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Chem 300 - Ch 19/#2 Today’s To Do List

Chem 300 - Ch 19/#2 Today’s To Do List. Adiabatic Processes & T Enthalpy More on Heat Capacity Heats of Transition. 3 paths to the same end-point (IG). Adiabatic Process. Adiabatic process: q = 0 No heat transfer Example: styrofoam cup. C v and U. Defn: C v = (  U/  T) v

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Chem 300 - Ch 19/#2 Today’s To Do List

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  1. Chem 300 - Ch 19/#2 Today’s To Do List • Adiabatic Processes & T • Enthalpy • More on Heat Capacity • Heats of Transition

  2. 3 paths to the same end-point (IG)

  3. Adiabatic Process • Adiabatic process: q = 0 • No heat transfer • Example: styrofoam cup

  4. Cv and U • Defn: Cv = (U/ T)v • In general: U = f(T, V) • Total differential: • dU = (U/ T)vdT + (U/ V)TdV = CvdT + (U/ V)TdV

  5. Energy & Ideal Gas • For IG, U only depends on T • U = f (T) (prove this later) • Specifically: dU = Cv dT • Cv = heat capacity • U = Cv (Tf - Ti) (if Cv is constant) • For isothermal process, U for IG is constant

  6. T-Change in the Adiabatic expansion of an IG • dU = dq + dw • for adiabatic: dq = 0 • for IG: dU = Cv dT • PV work: dw = - PextdV • CvdT = - PextdV = - RTdV/V • Cv (dT/T) = - R(dV/V) (Integrate) • Cv ln(T2/T1) = R ln(V1/V2) • Adiab. Expansion makes T decrease

  7. Adiabatic Process for different values of Cp

  8. Enthalpy • Definition: H = U + PV • H = f (T,P)

  9. T-dependence of Cpmfor C6H6

  10. Molar Enthalpy of Benzene

  11. Enthalpy Diagrams: (a) exothermic rxn (b) endothermic rxn

  12. Hess’s Law • rH values are additive

  13. Next Time • Heats of reactions from tables • Ht capacity & T-dependence of rH

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