STATE FUNCTIONS and ENTHALPY To play the movies and simulations included, view the presentation in Slide Show Mode.

FIRST LAW OF THERMODYNAMICS heat energy transferred ∆E = q + w energy change work done by the system Energy is conserved!

Chemical Reactivity What drives chemical reactions? How do they occur? The first is answered by THERMODYNAMICS and the second by KINETICS. Have already seen a number of “driving forces” for reactions that are PRODUCT-FAVORED. • formation of a precipitate • gas formation • H2O formation (acid-base reaction) • electron transfer in a battery

Energy transfer allows us to predict reactivity. Chemical Reactivity Energy transfer allows us to predict reactivity. In general, reactions that transfer energy to their surroundings are product-favored. So, let us consider heat transfer in chemical processes.

ENTHALPY: ∆H ∆H = Hfinal - Hinitial Process is ENDOTHERMIC If Hfinal > Hinitial then ∆H is positive Process is ENDOTHERMIC If Hfinal < Hinitial then ∆H is negative Process is EXOTHERMIC

∆H along one path = ∆H along another path This equation is valid because ∆H is a STATE FUNCTION Depend ONLY on the state of the system NOT how it got there. Can NOT measure absolute H, ONLY measure ∆H.

Standard Enthalpy Values Most ∆H values are labeled ∆Ho Measured under standard conditions P = 1 bar Concentration = 1 mol/L T = usually 25 oC with all species in standard states e.g., C = graphite and O2 = gas

Standard Enthalpy Values ∆Hfo = standard molar enthalpy of formation — the enthalpy change when 1 mol of compound is formed from elements under standard conditions. See Table 6.2 and Appendix L

∆Hfo, standard molar enthalpy of formation H2(g) + 1/2 O2(g) --> H2O(g) ∆Hfo (H2O, g)= -241.8 kJ/mol By definition, ∆Hfo = 0 for elements in their standard states.

HESS’S LAW ∆Horxn =  ∆Hfo (products) -  ∆Hfo (reactants) In general, when ALL enthalpies of formation are known, Calculate ∆H of reaction? ∆Horxn =  ∆Hfo (products) -  ∆Hfo (reactants) Remember that ∆ always = final – initial

Using Standard Enthalpy Values Calculate the heat of combustion of methanol, i.e., ∆Horxn for CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g) ∆Horxn =  ∆Hfo (prod) -  ∆Hfo (react)

Using Standard Enthalpy Values CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g) ∆Horxn =  ∆Hfo (prod) -  ∆Hfo (react) ∆Horxn = ∆Hfo (CO2) + 2 ∆Hfo (H2O) - {3/2 ∆Hfo (O2) + ∆Hfo (CH3OH)} = (-393.5 kJ) + 2 (-241.8 kJ) - {0 + (-201.5 kJ)} ∆Horxn = -675.6 kJ per mol of methanol