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Thermochemistry

Thermochemistry. Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances

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Thermochemistry

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  1. Thermochemistry

  2. Energyis the capacity to do work • Thermal energy is the energy associated with the random motion of atoms and molecules • Chemical energy is the energy stored within the bonds of chemical substances • There are also other forms of energy, such as nuclear, potential, and electrical

  3. Temperature = Thermal Energy 900C 400C Energy Changes in Chemical Reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. Which one has greater thermal energy? greater thermal energy 6.2

  4. Law of Conservation of Energy: • Energy can not be created or destroyed. It can only change form. • What types of energy are involved when you eat a burger?

  5. SURROUNDINGS SYSTEM Thermochemistry is the study of heat change in chemical reactions. The system is the specific part of the universe that is of interest in the study. closed isolated open energy nothing Exchange: mass & energy 6.2

  6. 2H2(g) + O2(g) 2H2O (l) + energy Here’s a video of this reaction: Can you tell energy is a product? H2O (g) H2O (l) + energy energy + 2HgO (s) 2Hg (l) + O2(g) energy + H2O (s) H2O (l) Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. Endothermic process is any process in which heat has to be supplied to the system from the surroundings. 6.2

  7. Exothermic and Endothermic Firewood burning Ice melting • Exothermic • Chemical  Thermal • Releases heat! • Endothermic • Thermal  Chemical • Absorbs heat!

  8. Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. DH = heat given off or absorbed during a reaction DH = H (products) – H (reactants) Products: 720 kJ/mol Reactants: 520.5 kJ/mol Exothermic: negative ΔH Endothermic: positive ΔH Product: 127 kJ/mol Reactants: 139.3 kJ/mol

  9. Reaction Endo or Exo? N2(g) + O2 (g) 2NO (g)∆H = 168 kJ/mol 2H2(g) + O2 (g) 2H2O (l)∆H = -571 kJ/mol 2 P(s) + 5 Cl2(g)  2 PCl5(s)   ∆H = -880 kJ/mol H2O(l)  H2O(g) ∆H = 44 kJ/mol Try It! Endo! Exo! Exo! Endo!

  10. The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. Heat (q) absorbed or released: q = mcDt Dt = tfinal - tinitial 6.4

  11. I Do: How much heat is given off in Joules when an 869 g iron bar cools from 940C to 50C? Is this process endothermic or exothermic? q = mcDt 6.4

  12. We Do: 71.5g of aluminum metal is heated from 20°C to 50°C in order to increase its malleability. What amount of heat is absorbed by the aluminum? Is this process endothermic or exothermic?_____________ q = mcDt

  13. You Do: A 2.0-gram sample of graphite cools from 10°C to 0°C. How much heat was released from the graphite sample?Endothermic or exothermic? _______________ q = mcDt

  14. Determining Endo/Exothermicity • To determine if a reaction is exothermic or endothermic, we measure the temperature of the surroundings.

  15. Calorimetry Calorimetry is the quantitative measurement of the heat required or evolved during a chemical process. For example, if the energy from an exothermic chemical reaction is absorbed in a container of water, the change in temperature of the water provides a measure of the amount of heat added.

  16. The enthalpy of solution (DHsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent. DHsoln = Hsoln - Hcomponents Which substance(s) could be used for melting ice? LiCl and CaCl2 Which substance(s) could be used for a cold pack? NaCl, KCl, NH4Cl, NH4NO3 6.6

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