are “state functions, so H is also a state function. • IF only PV work can take place: • ΔE = QV, and • ΔH = QP. • And, as most of practical chemistry takes place at constant P: “H” is an important function to measure, and to tabulate…. “Enthalpy” really is a state function… What does that mean? Enthalpy does not depend on path, or “history.” Enthalpy is a “property” of a chemical system. Just like Volume or Pressure, or Temperature, or Energy. Thus we can add and/or subtract enthalpies, and determine the enthalpies of new chemical reactions. Cou on rte  a! tof . AU Pe a tee eu Hel yy | \ Gve dey 2 Wr tom Aehwuris 3 A,B,C are different chemical “systems.” OR, by ALGEBRA: LT |wela : A: Cl) +3.(5) > Co, ¢s) AW= -TRS 8: Cly +7 2 ls)» co (4) Nu = =lle.c B-A: - + O'9) > c°( J) 09,09) AH = “110.54 33.5 Seemingly odd result may be re-arranged to give: C4 (8) > Co(s)4$ %65) Ah=+793. CY 1096010) (ata) AH=-233.0 Other reactions with O2(gas) • CH4(gas) + 2O2(gas)→ CO2(gas) + 2H2O(gas) • H2(gas) + 1/2 O2(gas) → H2O (gas) • What is the best way to organize this data? • Hess’s Law gets tedious……