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

2. Enthalpy Enthalpy (H)  the total E (KE + PE) of a system at constant P Enthalpy (H)  the total E (KE + PE) of a system at constant P when a system reacts,  H = H final - H initial when a system reacts,  H = H final - H initial for a chemical reaction: for a chemical reaction:  H rxn = H products - H reactants  H rxn = H products - H reactants

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

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

5. But, we do know... For an endothermic reaction,  H is (+) For an endothermic reaction,  H is (+) For an exothermic 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 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… 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?  H rxn = the heat that is either absorbed (+  H rxn ) or released by  H rxn = the heat that is either absorbed (+  H rxn ) or released by (-  H rxn ) the reactants during the course of a chemical reaction (-  H rxn ) the reactants during the course of a chemical reaction  H soln = the heat that is either absorbed (+  H soln ) or released by  H soln = the heat that is either absorbed (+  H soln ) or released by (-  H soln ) a substance when it dissolves (-  H soln ) a substance when it dissolves

9. What do all the  H’s mean?  H fus = the heat that must be added to change 1.0 mole of a substance from a solid to liquid at it’s melting point  H fus = the heat that must be added to change 1.0 mole of a substance from a solid to liquid at it’s melting point  H vap = the heat that must be added to change 1.0 mole of a substance from a liquid to gas at it’s boiling point  H vap = 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: Note: all  H ’s are usually kJ/mol all  H ’s are usually kJ/mol divide the number of kJ by the # of moles divide the number of kJ by the # of moles reverse process = same #, opposite sign reverse process = same #, opposite sign

11. calorimetry A calorimeter is a device used to measure the  T for a reacting system A calorimeter is a device used to measure the  T for a reacting system Often, filled with water to absorb or release heat 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… 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) …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 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 q rxn = -(q H2O + q cal ) q rxn = -(q H2O + q cal ) q rxn = -(ms  T + C  T) q rxn = -(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 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 absorbed or released the energy may be heat, sound, light, electricity, etc. 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 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. endothermic reaction heat is a reactant heat is a reactant  H is (+)  H is (+) products reactants  H = (+) number kJ EnergyEnergy

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

18. Example... When four moles of ammonia burns in air, 1170kJ of heat are produced. When four moles of ammonia burns in air, 1170kJ of heat are produced. 4 NH 3(g) + 5 O 2(g)  4 NO (g) + 6 H 2 O (l) + 1170 kJ 4 NH 3(g) + 5 O 2(g)  4 NO (g) + 6 H 2 O (l) + 1170 kJ  H rxn = -1170 kJ  H rxn = -1170 kJ

19. How many kJ are released into the surroundings for each gram of ammonia that reacts? How many kJ are released into the surroundings for each gram of ammonia that reacts? 1.00gNH 3 1.00gNH 3 x 1 mol/17.04g x 1 mol/17.04g x 1170kJ/4 mol NH 3 x 1170kJ/4 mol NH 3 = 17.2 kJ = 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? How many grams of nitric oxide have been produced according to the above reaction if 5000 kJ of heat has been generated? 5000 kJ 5000 kJ x 4 mol NO/1170 kJ x 4 mol NO/1170 kJ x 30.01g/1 mol x 30.01g/1 mol = 513g NO = 513g NO