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Thermodynamics

Thermodynamics. Thermodynamics is the study of the transformation of energy into heat and work Energy can be used to provide heat for mechanical work to produce electric work to sustain life. CH 4 (g) + 2 O 2 (g) --> CO 2 (g) + 2 H 2 O(g) + HEAT.

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Thermodynamics

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  1. Thermodynamics Thermodynamics is the study of the transformation of energy into heat and work Energy can be used • to provide heat • for mechanical work • to produce electric work • to sustain life

  2. CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(g) + HEAT The result of breaking C-H and O-O bonds and formation of C-O and H-O bonds is heat

  3. Why is propane (C3H8) a better fuel than benzene (C6H6)? What compounds are feasible as alternate fuels? Why are carbohydrates, proteins and fats fuels for our bodies? Why is ATP (adenosine triphosphate) an “energy-rich” molecule?

  4. Terminology - System, Surroundings & State Functions System: object under observation. In a chemical reaction, the reaction mixture is the system. Surrounding - everything else not considered to be the system.

  5. Transfer between system and surrounding System Energy Matter Yes Yes Open Yes No No No Closed Isolated

  6. State: macroscopic, measurable properties, like composition, volume, pressure, temperature, which define a system. Process - the path the system undergoes to change from an initial state to a final state.

  7. Energy Energy: capacity to do work Units of energy 1 Joule (J) = 1 kg m2 s-2 Objects can posses energy either as kinetic energy (KE) or potential energy (PE). Kinetic energy: energy of motion EK = 1/2 mv2 Potential energy: energy a body possesses on account of its position in a field of force.

  8. PE depends on the nature of the force being felt by the body. for example, electrostatic force, gravitational force EP = m g h (for a body experiencing a gravitational force) Total Internal Energy (E) = EK + EP

  9. Energy, Work and Heat Energy is the capacity to do work or transfer heat. Work : Energy used to move an object. Heat: Energy transferred from a hotter body to a colder body.

  10. Work Work : movement against an opposing force Work is done when a weight is raised against gravity. The reaction in a battery does work when it pushes electrons through a circuit. The hot gas mixture in the cylinder of an automobile engine does work when it pushes back the piston. Work (w) = force x distance Units: 1 J = 1 kg m2 s-2

  11. The total capacity of a system to do work is its INTERNAL ENERGY, E. Ewound spring > Eunwound spring Ecompressed gas > Eexpanded gas Echarged battery > Edischarged battery CHANGE in internal energy DE = Efinal - Einitial When the transfer of energy is only as work, DE = w When work is done by the system, w < 0 When work is done on the systen, w > 0

  12. Heat Heat (q) is energy transferred as a result of a temperature difference. Energy flows as heat from a high temperature region to a low temperature region. Units of Heat: Joule (J) 1 calorie (cal) = 4.184 J (nutritional calorie (Cal) = 1000 cal = 4184 J or 4.184 kJ)

  13. Heat is the transfer of energy as a result of a temperature difference. When the only transfer of energy is as heat, DE = q If energy enters the system as heat, q > 0 If energy leaves the system as heat, q < 0

  14. Heat, and hence changes in internal energy, accompany almost all chemical reactions. When a chemical reaction occurs by absorbing heat from its surroundings, the reaction is said to be endothermic. When a chemical reaction is accompanied by the release of heat, the reaction is said to be exothermic.

  15. Closed System (exchange of energy but not matter) A closed system with thermally insulating walls - ADIABATIC No flow of energy as heat into or out of the system. However, energy may be transferred into or out of the system as work. A closed system which permits transfer of energy as heat - NONADIABATIC.

  16. The First Law of Thermodynamics 1) Energy is conserved 2) Heat and work can produce equivalent effects 3) The only way that energy can be transferred is through heat and work. First Law DE = q + w where DE is the change in internal energy of a system that is not isolated q is the heat involved during the process w work done during the process Note: the internal energy of an ISOLATED system is constant Animation

  17. Sign convention If heat flows into the system q > 0 Heat flows out of the system q < 0 Work done by the system w < 0 Work done on the system w > 0

  18. DE is a state function; the change in DE depends only on the initial and final energies of the system and not on the details of the process. q and w are PATH FUNCTIONS.

  19. A gas at 25oC expanding at constant temperature (isothermal) by 100 cm3 by two different paths between the same initial and final states are the same w > 0 w = 0

  20. Mechanical Work mechanical work = force x distance If the direction of the applied force is in the same direction as the displacement, work is done on the body; w > 0 If the direction of the applied force is opposite to the direction of the motion of the body, work is done against the body ; w < 0

  21. 1 w = DKE = D ( m v2) 2 The work done in stopping a moving object, or pushing an object causes a change in the kinetic energy of the system The work done in raising an object from one height to another changes the potential energy of the object w = DPE = m g Dh

  22. Pressure-Volume Work Work done in expanding a gas against an external pressure, or to compress a gas causes a change in volume. w = - PextDV (constant P) where DV = Vfinal - Vinitial If the gas expands DV > 0 => work done by the gas (pushes against surrounding); work < 0 If the gas is compressed DV < 0 => work done on the gas (by the surrounding); work > 0

  23. w = Force x distance F = Pressure/Area w = d Pext A DV = d A w = - PextDV

  24. w = - PextDV = - (5.00 atm) (20.00 L - 10.00 L) = - 50.0 L-atm or - 5.06 kJ DE = q + w = 2.00 kJ + (-5.06 kJ) = -3.06 kJ Problem: A gas expands against a constant pressure of 5.00 atm, from 10.00 to 20.00 L, absorbing 2.00kJ of heat. Calculate the work done and the change in the internal energy of the gas. 1 L-atm = 101.3 J

  25. For a process which takes place at constant volume w = - PextDV = 0; if volume is constant DE = q + w For constant volume processes, no work can be done DE = qv where qv is the heat exchanged at constant volume If the external pressure is 0 (a vacuum), w = 0 A system does no expansion work when it expands into vacuum (free expansion).

  26. Calorimetry Calorimetry is the measurement of the amount of heat flow and change in temperature accompanying a process. Transfer of energy as heat accompanying a process can be measured in a calorimeter.

  27. q Heat capacity = CP = DT Heat capacity (at constant pressure), CP, of a body, is the amount of heat required to raise the temperature of the body by one degree at constant pressure. q is the amount of heat absorbed by the body DT is the rise in temperature Units of CP: joule/K or cal/K

  28. CP n Molar heat capacity = cP = CP specific heat capacity = cs = m Units of cP joule/(mole K) q = n cPDT Specific heat capacity, cs: the amount of heat absorbed per unit mass of body Molar heat capacity, cP: amount of heat absorbed per mole of sample Units of cs joule/(g K) q = m csDT

  29. If two objects at different temperatures are in contact with one another, heat flows from the hotter body to the colder body in an attempt for the system to reach an equilibrium. Assuming that there is no heat lost to the surroundings, then: heat lost by hotter object = heat gained by cooler object n1cP1DT1 = n2cP2DT2 n1cP1 (Tf1 - Ti1) = n2cP2 (Tf2 - Ti2)

  30. Any energy that leaves the system as heat, must be absorbed by the surrounding; energy absorbed by the system as heat is from the surrounding. qsys = - qsurr In calorimetry, the surroundings are the entire calorimeter assembly. qsys = - qcal qcal = CcalDT Ccal: heat capacity of calorimeter

  31. When methylhydrazine, CH6N2, a rocket fuel, undergoes combustion the following reaction occurs: CH6N2(l) + 5 O2(g) --> 2N2(g) + 2CO2(g) + 6H2O(g) When 4.00 g of CH6N2(l) is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00oC to 39.50oC. In a separate experiment, the heat capacity of the calorimeter is measured to be 7.794 kJ/oC. What is the heat of reaction for the combustion of a mole of CH6N2(l)? qrxn = - CcalorimeterDT = - (7.794 kJ/oC) (14.50oC) = -113.0 kJ Heat of reaction per mole of CH6N2(l) (-113.0 kJ)/(0.0868 mol) = - 1.30 x 103 kJ/mol

  32. Calorimetry of Foods Most of the energy our bodies need comes from the metabolism of carbohydrates, fats and proteins. Carbohydrates decompose into glucose, C6H12O6. Metabolism of glucose produces CO2 and H2O and energy C6H12O6(s) + 6O2 (g) --> 6CO2 (g) + 6H2O(l) + 2803 kJ

  33. The combustion of tristearin C57H110O6, a typical fat: 2C57H110O6 + 163O2 (g) --> 114 CO2(g) + 110 H2O(l) + 75,520 kJ On average, the metabolism of proteins produces ~ 4 Cal/g carbohydrates produces ~ 4Cal/g fats produces about ~ 9 Cal/g (1 Cal = 1000 cal; I Cal = 4.184 kJ)

  34. Calculate Calories from amount of carbohydrate, protein and fats (45g carb x 4 Cal/g carb) + (9g protein x 4 Cal/g protein) + (3g fat x 9 Cal/g fat) = 243 Cal

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