1. by HANN ILYANI ZULHAIMI hanna@unimap.edu.my ERT 108 PHYSICAL CHEMISTRY THE FIRST LAW OF THERMODYNAMICS

2.  The First Law of Thermodynamics  Enthalpy  Heat Capacities  The Joule and Joule – Thomson Experiments  Perfect gases and The First Law  Calculation of First Law Quantities OUTLINE

3.  Eventually the rock and ball come to rest.  What happened to its energy of motion?? THE FIRST LAW OF THERMODYNAMICS

4.  Total energy of a body: THE FIRST LAW OF THERMODYNAMICS

5. The internal energy of an isolated system is constant Change in internal energy of the system Heat flow into the system Work done on the system The first law of thermodynamics relates ∆U to heat (q) and work (w) that flows across the boundary between the system & surroundings. (closed system)

6. THE FIRST LAW OF THERMODYNAMICS Heat flow into the system from surroundings during a process Outflow of heat from the system to the surroundings Work done on the system by the surroundings Work done by the system on its surroundings

7.  What is internal energy?? THE FIRST LAW OF THERMODYNAMICS

8.  Internal energy is the energy that take on number of forms such as:  The translational the molecules  The molecular vibrations & rotations  The internal energy stored in the form of chemical bonds that can be released through a chemical reaction  The potential energy of interaction between molecules. THE FIRST LAW OF THERMODYNAMICS

9.  The internal energy, U is a state function.  For any process, ∆U depends only on the final and initial states of the system and independent of the path used to bring the system from initial state to the final state.  If the system goes from state 1 to state to by any process, then: THE FIRST LAW OF THERMODYNAMICS

10.  Cyclic process:  A process in which the final state of the system is the same as the initial state : U 2 = U 1 Thus, ∆ U = 0 THE FIRST LAW OF THERMODYNAMICS

11.  If an electric motor produced 15kJ of energy each second as mechanical work and lost 2kJ as heat to the surroundings, then the change in the internal energy of the motor each second is EXAMPLE

12.  If an electric motor produced 15kJ of energy each second as mechanical work and lost 2kJ as heat to the surroundings, then the change in the internal energy of the motor each second is EXAMPLE

13.  Suppose that, when a spring was wound, 100 J of work was done on it but 15 J escaped to the surroundings as heat. The change in internal energy of the spring is: EXAMPLE

14.  Suppose that, when a spring was wound, 100 J of work was done on it but 15 J escaped to the surroundings as heat. The change in internal energy of the spring is: EXAMPLE

15.  Enthalpy can be defined as:  Enthalpy is a state function (values depend on current state of the system, not on HOW the system acquired that state, which is independent of path.) ENTHALPY Internal energy Pressure Volume

16.  Energy transferred as heat at constant pressure is equal to the change in enthalpy of the system. HOW? ENTHALPY

17.  Rearranging,  For an infinitesimal change, ENTHALPY For a measurable change

18.  Consider a constant-volume process:  For an infinitesimal change, ENTHALPY Constant volume

19.  For a constant-pressure process;  For a constant-volume process; HEAT CAPACITIES Recall: -Is a measure of energy needed to change the temperature of a substance by a given amount. - Extensive property (doubles as the mass of the system doubled)

20.  At constant P,  At constant V, MOLAR HEAT CAPACITIES

21. Molar heat capacities for a number of gases.

22. RELATION BETWEEN C P & C V Recall: Can be related

23. RELATION BETWEEN C P & C V U is a function of T and V Thus; At constant P;

24. RELATION BETWEEN C P & C V Dividing by dT P ;

25. JOULE EXPERIMENT 1. A sample of a gas (the system) was placed in one side of the apparatus and the other side of the apparatus was evacuated. 2. The initial temperature of the apparatus was measured. 3. The stopcock was then opened and the gas expanded irreversibly into the vacuum. To determine

26. JOULE EXPERIMENT 1. Because the surroundings were not affected during the expansion into a vacuum, w was equal to zero. 2. The gas expanded rapidly so there was little opportunity for heat to be transferred to or from the surroundings. 3. If a change in temperature of the gas occurred, heat would be transferred to or from the surroundings after the expansion was complete, and the final temperature of the surroundings would differ from the initial temperature.

27. JOULE EXPERIMENT Joule coefficient, Joule experiment gave and because the changes in temperature that occurred were too small to be measured by the thermometer.

28. JOULE-THOMSON EXPERIMENT Insulated wall To determine

29. JOULE-THOMSON EXPERIMENT Adiabatic process Rearranging; Isenthalpic process To determine

30. JOULE-THOMSON EXPERIMENT  Joule-Thomson coefficient, To determine

31. PERFECT GASES  Perfect gas: one that obeys both of the following: U is not change with V at constant T If we change the volume of an ideal gas (at constant T), we change the distance between the molecules. Since intermolecular forces are zero, this distance change will not affect the internal energy.

32.  For perfect gas, internal energy can be expressed as a function of temperature (depends only on T):  An infinesimal change of internal energy, PERFECT GASES From (slide 19)

33. PERFECT GASES  For perfect gas, enthalpy depends only on T;  Thus;  An infinesimal change of enthalpy, This shows that H depends only on T for a perfect gas From (slide19)

34. PERFECT GASES  The relation of C P and C V for perfect gas; From slide 24 Perfect gas PV=nRT, thus or

35. PERFECT GASES  For perfect gas, - Since U & H depend only on T.

36. PERFECT GAS AND FIRST LAW First law; For perfect gas; Perfect gas, rev. process, P-V work only

37. REVERSIBLE ISOTHERMAL PROCESS IN A PERFECT GAS  First law:  Since the process is isothermal, ∆T=0;  Thus, ∆U = 0  First law becomes; 0

38. REVERSIBLE ISOTHERMAL PROCESS IN A PERFECT GAS  Since, thus:  Work done; OR rev. isothermal process, perfect gas

39. Since REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS  First law,  Since the process is adiabatic,  For perfect gas, becomes,

40. REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS  Since, thus  By integration;

41. REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS  If is constant, becomes; OR rev. adiabatic process, perfect gas

42.  Alternative equation can be written instead of  Consider REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS

43. Since REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS where rev. adiabatic process, perfect gas, C V constant

44. REVERSIBLE ADIABATIC PROCESS IN PERFECT GAS  If is constant, becomes rev. adiabatic process, perfect gas, C V constant

45. EXERCISE  A cylinder fitted with a frictionless piston contains 3.00 mol of He gas at P = 1.00 atm and is in a large constant- temperature bath at 400 K. The pressure is reversibly increased to 5.00 atm. Find w, q and ∆ U for the process.

48. 1. Reversible phase change at constant P & T.  Phase change: a process in which at least one new phase appear in a system without the occurance of a chemical reaction (eg: melting of ice to liquid water, freezing of ice from an aqueous solution.) CALCULATION OF FIRST LAW QUANTITIES

49. 2. Constant pressure heating with no phase change

50. 3. Constant volume heating with no phase change Recall: CALCULATION OF FIRST LAW QUANTITIES

51. 4. Perfect gas change of state: H & U of a perfect gas depends on T only; CALCULATION OF FIRST LAW QUANTITIES

52. 5. Reversible isothermal process in perfect gas H & U of a perfect gas depends on T only; Since, thus CALCULATION OF FIRST LAW QUANTITIES

53. 6. Reversible adiabatic process in perfect gas If C v is constant, the final state of the gas can be found from: where CALCULATION OF FIRST LAW QUANTITIES Adiabatic process

54. EXAMPLE C P,m of a certain substance in the temperature range 250 to 500 K at 1 bar pressure is given by C P,m = b+kT, where b and k are certain known constants. If n moles of this substance is heated from T 1 to T 2 at 1 bar (where T 1 & T 2 are in the range of 250 to 500 K), find the expression for ΔH.