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Thermal Energy & Heat

Thermal Energy & Heat. Temperature. Temperature measure of the average KE of the particles in a sample of matter. Temperature Conversions. o C to o F: o F = 9/5 o C + 32 o F to o C: o C = 5/9( o F – 32) o C to K: K = o C + 273 K to o C: o C = K – 273.

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Thermal Energy & Heat

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  1. Thermal Energy & Heat

  2. Temperature • Temperature • measure of the average KE of the particles in a sample of matter

  3. Temperature Conversions • oC to oF: oF = 9/5oC + 32 • oF to oC: oC = 5/9(oF – 32) • oC to K: K = oC + 273 • K to oC: oC = K – 273

  4. Going from Celsius to Fahrenheit Convert 37 oC to oF. oF = 9/5oC + 32 oF = 9/5(37oC) + 32 = 66.6 + 32 = 98.6oF

  5. Going from Fahrenheit to Celsius Convert 68oF to oC oC = 5/9(oF – 32) oC = 5/9(68 – 32) = 5/9(36) = 20 oC

  6. Going from Celsius to Kelvin Convert 100oC to K K = oC + 273 K = 100 + 273 = 373 K

  7. Going from Kelvin to Celsius Convert 310 K to oC oC = K – 273 oC = 310 – 273 = 37oC

  8. Thermal Energy • Thermal Energy • the total energy of the particles in a material • KE - movement of particles • PE - forces within or between particles due to position • depends on temperature, mass, and type of substance

  9. 80ºC 80ºC 400 mL A B 200 mL Thermal Energy • Which beaker of water has more thermal energy? • B - same temperature, more mass

  10. Heat Transfer • Heat • thermal energy that flows from a warmer material to a cooler material • Like work, heat is... • measured in joules (J) • a transfer of energy

  11. 80ºC 10ºC A B Heat Transfer • Why does A feel hot and B feel cold? • Heat flows from A to your hand = hot. • Heat flows from your hand to B = cold.

  12. Heat Transfer • Heat is always transferred from hot to cold. • Insulators slow the transfer of heat due to air pockets. • Conductors easily allow the transfer of heat, like metals. • Heat is transferred by conduction, convection, and radiation.

  13. Conduction • Heat is transferred due to objects touching each other or through collisions. • Occurs best in solids. • Heat continues to be transferred until both objects reach the same temperature, called a thermal equilibrium.

  14. Convection • Transfer of heat through a liquid or gas through moving currents, called convection currents. • The cause of wind and weather.

  15. Radiation • Transfer of heat through electromagnetic radiation (light from stars or light bulbs). • Transferred in all directions. • No contact required! • Dark or dull objects absorb more than light or shiny objects do.

  16. Heat Transfer • Specific Heat (Cp) • amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin • units: J/(kg·K)or J/(g·°C)

  17. 50 g Al 50 g Cu Heat Transfer • Which sample will take longer to heat to 100°C? • Al - It has a higher specific heat. • Al will also take longer to cool down.

  18. – Q = heat loss + Q = heat gain T = Tf - Ti Heat Transfer Q = m T  Cp Q: heat (J) m: mass (kg) T: change in temperature (K or °C) Cp: specific heat (J/kg·K or J/g.oC)

  19. Coffee cup Calorimeter Heat Transfer • Calorimeter • device used to measure changes in thermal energy • in an insulated system, heat gained = heat lost

  20. Heat Transfer • A 32-g silver spoon cools from 60°C to 20°C. How much heat is lost by the spoon? GIVEN: V m = 32 g Ti = 60°C Tf = 20°C Q = ? Cp = 235 J/kg·K WORK: F Q = m·T·Cp m = 32 g = 0.032 kg T = 20°C - 60°C = – 40°C T = -40oC + 273 = 233 K SQ = (0.032kg)(233K)(235J/kg·K) A Q = 1752 J

  21. Heat Transfer • How much heat is required to warm 230 g of water from 12°C to 90°C? GIVEN: V m = 230 g Ti = 12°C Tf = 90°C Q = ? Cp= 4.184 J/g·oC WORK: F Q = m·T·Cp m = 230 g T = 90°C - 12°C = 78°C SQ = (230 g)(78oC)(4.184 J/g·oC) AQ = 75,061 J

  22. When energy is changed from one form to another, all of the energy can be accounted for. This is called the Law of Conservation of Energy.

  23. Heat is energy. • If heat is released by a chemical system, an equal amount of heat will be absorbed by the surroundings

  24. When your body breaks down sugar completely, the body releases the same amount of heat as compared to burning the same amount of sugar in a flame.

  25. Heat is transferred due to a temperature difference. • Faster molecules speed up the slower molecules. • This slows the faster molecules.

  26. In an endothermic process, the system absorbs heat from the surroundings. A process that absorbs heat is a(n) endothermic process. • In an exothermic process, the system gives off heat to the surroundings.

  27. If you were to touch the flask in which an endothermic reaction was occurring, the flask would probably feel cooler than before the reaction started. • In an exothermic reaction, the energy stored in the chemical bonds of the reactants is greater than the energy stored in the bonds of the products.

  28. As perspiration evaporates from your skin, your body is cooled. • With respect to your body, this process is said to be exothermic.

  29. A calorie is the quantity of heat that raises the temperature of 1 gram of pure water 1C. • 1 Calorie = 4.18 kJ

  30. How many kJ of energy can be released by a banana containing 150 Cal?

  31. An object's heat capacity is the amount of heat required to change the temperature of an object by exactly 1°C.

  32. Chem Notes 17B • The specific heat capacity or specific heat is the heat divided by the mass and temperature change.

  33. Q = m T  Cp Cp or c = specific heat Q= heat m= mass T=change of temperature

  34. The temperature of an 10. g sample of metal changed from 25°C to 50°C when it absorbed 500. J of heat. What is the specific heat of this sample?

  35. V c=? m = 10. g • T = 50°C-25°C = 25°C • Q = 500. J F Q = m T  Cp S A

  36. Determine the specific heat of a material if an 18 g sample absorbed 75 J as it was heated from 15°C to 40.°C.

  37. V c = ? m = 18 g • T = 40.°C-15°C = 25°C Q = 75 J • FQ = m·T·Cp • SA

  38. What is the specific heat of a substance if 2000. cal are required to raise the temperature of a 300.-g sample by 20.C?

  39. Vc = ? Q = 2000. cal m = 300. g T = 20.C • F Q = m·T·Cp • S A

  40. What is the amount of heat required to raise the temperature of 200.0 g of aluminum by 10.C? • (specific heat of aluminum = 0.21 cal/gC)

  41. VQ = ? m = 200. g • c = 0.21 cal/gC T = 10.C • F Q = m·T·Cp • S • (200. g)(0.21 cal/gC)( 10.C) = • A 420 cal

  42. Water has the highest specific heat of all common substances.

  43. Chem Notes 17C • The symbol H stands for the heat of reaction for a chemical reaction. • This is also called the change in enthalpy. H = Q

  44. The heat content of a system is equal to the enthalpy only for a • system that is at constant pressure.

  45. Calorimetry depends on the law of conservation of energy.

  46. If 27.0 mL of water containing HCl is mixed with 28.0 mL of water containing NaOH in a calorimeter such that the initial temperature of each solution was 24.0°C and the final temperature of the mixture is 33.0°C, how much heat (in kJ) is released in the reaction? Assume that the densities of the solutions are 1.00 g/mL.

  47. V H = ? • m = 27.0 g + 28.0 g = 55.0 g • T = 33.0°C - 24.0°C = 9.0°C c = 0.00418 kJ/g°C • F H =m Tc • S H = (55.0g)(9.0°C)(.00418kJ/g°C) • A

  48. A lead mass is heated and placed in a foam cup calorimeter containing 40.0 mL of water at 17.0°C. The water reaches a temperature of 20.0°C. How many joules of heat were released by the lead?

  49. VH = ? m = 40.0 g • T = 20.0°C - 17.0°C = 3.0°C • c = 4.18 J/g°C • FH =m Tc • SA

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