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THERMOCHEMISTRY

THERMOCHEMISTRY. Courtesy of lab-initio.com. Definitions #1. Energy : The capacity to do work or produce heat. Potential Energy : Energy due to position or composition. Kinetic Energy : Energy due to the motion of the object. Definitions #2.

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THERMOCHEMISTRY

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  1. THERMOCHEMISTRY Courtesy of lab-initio.com

  2. Definitions #1 Energy: The capacity to do work or produce heat Potential Energy: Energy due to position or composition Kinetic Energy: Energy due to the motion of the object

  3. Definitions #2 Law of Conservation of Energy: Energy can neither be created nor destroyed, but can be converted between forms The First Law of Thermodynamics: The total energy content of the universe is constant

  4. State Functions depend ONLY on the present state of the system ENERGYIS A STATE FUNCTION A person standing at the top of Mt. Everest has the same potential energy whether they got there by hiking up, or by falling down from a plane  WORKIS NOT A STATE FUNCTION WHY NOT???

  5. E = q + w E= change in internal energy of a system q= heat flowing into or out of the system -qif energy is leaving tothe surroundings +qif energy is entering fromthe surroundings w= work done by, or on, the system -wif work is done bythe systemonthe surroundings +wif work is done onthe system bythe surroundings

  6. Work, Pressure, and Volume Compression Expansion +V(increase) -V(decrease) -wresults +wresults Esystemdecreases Esystemincreases Work has been done on the system by the surroundings Work has been done by the system on the surroundings

  7. Example

  8. Quiz #5

  9. Energy Change in Chemical Processes Endothermic: Reactions in which energy flows into the system as the reaction proceeds. + qsystem - qsurroundings

  10. Energy Change in Chemical Processes Exothermic: Reactions in which energy flows out of the system as the reaction proceeds. - qsystem + qsurroundings

  11. Calorimetry The amount of heat absorbed or released during a physical or chemical change can be measured, usually by the change in temperature of a known quantity of water in a calorimeter.

  12. Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree 1 BTU is the heat required to raise the temperature of 1 pound of water by 1 F

  13. The amount of heat required to raise the temperature of one gram of substance by one degree Celsius. Specific Heat

  14. Calculations Involving Specific Heat q = Heat lost or gained s = Specific Heat Capacity T= Temperature change

  15. Hess’s Law Germain Henri Hess

  16. Hess’s Law “In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.”

  17. Hess’s Law Example Problem CH4 C + 2H2 +74.80 kJ Step #1: CH4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H.

  18. Hess’s Law Example Problem CH4 C + 2H2 +74.80 kJ C + O2 CO2 -393.50 kJ Step #2: Keep reaction #2 unchanged, because CO2 belongs on the product side

  19. Hess’s Law Example Problem CH4 C + 2H2 +74.80 kJ C + O2 CO2 -393.50 kJ 2H2 + O2 2 H2O -571.66 kJ Step #3: Multiply reaction #2 by 2

  20. Hess’s Law Example Problem CH4 C + 2H2 +74.80 kJ C + O2 CO2 -393.50 kJ 2H2 + O2 2 H2O -571.66 kJ CH4 + 2O2 CO2 + 2H2O -890.36 kJ Step #4: Sum up reaction and H

  21. Calculation of Heat of Reaction The standard enthalpy of formation, ΔHof ,is the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states (The degree symbol indicates it is calculated at standard temperature and pressure)

  22. Calculation of Heat of Reaction Hrxn = Hf(products) - Hf(reactants) Hrxn= [-393.50kJ + 2(-285.83kJ)] – [-74.80kJ] Hrxn = -890.36 kJ

  23. Calculation of Heat of Reaction

  24. Calculation of Heat of Reaction

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