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Thermochemistry

Thermochemistry. Thermochemistry: Study of energy changes that occur during chemical reactions and changes in state Chemical Potential Energy: Energy stored in chemical bonds of a substance. What is heat, how does it flow?.

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Thermochemistry

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  1. Thermochemistry

  2. Thermochemistry: Study of energy changes that occur during chemical reactions and changes in state • Chemical Potential Energy: Energy stored in chemical bonds of a substance

  3. What is heat, how does it flow? • Heat (q) : energy transferred from one object to another due to temperature difference • Heat always flows from warmer object to cooler object! Why? Hint – Energy…

  4. Heat vs. Temperature… • Heat: • total energy of molecular motion in a substance • Depends on size and mass • Temperature • average energy of molecular motion in a substance • Does not depend on size or mass • Can one object have more heat, but a lower temperature than another object? • Hot cup of water vs. the ocean

  5. Energy • Law of conservation of energy: Energy is not created or destroyed • Endothermic Process: absorbs heat from the surroundings Reactants + Heat  Products • Exothermic Process: releases heat to surroundings Reactants  Products + Heat

  6. Units for Heat • calorie: heat needed to raise 1.0 gram of water 1.0ᵒC 1 Calorie = 1 kilocalorie = 1000 calories Ex. Kudos Bar 130 Calories = 130 kcal = 130,000 calories • Joule – SI unit for energy 1 J = 0.2390 cal 4.184 J = 1 cal How many joules are in a Kudos bar?

  7. Specific Heat • Specific Heat: Amount of heat needed to increase the temperature of 1g of a substance exactly 1ᵒC. specific heat (C) = Heat (Joules or calories) Mass (grams) x change in temperature (C) C = q . ( m ) (T)

  8. The temperature of a 95.4g piece of copper increases from 25.0ᵒC to 48.0ᵒC when the copper absorbs 849J of heat. What is the specific heat of copper?

  9. Calorimetry • Calorimetry: measure of heat flow into or out of a system • Enthalpy (H): the internal energy of the molecules of a material Heat(q) = H = m (C) T H = change in the heat m = mass C = specific heat T = change in the temperature

  10. Thermochemical Equations • Enthalpy change (ΔH) can be written as a product or a reactant. Endothermic 2NaHCO3(s) +129kJ Na2CO3(s) + H2O(g) + CO2(g) 2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)ΔH=129kJ Exothermic CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ CaO(s) + H2O(l)  Ca(OH)2(s)ΔH=-65.2kJ

  11. 2NaHCO3(s) + 129kJ  Na2CO3(s) + H2O(g) + CO2(g) ΔH=129kJ How much heat (in kJ) is required to decompose 2.24mol of NaHCO3? ΔH = 2.24mol NaHCO3 x 129kJ 2mol NaHCO3 = 144kJ

  12. C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)ΔH=-1368kJ How much heat (in kJ) is released when 12.5g of ethanol burns?

  13. Fusion and Solidification • Molar Heat of Fusion (ΔHfus): heat absorbed by 1 mol of a substances as it melts (solid  liquid) ENDOTHERMIC solid + heat  liquid • Molar Heat of Solidification (ΔHsolid): heat lost when 1 mol of a liquid solidifies (liquid  solid) EXOTHERMIC liquid  solid + heat ΔHfus =-ΔHsolid

  14. Vaporization and Condensation • Molar Heat of Vaporization (ΔHvap): amount of heat needed to convert 1 mol of liquid to gas (liquid  gas) ENDOTHERMIC liquid + heat  gas • Molar Heat of Condensation: (ΔHcond): amount of heat released when 1 mol of vapor condenses (gas  liquid) EXOTHERMIC gas  liquid + heat ΔHvap= -ΔHcond

  15. H2O(s) H2O(l) ΔHfus = 6.01kJ/mol How many grams of ice at 0ᵒC will melt if 2.25kJ of heat are added?

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