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Energy & Chemical Change

Learn about energy transfer, specific heat, thermochemical equations, Hess's Law, and heat in chemical reactions. Solve practice problems to grasp concepts easily.

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Energy & Chemical Change

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  1. Energy & Chemical Change Chapter 16

  2. 16.1 Energy • Energy is the ability to do work or produce heat. Energy exists as either potential or kinetic.

  3. Physical and chemical changes always involve energy transfer. • Energy is measured in joules or calories • 1 calorie = 4.184 joules

  4. Endothermic reactions absorb energy • Exothermic reactions release energy

  5. Energy transfer can be calculated by the equation: q = m·ΔT·C • q= energy (joules) • m = mass (grams) • ΔT = change in temperature (Celsius) • C = specific heat (Joules/grams·Celsius)

  6. What is specific heat?

  7. The specific heat of a substance is the heat required to raise the temperature of 1.0 gram by 1.0 °C. • Specific heat of water: • Gas: 2.02 J/g °C • Liquid: 4.20 J/g °C • Solid: 2.06 J/g °C

  8. Example 1: How many Joules are required to heat 690 grams of nickel from 22 °C to 322 °C? (C = 0.44 J/g °C)

  9. Example 2: How many joules are needed to raise the temperature of 20.0 grams of water from 12°C to 25°C? (C = 4.20 J/g °C)

  10. Practice Problem 1: Calculate the amount of energy needed to raise the temperature of 20 grams of iron from 10 °C to 40 °C. (C = 0.45 J/g °C)

  11. Practice Problem 2: A total of 260 joules of energy were added to a sample of gold. The temperature increased from 10 °C to 20 °C. What is the mass of the sample? • (C = 0.13 J/g °C)

  12. Energy & Chemical Change Chapter 16

  13. 16.2 Heat in Chemical Reactions • Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. • The system is a specific part of the universe that contains the reaction being studied. • The surroundings is everything else outside the system. System + Surrounding = Universe

  14. 16.3 Thermochemical Equations • Phase changes require energy. • The change from solid to liquid is called the enthalpy (heat) of fusion (ΔHfus). • The change from liquid to gas is called the enthalpy (heat) of vaporization (ΔHvap) q = m·ΔH ΔHfus = 334 J/g (s-l) ΔHvap = 2260 J/g (l-g)

  15. EX: Calculate the amount of heat required to convert 25.0 grams of ice at -50 °C to vapor at 140 °C. • Specific heat of water: • Gas: 2.0 J/g °C • Liquid: 4.2 J/g °C • Solid: 2.1 J/g °C

  16. EX: Calculate the amount of heat required to convert 10.0 grams of ice at -5 °C to water at 20 °C.

  17. PP: Calculate the amount of heat required to convert 10.0 grams of ice at -20 °C to vapor at 120 °C.

  18. PP: Calculate the amount of heat required to convert 50.0 grams of ice at - 20 °C to vapor at 300 °C.

  19. Energy & Chemical Change Chapter 16

  20. 16.4 Energy Transfer • The amount of energy transferred in a reaction can be calculated by the equation: m·(Ti - Tf)·C =m·(Tf - Ti)·C

  21. EX: A 25 gram sample of aluminum at 90 °C is dropped into a 100 gram sample of water at 20 °C. What will be the final temperature of the system? • (Specific heat of aluminum is 0.90 J/g°C)

  22. EX: A sample of copper at 75 °C is dropped into a 50 gram sample of water at 10 °C. The final temperature is 14 °C. What is the initial mass of the metal? (Specific heat of copper is 0.38 J/g °C)

  23. PP: A 50 gram sample of aluminum at 80 °C is dropped into a 150 gram sample of water at 10 °C. What will be the final temperature of the system? (Specific heat of aluminum = 0.90 J/g °C)

  24. PP: A sample of copper at 99 °C is dropped into a 60 gram sample of water at 15 °C. The final temperature is 16 °C. What is the initial mass of the metal? (Specific heat of copper is 0.38 J/g °C)

  25. Energy & Chemical Change Chapter 16

  26. Hess’s Law • Hess' Law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps.

  27. EX1: Find the enthalpy for the reaction: CO2(g) + 2H2O(l) => CH4(g) + 2O2(g) using the following data: C(s) + O2(g) => CO2(g) H=-196.7KJ H2O(l) => H2(g) + ½ O2(g) H=142.9KJ CH4(g) => C(s) + 2H2(g) H=37.4KJ

  28. EX2: Find th enthalpy for the reaction: 2CO2(g) + 3H2O(l) => C2H6O(l) + 3O2(g) using the following data: 2C2H6O(l) +O2(g) => 2C2H4O(l) + 2H2O(l) H=-610.5KJ 2CO2(g) + 2H2O(g) => C2H4O(l) + 5/2 O2(g) H=1750.5KJ

  29. PP1: Find the enthalpy of the reaction: CH4(g) => C(s) + 2H2(g) using the following data: CO2(g) => C(s) + O2(g) H=147.5KJ H2(g) + ½ O2(g) => H2O(l) H=-107.2KJ CO2(g) + 2H2O(l) => CH4(g) + 2O2(g) H=333.9KJ

  30. PP2: Find the enthalpy of the reaction: 2HCl(g) + ½ O2(g) => H2O(g) + Cl2(g) Using the following data: CH2Cl2(l) + O2(g) => COCl2(g) + H2O(l) H=-47.5KJ ½ H2(g) + ½ Cl2(g) => HCl(g) H=-230KJ COCl2(g) + 2H2O(l) => CH2Cl2(l) + H2(g) + 3/2O2(g) H=402.5KJ

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