The first law of Thermodynamics Conservation of energy 2 3 4 Expansion work 5 Compression work 6 If P ext is not constant 7 Work is the area under P ext vs. V 8 Reversible isothermal compression 9 Reversible isothermal expansion 10 Reversible isothermal expansion 11 Energy is a state function 12 Path function 1 st law: Conservation of energy 13 All state functions are exact differentials 3 different reversible paths 14 Path A: isothermal expansion 15 Path B: Adiabatic expansion 16 Path C: heat at constant V 17 Path B+C 18 Path D+E 19 Compare q, w, and U 20 Compare q, w, and U 21 Adiabatic expansion 22 Adiabatic vs. isothermal expansion 23 w and q from molecular concepts 24 w and q from molecular concepts 25 Pressure from molecular concepts 26 Processes at constant P 27 Enthalpy: a state function 28 Melting ice 29 Boiling water 30 Heat capacity 31 32 Calculate Cp-Cv for a ideal gas How do you expect the difference Cp-Cv to be for solids and liquids? Phase transitions 33 Example: benzene 34 Thermochemistry 35 Examples 36 H is additive 37 Hesss law 38 Standard reaction enthalpy 39 Standard molar enthalpy of formation 40 f H o for elements 41 Using f H o to get r H 42 r H at different T 43 Summary Energy is conserved! U is a state function, w and q are path functions Since U and H are state functions, we can tabulate and calculate thermochemical values Next: Energy is not enough to predict the direction of a spontaneous process 44