Chapter 14 Thermochemistry - PowerPoint PPT Presentation

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Chapter 14 Thermochemistry

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  1. Chapter 14 Thermochemistry

  2. OBJECTIVES: students will be able to understand… Concept of energy and its various forms Relationship btwn energy, work, and heat The key features of a state function How to use Hess’s law & thermochemical cycles to calculate enthalpy D’s of chem rxtns How to use calorimetric data to calculate enthalpy D’s

  3. Thermochemistry Energy and Work: Describing energy D’s during a rxtn. Energy D’s are often of prime importance:

  4. Thermite rxtn Gives off heat even under water

  5. Chemical Hand Warmers Most hand warmers  heat released from slow oxidation of Fe 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)

  6. Energy – Takes many forms – Some can be seen or felt – Defined by its effect on matter – How many can you name?

  7. Forms of energy

  8. Chemical: particular arrangement of atoms in a chem compound. Heat & light produced in this rxtn due to E released during

  9. Isopropyl alcohol and oxygen Chemical Energy: C3H7OH(g) + 3O2(g) 2CO2(g) + 3H2O(g) + E C3H7OH(g) E (kJ/mol) CO2(g)

  10. Nature of Energy Chemistry studies matter energy affects matter Energy = anything w/ capacity to do work Work= ___________ x ______________ Heat = flow of E  difference in temp E exchanged btwn objects thru contact = collisions

  11. Energy, Heat, and Work Think of energy as a quantity an object(s) can possess Think of heat & work as 2 diff ways that an object can exchange E with other objects:

  12. Classification of Energy • Kinetic energy (KE) = • E of motion or • E being transferred • Thermal energy = E associated with temp • thermal E = a form of KE

  13. Classification of Energy Potential energy (PE) = stored E in an object, or energy associated with

  14. Manifestations of Energy

  15. Surroundings Surroundings System System System and Surroundings • system material or process being studied • Surroundings:everything else • system can exchange energy w/ surroundings

  16. System + Surroundings =Universe forms of E that flow: qsys = and wsys = Work can be expressed as: w = “PV work” is either expansion or compression

  17. When is q positive? When is it negative? When is w positive? When is it negative?

  18. q is pos when heat flows q isneg when heat flows w is pos when work is done w is neg when work is done __

  19. Comparing Amount of E in System & Surroundings During Transfer Conservation of E: amount of E gained or lost by system must = amount of E lost or gained by surroundings

  20. Law of Conservation of Energy E cannot be created nor destroyed in a chem rxtn When E is transferred btwn objects, or converted from one form to another, total amount of

  21. Units of Energy • KE of an object directly a its mass & velocity KE = ½mv2 When mass is kg & velocity is m/s, then unit for

  22. Units of Energy 1 Joule of E = Energy needed to move 1 kg mass at speed = to 1 m/s 1J=

  23. Units of Energy • joule (J) = amount of E needed to move a 1-kg mass a distance of 1 meter • 1 J =

  24. Units of Energy • calorie (cal) = energy needed to raise temp of one gram of water 1°C • kcal = • food Calories =

  25. Units of Energy

  26. The First Law of ThermodynamicsLaw of Conservation of Energy Thermodynamics: study of E & its _______________________ Total amount of E in universe is constant No system can be designed that will continue to produce E without some source of E

  27. Energy Flow and Conservation of Energy • sum of ED’s in system & surroundings must be • DE universe = DE system + DE surroundings • DE universe =

  28. Internal Energy • internal energy= sum of KE & PE of all particles that compose the system • D in internal Esystem only depends on amount of E in system at beginning & end

  29. Internal Energyis a state function: • depends only on initial & final conditions, • noton the process used; • E = Efinal – Einitial • DErxtn = Eproducts − Ereactants

  30. State Functions: a thermodynamic property that depends only on the state of the system, noton the pathway used to get there the opposite is an energy transfer function

  31. Either of 2 trails to reach top of the mountain: 1st = long & winding, 2nd = short but steep. State Function

  32. Regardless of which trail you take, when you reach the top you will be 10,000 ft above base. State Function Dist from base to peak is a state function. Depends only on diff in elevation btwn base & peak, not on how you arrive there!

  33. A lake’s water level is a state function: a property of the system Outflow/inflow & evaporation/precipitation Are processes that D the water level = path functions. We may not know which process causes a D, but we do know the sum of the processes (water level)

  34. Internal E (DU), a state function, is affected by q & w (path functions). They are processes that alter the system. Once a D occurs in U, we can’t say which process caused the D. We can see the sum of these processes reflected in the D in internal energy

  35. final energy added DE = + Internal Energy initial initial energy removed DE = ─ Internal Energy final Energy Diagrams E diagrams show the direction of E flow during a process

  36. final energy added DE = + Internal Energy initial initial energy removed DE = ─ Internal Energy final Energy Diagrams If final condition has larger amount of internal E than initial condition, D in internal E will be (+)

  37. initial energy removed DE = ─ Internal Energy final Energy Diagrams If final condition has smaller amount of internal E than initial condition, D in internal E will be (─)

  38. energy released DErxn = ─ E Flow in a ChemRxtn • Total amount of Einternal in 1mol of C(s) & 1 mole of O2(g) is > Einternal in 1 mole of CO2(g) • In rxtn C(s) + O2(g) → CO2(g), there’s a net release of E into the surroundings C(s), O2(g) Internal Energy CO2(g) Surroundings System C + O2→ CO2

  39. E Flow in a ChemRxtn energy absorbed DErxn = + C(s), O2(g) Internal Energy CO2(g) Surroundings System CO2 →C + O2 System C + O2→ CO2 • Total amount of Einternal in 1mol of C(s) & 1 mole of O2(g) is > Einternal in 1 mole of CO2(g) (at same T and P) • In rxtn CO2(g) → C(s) + O2(g), E is absorbedfrom surroundings into the system

  40. Surroundings DE + System DE ─ Energy Flow • When E flows out of system, DEsystem is (─) • When energy flows into the surroundings, DEsurroundings is (+) • Therefore:

  41. Surroundings DE ─ System DE + Energy Flow • When E flows into a system, it must come from surroundings • When E flows into a system, DEsystem is

  42. Surroundings DE ─ System DE + Energy Flow • When E flows out of the surroundings, DEsurroundingsis • Therefore: DEsystem= ─DEsurroundings

  43. Energy Exchange • Energy is exchanged btwn system & surroundings thru heat&work • q = heat (thermal) energy • w = work energy • q and w are NOT state functions, their value depends on the process DE = q + w

  44. Energy Exchange Energy exchanged btwn system & surroundings by either heat exchangeorwork being done

  45. Heat & Work White ball = initial amount of 5.0 J of KE As it rolls some E converted to heat by friction Rest of KE transferred to purple ball by collision

  46. Heat & Work On a smooth table, most KEwhite ball is transferred from white to purple ball. Small amount lost thru friction as heat DE for white ball: DE= KEfinal − KEinitial = 0 J − 5.0 J = −5.0 J KE transferred to purple ball, w = −4.5 J KE lost as heat, q = −0.5 J q + w = (−0.5 J) + (−4.5 J)

  47. Heat & Work DE for white ball: DE = KEfinal − KEinitial = 0 J − 5.0 J = −5.0 J KE transferred to purple ball, w = −2.0 J KE lost as heat, q = −3.0 J q + w = (−3.0 J) + (−2.0 J) On a rough table, most KEwhiteball is lost thru friction: < 1/2 is transferred to purple ball

  48. Heat, Work, and Internal Energy DE of white ball is = for both cases, but q and w are not On rougher table, heat loss, q, is greater (q = more neg #)

  49. Heat, Work, and Internal Energy But on rougher table, less KE transferred to purple ball, so work,w, done by white ball, is less (less neg #)

  50. Heat, Work, and Internal Energy DE is a state function depends only on velocity of white ball before and after the collision.