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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. Heat flow. Heat flowing into or out of a system always results in some kind of change to the system The temperature of the system could change

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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. Heatflow • Heat flowing into or out of a system always results in some kind of change to the system • The temperature of the system could change • There could be some other change, like a change in physical state, for instance

  3. Heatflow • When heat flowing into or out of a substance results in a T, we can calculate the amount of heat with the equation: • q = ms T • But sometimes heat flowing into or out of a substance results in a different kind of change – without a temperature change • Melting, freezing, chemical reactions, etc. • Measured as H – a change in enthalpy

  4. 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

  5. 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

  6. 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

  7. H=q/n At constant pressure

  8. 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...

  9. Measuring H • Because H = q/n, the heat lost or gained per mole, if we can measure the heat lost or gained, we can know the value of H

  10. What are all the H’s? • Any energy change for a system that doesn’t result in a T for the system is measured as a H • Ex: melting/freezing, boiling/condensing, dissolving, or the energy that flows into or out of a reacting system

  11. Changes in state require changes in energy (H)

  12. What do all the H’s mean? • Hfus = the heat that must be added to change 1.0mol of a substance from a solid to liquid at it’s melting point • For freezing  use a (-) number • Freezing is exothermic • For melting  use a (+) number • Melting is endothermic

  13. What do all the H’s mean? • Hvap = the heat that must be added to change 1.0mol of a substance from a liquid to gas at it’s boiling point • For condensing  use a (-) number • condensing is exothermic • For boiling  use a (+) number • boiling is endothermic

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

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

  16. Heat flow can result in several things… • If the heat flow results in a T, the equation used is: • q = ms T • IF the heat flow results in a different change – like melting or freezing, the equation is: • q = nH

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

  18. 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

  19. Heat of reaction • The entire energy change (E) for a reaction is often called the heat of reaction (Hrxn ) • The energy may be absorbed(+ Hrxn) or released (- Hrxn ) • the energy may be heat, sound, light, electricity, etc.

  20. Heat of reactionHrxn • Chemical changes involve breaking bonds in the reactants (endothermic) and forming bonds in the products (exothermic) • for a chemical reaction: Hrxn = Hproducts – Hreactants • Whether the Hrxnis a + or - number depends on the relative strengths of the bonds in the reactants and products.

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

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

  23. 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

  24. 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

  25. 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

  26. 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

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

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