Chemistry II Chapter 5 Thermochemistry - PowerPoint PPT Presentation

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Chemistry II Chapter 5 Thermochemistry

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  1. Chemistry IIChapter 5Thermochemistry

  2. 1. The study of energy and its transformations is known as _thermodynamics__.

  3. 2. __Thermochemistry_____ considers the relationships between chemical reactions and the energy changes involving heat.

  4. Nature of Energy • The magnitude of the kinetic energy, Ek, of an object depends on its __mass__ (m) and __velocity___(v)

  5. Formula for kinetic energy: • Ek = ½ mv2

  6. 4. Potential energy is the energy an object has by virtue of its ___position__ relative to other objects.

  7. Types of potential energy • gravitational • elastic • chemical • electrical

  8. 5.__Electrostatic Forces_________ results from the interactions between charged particles.

  9. The formula for electrostatic force is: • Ee1 = kQ1Q2/ d

  10. k is the constant of proportionality (8.99 x 109 J m /C2) - C is the _coulomb______, the unit for electrical charge.- Q1 and Q2 are the _charges__ on the two particles involved- d is the distance between the particles

  11. 8. When the charges are the same, the electrostatic force, Ek, is ______positive_______and the charges __repel____.

  12. 9. When the charges are opposite the electrostic force, Ek, is ____negative_____ and the charges __attract____.

  13. The energy of a substance is due to • 1.) The thermal (kinetic) energy of the molecules. • 2.) The potential energy of the bonds.

  14. 11. The SI unit for energy is the __joule____ (J) named in honor of ___James Joule____ ( 1818 -1889) – a British Scientist who investigated heat and work.

  15. 12. A __calorie_______ was originally defined as the amount of energy required to raise the temperature of 1gram of water from 14.5 to 15.5 degrees Celsius

  16. 13. Today energy is defined in terms of the joule: 1 calorie = 4.184 joules

  17. 14. When we use thermodynamics to analyze energy changes, we focus our attention on a limited and well defined part of the universe. a. The portion we single out to look at is called the __system___. b. Everything else is the _surroundings____. c. A ___closed system_____ can exchange energy but not material with its surroundings.

  18. Transferring Energy: Work and HeatEnergy is transferred in two ways:1.) To cause the motion of an object against a force2.) To cause a temperature change

  19. 15. The energy used to cause an object to move against a force is called __work_____. • W= F x D

  20. 16. The other way that energy is transferred is __heat__- the energy transferred by a hotter object to a colder one.

  21. 17. _Energy______is the capacity to do work or to transfer heat.

  22. Section 2The First Law of Thermodynamics

  23. 18. The First Law of Thermodynamics – Energy is conserved

  24. 19. The _Internal_____ __energy___ of a system is the sum of all of the kinetic and potential energy in the components of the system.

  25. 20. We represent the internal energy with the symbol _E__.

  26. 21. We can never really know the internal energy of a system, E, but we can find the ____Change____ in E (ΔE) • ΔE = Ef - Ei

  27. 22. A __Positive_____ value for ΔE results when Efinal> Einitial which means that energy has been put into the system ( __Endothermic___)

  28. 23. A ___negative__ value for ΔE results when Efinal< Einitial which means that energy has been lost to the surroundings ( _exothermic___)

  29. Energy DiagramsExothermic * Endothermic

  30. 24. When a system undergoes any chemical or physical change, the change in internal energy ΔE is given by the heat, q, added to or released by the system plus the work done on or by the system, w. Formula: ΔE = q + w

  31. When heat, q, is added to a system q is positiveWhen work, w, is done on a system it has a positive value

  32. Sample exercise 5.3The hydrogen and oxygen gases in the cylinderillustrated in figure 5.3 are ignited. As the reaction occurs, the system loses 1150 J of heat to the surroundings. The reaction also causes the pistion to rise as the hot gases expand. The expanding gas does 480 J of work on the surroundings as it pushes against the atmosphere. What is the change in the internal energy of the system ?

  33. Endothermic – heat flows into the system Exothermic- heat flows out of the system

  34. State FunctionsAlthough we cannot know exactly what the internal energy, E, of a system, it does have a definite value, Δ E can be determined.

  35. 26. Internal energy is a __state___ _function___. A state function is a property of a system that is determined by specifying the conditions of state.

  36. 27. The value of a state function depends only on its _present____ __conditon__ and not on the particular ___history___ of the sample.

  37. 28. B/c internal energy, E, is a state function Δ E depends only on the __initial_____ and _final___ states and not the change that occurred.

  38. 29.)Internal energy is a state function, but q+w is not a state function.

  39. Section 3: Enthalpy • 30.)Most commonly the only kind of work produced by chemical change is _____mechanical_ work.

  40. 31.)Usually reactions are carried out at __constant___atmospheric pressure.

  41. For example: Consider the following equation Zn + 2 HCl → ZnCl2 + H2 Net Ionic Zn + 2H+ → Zn+2 + H2

  42. 32.) As the hydrogen gas is produced it does positive__against the atmosphere. We can better see this work if the reaction takes place in a reaction vessel like the one below.

  43. 33.) This kind of work is called _Pressure__ - __volume__ work (P-V Work). • Formula: • W= -PΔV

  44. 34. When the change in volume is positive the work done is ___negative__.

  45. 35. The thermodynamic function called ___enthalpy___ accounts for the heat flow in chemical changes occurring at constant pressure when no forms of work are performed other than P-V work.

  46. Enthalpy equals the internal energy plus the product of the pressure and the change in volume of the system. • 36.) The symbol for enthalpy is _____H_____.

  47. Formula for enthalpy: H = E + PV • Enthalpy (H) equals the total internal energy ( E) and pressure volume work.

  48. 37.) Enthalpy is a __state____ function because internal energy, E, pressure and volume are all state functions. ΔH = ΔE + ΔPV W = - PΔV -W = P ΔV ΔH = ΔE + ΔPV= q + w-w=q ( at constant pressure) ΔH = qp

  49. Change in enthalpy equals the heat gained or lost at constant pressure.It is easier to measure the change in temperature than the change in internal energy. • 38.) When ΔH is ____positive____ (qp is positive) the system has gained heat from the surroundings.