1. IIIIIIIVV I. Enthalpy Ch. 5 – Thermochemistry

2. A. Reaction Energy (Chp. #2) n Heat ( q ) – Units: J (joules)  Total amount of thermal energy  heat or energy flows from something hot to something cold. n Temperature – intensity of the thermal energy; measure of the average KE

3. A. Reaction Energy ºF ºC K -45932212 -2730100 0273373 K = ºC + 273 n SI Unit of Temperature is Kelvin. Absolute Zero

4. A. Reaction Energy n Thermodynamics –  Thermo – heat  Dynamic – motion  All energy changes that accompany physical and chemical processes.  Nature favors changes that proceed toward lower energy and greater disorder.

5. A. Reaction Energy n Entropy (S ): Units: J/mol  A measure of the total disorder in a substance or system. n Molar Entropy – Entropy possessed by 1 mol of a substance.  Always positive  Increases with temperature – Entropy of gas is greater than liquid  At 0 K, entropy of a pure substance is zero.

6. A. Reaction Energy n Reaction Pathway – shows the progress & energy change in a reaction n E a – activation energy: energy needed to “start” a reaction n Energy changes because:  Bond breaking – requires energy  Bond formation – releases energy

7. A. Reaction Energy n Enthalpy ( H ) Units: J/mol  The heat of a reaction  Depends on: temperature, physical state, and composition  Given in a problem or table for standard states of reactants & products – - Example water: standard is liquid state not solid state

8. A. Reaction Energy n Change in Enthalpy ( H ): The difference between the enthalpy of the reactants and products n It indicates the heat absorbed or heat released in a chemical reaction H = H products – H reactants Positive value – heat absorbed, endothermic Negative value – heat released, exothermic

9. Exothermic Reaction (exit…) Release of heat so surroundings (water) gets warmer. Exothermic Reaction

10. B. Exothermic Reaction n reaction that releases energy n products have lower PE than reactants n H is negative 2H 2 (l) + O 2 (l)  2H 2 O(g) + energy

11. All combustion reactions are exothermic. C 3 H 8 + 5O 2 3CO 2 + 4H 2 O + 2043 KJ 1 mole of propane produces 2043 kilojoules of heat C 3 H 8 + 5O 2 3CO 2 + 4H 2 O H = -2043 KJ negative B. Exothermic Reaction

12. Endothermic Reaction (in…. ) Absorption of heat so surroundings (water) gets colder.

13. C. Endothermic Reaction n reaction that absorbs energy n reactants have lower PE than products n H is positive 2Al 2 O 3 + energy  4Al + 3O 2 energy absorbed

14. Most decomposition reactions are endothermic. 2H 2 O + 572 KJ 2H 2 + O 2 2 moles of water requires 572 KJ of energy (electricity) to decompose 2H 2 O 2H 2 + O 2 H = +572 KJ positive C. Endothermic Reaction

15. Enthalpy Problem How much heat will be released if 1.0g of H 2 O 2 decomposes according to the following equation? 2H 2 0 2 2H 2 0 + 0 2 H = -190 KJ 1.0 g ? KJ

16. 2H 2 0 2 2H 2 0 + 0 2 H = -190 KJ 1.0 g 1.0 g H 2 0 2 1 mole H 2 0 2 -190 KJ 34.0 g H 2 0 2 2 moles H202H202 = -2.8 KJ

17. © 2012 Pearson Education, Inc. Changes in Internal Energy n When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is,  E = q + w.

18. © 2012 Pearson Education, Inc.  E, q, w, and Their Signs

19. Calorimetry: n The study of heat flow and heat measurement. n We measure temperature changes in a reaction in a calorimeter while in the lab.

20. Calorimeter: n Used to determine enthalpy changes (heat lost or gained) of chemical reactions The heat lost in a reaction is gained by the water: q rxn = -q sur

21. Exothermic Reaction (exit…) Release of heat so surroundings (water) gets warmer. Exothermic Reaction

22. Endothermic Reaction (in…. ) Absorption of heat so surroundings (water) gets colder.

23. Heat Capacity: n The amount of heat (joules) needed to raise the temperature of the object 1 o C n Depends on mass & composition n For example: 1 drop of water vs. 1 liter of water

24. Specific Heat (c) :  The heat capacity of 1g of a substance  For example: water’s specific heat 4.184 J See a table in the book or a periodic table for these values. Notice, metals have low specific heats. g o C

25. Heat equation: q= m. ΔT. c Heat lost or gained Mass of water + salt T f – T i Final temperature – initial temperature Specific heat of water To find enthalpy: Δ H = q/mole

26. When 13.7 g of solid lead (II) nitrate is dissolved in 85.0 g of water, the temperature of the water is lowered from 23.4 o C to 19.7 o C. Is this reaction exothermic or endothermic? Is H positive or negative? Find the heat absorbed or released and the change in enthalpy.