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Thermochemistry

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

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Thermochemistry

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  1. Thermochemistry

  2. 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.

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

  4. (For example) - 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.

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

  6. In an endothermic process, the system absorbs heat from the surroundings. A process that absorbs heat is a(n) endothermic process.

  7. In an exothermic process, the system gives off heat to the surroundings.

  8. If you were to touch the flask in which an endothermic reaction was occurring, the flask would probably feel warmer than before the reaction started.

  9. 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.

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

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

  12. How many kJ of energy can be released by a banana containing 150 Cal? (1 Calorie = 4.18 kJ)

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

  14. The specific heat capacity or specific heat is the heat divided by the mass and temperature change. • Where c = specific heat Q = heat m = mass and T = change of temperature • The temperature of a 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? • c = ? • m = 10. g • T = 50°C-25°C = 25°C • Q = 500. J

  15. 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. • c = ? • m = 18 g • T = 40.°C-15°C = 25°C • Q = 75 J

  16. 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? • c = ? • Q = 2000. cal • m = 300. g • T = 20.C

  17. 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) • Q = ? • m = 200. g • c = 0.21 cal/gC • T = 10.C

  18. Q = mcT • Q = (200. g)(0.21 cal/gC)( 10.C) = 420 cal

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

  20. The symbol H stands for the heat of reaction for a chemical reaction. • This is also called the change in enthalpy. H = Q • The heat content of a system is equal to the enthalpy only for a system that is at constant pressure. • Calorimetry depends on the law of conservation of energy.

  21. 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.

  22. 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

  23. 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?

  24. H = ? • m = 40.0 g • T = 20.0°C - 17.0°C = 3.0°C • c = 4.18 J/g°C

  25. Fusion = solid  liquid • Solidification = liquid  solid • Molar heat of fusion – • heat required to melt a mole = Hfusion • Molar heat of solidification – • heat required to freeze a mole =Hsolid • Hfusion = -Hsolid

  26. Vaporization is liquid  vapor • Condensation is vapor  liquid • Molar heat of vaporization – • heat required to vaporize a mole = Hvap • Molar heat of condensation – • heat required to condense a mole = Hcond • Hvap = -Hcond

  27. Given the equation I2(s) + 62.4 kJ  I2(g)H = +62.4 kJ

  28. How much heat, in kJ, is released when 108 g of water at O°C freezes to ice at O°C if Hsolid for water = -6.01 kJ/mol?

  29. How much heat is released in the condensation of 27.0 g of steam at 100°C to water at 100°C if Hcond for water = -40.7 kJ/mol?

  30. How many grams of ice at 0°C can be melted into water at 0°C by the addition of 75.0 kJ of heat? Hfus for water = 6.01 kJ/mol

  31. The heat of solution is the amount of heat absorbed or released when a solid dissolves. • If the molar heat of solution of NaOH is -445.1 kJ/mol, how much heat (in kJ) will be released if 80.00 g of NaOH are dissolved in water?

  32. Hess's law makes it possible to calculate H for complicated chemical reactions. • Hess’s Law – If you add 2 or more thermochemical equations to give a final equation, then you also ADD the heats of reactions to give the final heat of reaction.

  33. Given the equation: • C2H4 (g) + 3O2(g) + 2H2O(l) + 1411 kJ • How much heat is released when 8.00 g of O2 react?

  34. What is the heat of reaction (H) for the combustion (with O2) of benzene, C6H6 to form carbon dioxide and water? Write the balanced equation for the reaction.

  35. Standard heats of formation: • C6H6 = +48.50 kJ • O2 = 0.0 kJ • CO2 = -393.5 kJ • H20 = -285.8 kJ

  36. 2 C6H6 + 15 O2 12 CO2 + 6 H2O • H + (2) (48.50 kJ) + (15)(0.0 kJ) = (12) (-393.5 kJ) + (6) (-285.8 kJ) • H = -3266.9 kJ

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