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Thermochemistry. Thermochemistry. Chemical reactions are accompanied by changes in energy. Definitions:. Energy:. The ability to do work, w = f x d. K.E. =1/2mv 2 energy in motion. What are some forms of kinetic energy?. Mechanical, electrical, thermal, sound, etc (Motion).

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## Thermochemistry

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**Thermochemistry**Chemical reactions are accompanied by changes in energy.**Definitions:**Energy: The ability to do work, w = f x d K.E. =1/2mv2 energy in motion What are some forms of kinetic energy? Mechanical, electrical, thermal, sound, etc (Motion) P.E. potential energy What are some forms of potential energy? Chemical Positional**Energy Units:**Joule = J 1 J = kg ·m/s2 Since a J is very small we usually use kJ’s calorie = cal 1 cal = 4.184 J (exact by definition) 1 cal 4.184 J A conversion factor! 1 cal is the amount of heat necessary to heat 1 g of water 1oC (from 14.5 to 15.5oC)**Food Calories:**1 Cal (note capital for food) = 1000 cal = 1 kcal So... That donut with only 500 Calories is really.... 500,000 calories... (eat 2 to make it an even million).**1st Law of Thermodynamics:**Energy cannot be created nor destroyed. System Surroundings System: whatever we are interested in. Surroundings: the rest of the universe**Internal Energy (E): The sum of all K.E. and P.E. of a**system. Internal energy is too complex to be measured exactly, so we look at the change in internal energy: Esystem = Efinal - Einitial Sign of E: (+) means: system has gained energy (-) means: system has lost energy From the 1st Law of Thermodynamics: Esystem = -Esurroundings**Sign of E:**(+) means: system has gained energy (-) means: system has lost energy 2H2(g) + O2(g) 2H2O(g) 2H2O(g) 2H2(g) + O2(g)**Sign of E:**(+) means: system has gained energy (-) means: system has lost energy + energy (given off) 2H2(g) + O2(g) 2H2O(g) 2H2O(g) 2H2(g) + O2(g) + energy (added to system)**Heat and Work: energy exchange**E = q + w Work done on (+) or by (-) the sys. Heat gained (+) or lost (-) by the sys. Internal energy of the sys. (both K.E. and P.E.)**Reactions are Energy Driven:**Energy given off Exothermic (-) sign “Exit” Energy absorbed Endothermic (+) sign “Enter” Remember: 1st Law of Thermo.: Esystem = -Esurroundings So for every endothermic process there is an exothermic process.**Decide if each of the following are Endo or Exothermic**processes: Ice melting? Water freezing? A candle burning? Dynamite exploding? A plant growing?**Misconception: Energy vs Temperature**Both containers are at the same temperature, which one contains the most energy? Temperature is an intensive property (amount independent) Energy is an extensive property(amount dependent)**Enthalpy (H): heat content per amount**Extensive property Enthalpy Change (H) In a reaction: H = HProducts - Hreactants H (+) endo H (-) exo**Enthalpy Change (H)**In a reaction: H = HProducts - Hreactants H (+) endo H (-) exo Enthalpy is a state function. “Net Change” “Path indepedent” Both accomplished the same net result! A B**Problem:**Given the reaction, which occurs in the presence of the catalyst MnO2): 2KClO3(s) 2KCl(s) + 3O2(g) H = -89.7 kJ a. What would be the value of H for the reverse RXN? +89.7 kJ**Problem:**Given the reaction (which occurs in the presence of the catalyst MnO2): 2KClO3(s) 2KCl(s) + 3O2(g) H = -89.7 kJ The H value listed is really a conversion factor! -89.7 kJ -89.7 kJ -89.7 kJ 2 mol KClO3 2 mol KCl3 mol O2**Problem:**Given the MnO2 catalyzed reaction: 2KClO3(s) 2KCl(s) + 3O2(g) H = -89.7 kJ -89.7 kJ -89.7 kJ -89.7 kJ 2 mol KClO3 2 mol KCl3 mol O2 Calculate the value of Hfor the decomposition 9 moles of KClO3 ? 2KClO3(s) 2KCl(s) + 3O2(s) H = -89.7 kJ 9 moles KClO3 -------------------------------------------------- = kJ -403.7**Problem:**Given the MnO2 catalyzed reaction: 2KClO3(s) 2KCl(s) + 3O2(g) H = -89.7 kJ -89.7 kJ -89.7 kJ -89.7 kJ 2 mol KClO3 2 mol KCl3 mol O2 Calculate the value of H for the formation 25.0 grams of KCl . 2KClO3(s) 2KCl(s) + 3O2(s) H = -89.7 kJ 25.0 g KCl mol KCl -89.7 kJ -------------------------------------------------- = kJ -15.0 74.6 g KCl 2 mol KCl**CS2(l) + 3O2(g) CO2(g) + 2SO2(g)**From tables: 87.9kJ/mol 0 kJ/mol -393.5 kJ/mol -296.8 kJ/mol H = H(products) - H(reactants) Each value must be multiplied by the number of moles in the equation/**CS2(l) + 3O2(g) CO2(g) + 2SO2(g)**From tables: 87.9kJ/mol 0 kJ/mol -393.5 kJ/mol -296.8k J/mol H = H(products) - H(reactants) H = H(products) - H(reactants) = [-393.5 + 2(-296.8)] - [87.9 + 3(0)] H = -1073.4 kJ

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