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T h e r m o d y n a m i c s

T h e r m o d y n a m i c s. The study of energy transfers and chemical driving forces. Enthalpy. Enthalpy (H)  the total E (KE + PE) of a system at constant P when a system reacts, H = H final - H initial for a chemical reaction: H rxn = H products - H reactants. H.

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T h e r m o d y n a m i c s

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  1. Thermodynamics The study of energy transfers and chemical driving forces

  2. Enthalpy • Enthalpy (H) the total E (KE + PE) of a system at constant P • when a system reacts, H = Hfinal - Hinitial • for a chemical reaction: • Hrxn = Hproducts - Hreactants

  3. H • H is a state function - • that is, what is the absolute difference? • the “history” of how it got there isn’t important • ex: T, P, V, etc...

  4. The only problem is... • The enthalpy of a system (H) cannot actually be measured • KE = 1/2mv2 • the velocity of any object is always relative to a frame of reference • the absolute velocity of the earth cannot be determined

  5. But, we do know... • For an endothermic reaction, H is (+) • For an exothermic reaction, H is (-) • so, H is all that is really important, and it can be measured if we assume all the energy gained or lost is heat

  6. Measuring H • Because H = the heat lost or gained for each mole, if we can measure the heat lost or gained, we can know the value of H…

  7. H=q/n At constant pressure

  8. What do all the H’s mean? • Hrxn = the heat that is either absorbed (+ Hrxn ) or released by (- Hrxn ) the reactants during the course of a chemical reaction • Hsoln = the heat that is either absorbed (+ Hsoln ) or released by (- Hsoln ) a substance when it dissolves

  9. What do all the H’s mean? • Hfus = the heat that must be added to change 1.0 mole of a substance from a solid to liquid at it’s melting point • Hvap = the heat that must be added to change 1.0 mole of a substance from a liquid to gas at it’s boiling point

  10. What do all the H’s mean? • Note: • all H’s are usually kJ/mol • divide the number of kJ by the # of moles • reverse process = same #, opposite sign

  11. calorimetry • A calorimeter is a device used to measure the T for a reacting system • Often, filled with water to absorb or release heat

  12. calorimetry • Because the calorimeter (the water inside it and the device itself) is the surroundings, calculating the heat (q) that flows into or out of the water allows us to infer the heat that flowed into or out of the system… • …which allows us to know the H for the system itself (H = q/n)

  13. Because the heat is absorbed by or released mostly from the water, and a bit from the calorimeter, measuring T of the water allows one to measure q for the reaction • qrxn = -(qH2O + qcal) • qrxn = -(msT + CT)

  14. Heat of reaction • The entire energy change (E) for a reaction is often called the heat of reaction (why? We’ll see) • The energy may be absorbed or released • the energy may be heat, sound, light, electricity, etc.

  15. Thermochemical Equations • Since reactions involve the gain of heat from or loss of heat to the surroundings, an energy term may be included on the reactant or product side of a chemical equation

  16. H = (+) number kJ reactants endothermic reaction • heat is a reactant • H is (+) products Energy

  17. exothermic reactions • heat is a product • H is (-) reactants Energy H = (-) number kJ products

  18. Example... • When four moles of ammonia burns in air, 1170kJ of heat are produced. • 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(l) + 1170 kJ • Hrxn = -1170 kJ

  19. How many kJ are released into the surroundings for each gram of ammonia that reacts? • 1.00gNH3 • x 1 mol/17.04g • x 1170kJ/4 mol NH3 • = 17.2 kJ

  20. How many grams of nitric oxide have been produced according to the above reaction if 5000 kJ of heat has been generated? • 5000 kJ • x 4 mol NO/1170 kJ • x 30.01g/1 mol • = 513g NO

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