Thermodynamics

1. Thermodynamics

2. Intensive and extensive properties • Intensive properties: • System properties whose magnitudes are independent of the total amount, instead, they are dependent on the concentration of substances • Extensive properties • Properties whose value depends on the amount of substance present

3. State and Nonstate Functions • Euler’s Criterion • State functions • Pressure • Internal energy • Nonstate functions • Work • Heat

6. Energy • Capacity to do work • Internal energy is the sum of the total various kinetic and potential energy distributions in a system.

16. Heat • The energy transferred between one object and another due to a difference in temperature. • In a molecular viewpoint, heating is: • The transfer of energy that makes use of disorderly molecular motion • Thermal motion • The disorderly motion of molecules

17. Exothermic and endothermic • Exothermic process • A process that releases heat into its surroundings • Endothermic process • A process wherein energy is acquired from its surroundings as heat.

19. Work • Motion against an opposing force. • The product of an intensity factor (pressure, force, etc) and a capacity factor (distance, electrical charge, etc) • In a molecular viewpoint, work is: • The transfer of energy that makes use of organized motion

35. Adiabatic Changes • q=0! • Therefore ΔU = w • In adiabatic changes, we can expect the temperature to change. • Adiabatic changes can be expressed in terms of two steps: the change in volume at constant temperature, followed by a change in temperature at constant volume.

36. Adiabatic changes • The overall change in internal energy of the gas only depends on the second step since internal energy is dependent on the temperature. • ΔUad = wad = nCvΔT for irreversible conditions

37. Adiabatic changes • How to relate P, V, and T? during adiabatic changes? Use the following equations! • ViTic = VfTfc • Where c = Cv/R • PiViγ = PfVfγ • Where γ = Cp/Cv

38. Adiabatic changes • What about for reversible work? • Wad,rev=

42. Exercise • 10 g of N2 is obtained at 17°C under 2 atm. Calculate ΔU, q, and w for the following processes of this gas, assuming it behaves ideally: (5 pts each) • Reversible expansion to 10 L under 2 atm • Adiabatic free expansion • Isothermal, reversible, compression to 2 L • Isobaric, isothermal, irreversible expansion to 0.015 m3 under 2 atm • Isothermal free expansion • (Homework) 2 moles of a certain ideal gas is allowed to expand adiabatically and reversibly to 5 atm pressure from an initial state of 20°C and 15 atm. What will be the final temperature and volume of the gas? What is the change in internal energy during this process? Assume a Cpof 8.58 cal/mole K (10 pts) 1 cal = 4.184 J

47. Enthalpy • As can be seen in the previous derivation, at constant pressure: • ΔH =nCpΔT

48. Relating ΔH and ΔU in a reaction that produces or consumes gas • ΔH = ΔU + pΔV, • When a reaction produces or consumes gas, the change in volume is essentially the volume of gas produced or consumed. • pΔV = ΔngRT, assuming constant temperature during the reaction • Therefore: • ΔH = ΔU + ΔngRT

49. Dependence of Enthalpy on Temperature • The variation of the enthalpy of a substance with temperature can sometimes be ignored under certain conditions or assumptions, such as when the temperature difference is small. • However, most substances in real life have enthalpies that change with the temperature. • When it is necessary to account for this variation, an approximate empirical expression can be utilized

50. Dependence of Enthalpy on Temperature • Where the empirical parameters a, b, and c are independent of temperature and are specific for each substance • Integrate resulting equation for Cp appropriately in order to get ΔH