1. Thermochemistry

2. Thermochemistry is concerned with the heat changes that occur during chemical reactions and changes in state. Energy is the capacity to do work or supply heat measured in Joules or calories

3. Energy Transformations Kinetic energy –Energy of motion Gas Molecule Motion Potential Energy stored energy –Atomic - energy stored in the nucleus of an atom –Chemical energy -energy stored in chemical bonds –TE = PE + KE 17.1

4. Energy Transformations When fuel is burned in a car engine, chemical potential energy is released and is used to do work. 17.1

5. Energy Transformations –In what direction does heat flow? –Heat FlowHeat Flow 17.1

6. Energy Transformations Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between them. –Heat always flows from a warmer object to a cooler object. »O C »37 C 17.1

7. Exothermic and Endothermic Processes –What happens in endothermic and exothermic processes? Heat FlowHeat Flow 17.1

8. Exothermic and Endothermic Processes Exothermic (-) A process that releases heat to its surroundings Liquid  Solid –A + B  AB + nrg Endothermic (+) A process that absorbs heat from its surroundings. Solid  Liquid –AB + nrg  A + B 17.1

9. Exothermic and Endothermic Processes The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. All energy can be accounted for as work, stored energy or heat 17.1

10. Temperature Temperature is not a form of energy it is a measurement of the average KE of the particles in the substance Two scales in Chemistry

11. Temperature Scales and Interconversions Kelvin ( K ) - The “Absolute temperature scale” begins at absolute zero and only has positive values. Celsius ( o C ) - The temperature scale used by science, formally called centigrade, most commonly used scale around the world; water freezes at 0 o C, and boils at 100 o C. Fahrenheit ( o F ) - Commonly used scale in the U.S. for our weather reports; water freezes at 32 o F and boils at 212 o F. Kelvin = o C + 273 o C = Kelvin - 273 o F = (9/5) o C + 32 o C = [ o F - 32 ] 5/9

12. Kelvin Scale Absolute Zero is the lowest point at which there is no motion or kinetic energy C = K – 273 K = C + 273

13. Theory of Physical Phase Solids –Strong attractive forces Liquids intermediate attractive forces Gases -very weak forces therefore no definite shape or volume

15. A Model for Solids –The general properties of solids reflect the orderly arrangement of their particles and the fixed locations of their particles. 13.3

16. A Model for Solids The melting point (mp) is the temperature at which a solid changes into a liquid. Occurs at 0 Celsius or 273 Kelvin 13.3

17. Crystal Structure and Unit Cells In a crystal, the particles are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice. 13.3

18. Sublimation Process in which a solid changes directly to a vapor without passing through the liquid phase Solid CO 2 Solid I 2

19. A Model for Liquids Substances that can flow are referred to as fluids. Both liquids and gases are fluids. 13.2

20. Evaporation In an open container, molecules that evaporate can escape from the container.

21. Evaporation –During evaporation, only those molecules with a certain minimum kinetic energy can overcome the attraction of nearby molecules and escape from the surface of the liquid 13.2

22. Evaporation –The conversion of a liquid to a gas or vapor is called vaporization. –When such a conversion occurs at the surface of a liquid that is not boiling, the process is called evaporation. –Evaporation occurs at the surface of the liquid and is the process a liquid changing to a gas –It occurs below the boiling point –It is a cooling process 13.2

23. Evaporation –Animation 15 –Observe the phenomenon of evaporation from a molecular perspective.

24. Evaporation In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid. 13.2

25. Condensation Process in which vapor or gas changes to a liquid

26. Phase Changes Heating & Cooling Curves As heat is added & temperature increases, KE Increases and PE remains constant As heat is removed & temperature decreases, KE decreases and PE remains the same As you continue to heat and a phase change occurs, PE increases and KE and temperature remains constant

27. Phase Changes Heating & Cooling Curves During a phase change, none of the heat is being used to increase the randomly moving particles. Heat is needed to change ice at 0 o C to water at 0 o C. The particles are rearranging themselves. Energy is stored as potential energy. The mp and fp take place at the same temperature

28. Phase Change Examples Exothermic: –Gas, Liquid, Solid –KE decreases on diagonal lines and PE decreases during phase changes Endothermic: –Solid, Liquid, Gas –KE increases on diagonal lines and PE increases on during phase change (horizontal lines)

29. Heating & Cooling Curve

30. Heats of Vaporization and Condensation 17.3

31. Latent Heat Heats of Fusion and Solidification –How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies? 17.3

32. Heats of Fusion and Solidification The heat of fusion (H fus ) is the quantity of heat needed to change a unit mass of a solid to a liquid at constant temperature. It takes 334J/g to change ice to liquid water at 0 o C at STP. The heat of solidification (H solid ) is the quantity of heat needed to change a unit mass of a liquid to a solid at constant temperature. 334J/g is released 17.3

33. Heats of Fusion and Solidification –The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies; that is, H fus = – H solid. 17.3

35. Heats of Vaporization and Condensation –How does the quantity of heat absorbed by a vaporizing liquid compare to the quantity of heat released when the vapor condenses? 17.3

36. Heats of Vaporization and Condensation –The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, ∆H vap = –∆H cond. –Heat of vaporization is the quantity of energy needed to vaporize a unit mass of liquid at a constant temperature –2260J/g is needed to change liquid water to steam 17.3

37. Heats of Vaporization and Condensation –Animation 21 –Observe the phase changes as ice is converted to steam when heat is added.

38. Vapor Pressure Vapor pressure is a measure of the force exerted by a gas above a liquid. See Table H 13.2

39. Would you need more energy to increase the temperature of a bathtub filled with 400,000 ml of water 0.5 o C or to increase a glass of a water filled with 250ml by 80 o C?

40. Calorimetry Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. Measured by a Calorimeter 17.2

41. Calorimetry The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter. 17.2

42. Units for Measuring Heat Flow Heat flow is measured in two common units, the calorie and the joule. –The energy in food is usually expressed in Calories. Food calories A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water by 1 Celsius 17.1

43. Joule The SI unit of heat and energy Increases 1 gram of water by 0.239 C 4.18 J = 1 calorie When one pound of coal burns, it releases 3.5 x 10 6 calories. Convert this amount to joules. Convert to kilojoules 1.5 x 10 7 Joules 1.5 x 10 4 KJ

44. Calorimetry Calorimetry experiments can be performed at a constant volume using a bomb calorimeter. 17.2

45. PROBLEMS Quantity of heat transferred can be measured by 3 factors 1.Mass (m) of water in the calorimeter 2.The change in the temperature (T final – T initial ) 3.Constant (specific heat of a substance) 1.q = mc t

47. Calculate the amount of heat energy that must be added to a 48g sample of liquid water in order to raise its temperature 30 o C

48. Calculate the maximum mass of water that can be heated from room temperature (25 o C) to its boiling point by the addition of 20475 Joules of heat energy. 65 g

49. Calculate the amount of heat energy that must be removed from a 55g sample of water at 351K in order to lower its temperature to 298K

50. Heat Capacity and Specific Heat The heat capacity of an object depends on both its mass and its chemical composition. –The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object. –Specific HeatSpecific Heat 17.1

51. Heat Capacity and Specific Heat The specific heat capacity, or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C. 17.1

52. Heat Capacity and Specific Heat Water releases a lot of heat as it cools. During freezing weather, farmers protect citrus crops by spraying them with water. 17.1

53. Heat Capacity and Specific Heat 17.1

54. Heat Capacity and Specific Heat Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust. 17.1

59. 1. The energy released when a piece of wood is burned has been stored in the wood as sunlight. heat. calories. chemical potential energy. Section Quiz.

60. 2. Which of the following statements about heat is false? Heat is the same as temperature. Heat always flows from warmer objects to cooler objects. Adding heat can cause an increase in the temperature of an object. Heat cannot be specifically detected by senses or instruments. Section Quiz.

61. 3. Choose the correct words for the spaces: In an endothermic process, the system ________ heat when heat is ________ its surroundings, so the surroundings _____________. gains, absorbed from, cool down. loses, released to, heat up. gains, absorbed from, heat up. loses, released to, cool down. 17.1 Section Quiz.

62. 5. Assuming that two samples of different materials have equal mass, the one that becomes hotter from a given amount of heat is the one that has the higher specific heat capacity. has the higher molecular mass. has the lower specific heat capacity. has the higher density. 17.1 Section Quiz.

63. Theory of Physical Phase Solids –Close –Strong attractive forces Liquids -the molecules that have higher KE escape and enter gas phase Gases -very weak forces therefore no definite shape or volume