1. Energy and Chemical Reactions Chapter 5

2. 5.1 Energy the science of heat and work is called thermodynamics Kinetic energy thermal, mechanical, electric, sound Potential energy chemical, gravitational, electrostatic

3. Energy law of conservation of energy states that energy can be converted from one form to another, but can neither be created nor destroyed also called the first law of thermodynamics

4. Temperature and Heat heat is not the same as temperature more thermal energy = more particle motion total thermal energy is sum of individual energies of all particles

5. Thermal Equilibrium thermal equilibrium means that two objects have reached the same temperature heat always transfers from hotter object to cooler object transfer continues until equilibrium is reached heat lost = heat gained

6. Thermal Equilibrium exothermic process occurs when heat transfers from system to surroundings heat is released as a product endothermic process occurs when heat transfers from surroundings into system heat is required as a reactant

7. Energy Units SI unit of thermal energy is the joule (J) 1 J = 1 kg(m 2 /s 2 ) usually used the kilojoule (kJ) 4.184 joule (J) = 1 calories (cal) dietary Calorie is equal to 1000 cal or 1 kcal

8. 5.2 Specific Heat specific heat is the quantity of heat required to raise 1 gram of substance 1 kelvin (J/g∙K) Q = CmΔT Where ΔT = T final - T initial

9. Practice Problem In an experiment it was determined that 59.8 J was required to change the temperature of 25.0 g of ethylene glycol by 1.00 K. Calculate the specific heat capacity of ethylene glycol.

10. Quantitative Aspects In an isolated system, the sum of the energy changes must be zero! Energy is CONSERVED!!

11. Practice Problem A 15.5 g piece of chromium, heated to 100.0 o C, is dropped into 55.5 g of water at 16.5 o C. The final temperature of the metal and the water is 18.9 o C. What is the specific heat of chromium? Assume no heat is lost to container or air.

12. Homework After reading sections 5.1 and 5.2, you should be able to do the following problems… p. 245 (13-18) p. 247 (66,67,83,84)

13. 5.3 Energy and Changes of State Heat of fusion: solid to liquid (endothermic) Heat of vaporization: liquid to gas (endothermic) Heat of solidification: liquid to solid (exothermic) Heat of condensation: gas to liquid (exothermic)

14. Energy and Changes of State Temperature is held constant during a state change!! See figure 5.9 on p. 219

15. Practice Problem How much heat must be absorbed to warm 25.0 g of liquid methanol, CH 3 OH, from 25.0 o C to 64.6 o C and then to evaporate the methanol completely at that temperature? The specific heat of methanol is 2.53 J/g∙K. ΔH vap = 2.00 x 10 3 J/g

16. Practice Problems To make a glass of iced tea, you pour 250 mL of tea, whose temperature is 18.2 o C, into a glass containing 5 ice cubes. Each cube has a mass of about 15 g. What quantity of ice will melt, and how much ice will remain to float at the surface in this beverage? Iced tea has a density of 1.0 g/cm 3 and a specific heat of 4.2 J/g∙K.

17. 5.4 First Law of Thermodynamics conservation of energy ΔU = q + w Where ΔU is change in internal energy q is heat transfer to or from system w is work transfer to or from system, called P-V work

18. P-V work work done on or by the system equals the volume change that occurs against resisting external pressure w = - P(ΔV)

19. Enthalpy Enthalpy is heat content of a substance at constant pressure Enthalpy change (ΔH) negative ΔH and ΔU mean that energy is transferred from system to surroundings positive signs mean that energy is transferred from surroundings to system

20. State Functions changes in ΔH or ΔU depend only on final and intial states the pathway to go between is not relevent (bank accounts, balloons, etc)

21. Homework After reading sections 5.3 and 5.4, you should be able to do the following problems… p. 243 (25-28)(69-72,85)

22. 5.5 Enthalpy Changes enthalpy changes are specific to the reactants and products and their amounts (states of matter are important) ΔH is positive when endothermic and negative when exothermic (reverse reactions have opposite sign)

23. Enthalpy Changes Standard reaction enthalpy: enthalpy change accompanying a specific reaction Standard state: most stable form of the substance in the physical state at a given pressure and temperature

24. Enthalpy Changes change depends on molar amounts of substances calculate moles of substance and then multiply that by heat transfer per mole

25. Practice Problem What quantity of heat energy is required to decompose 12.6 g of liquid water to the elements? The combustion of ethane, C 2 H 6, has an enthalpy change of -2857.3 kJ for the reaction. Calculate ΔH when 15.0g is burned. 2C 2 H 6 (g) + 7O 2 (g)  4CO 2 (g) + 6H 2 O(g)

26. 5.6 Calorimetry technique to determine heat transfer constant pressure measures change in enthalpy constant volume measures change in internal energy

27. Constant Pressure Calorimetry can be used to determine heat gained or lost by solution can be used to determine heat required or released by reaction at constant pressure, the heat measured is ΔH change in heat content of soluntion can be measure and used to calculate the heat of reaction q r + q soln = 0

28. Practice Problem Assume you mix 200. mL of 0.400 M HCl with 200. mL of 0.400 M NaOH in a coffee-cup calorimeter. The temperature of the solutions before mixing was 25.20 o C; after mixing and allowing reaction to occur, the temperature is 27.78 o C. What is the molar enthalpy of neutralization of the acid? (assume densities of all solutions are 1.00 g/mL and their specific heats are 4.20 J/gK)

29. Constant Volume Calorimetry used to calculate heats of combustion and caloric value of foods – use a “bomb” constant volume, so energy transfer as work doesn’t occur and heat is therefore the change in internal energy q r + q bomb + q water = 0

30. Practice Problem A 1.00g sample of sucrose is burned in a bomb calorimeter. The temperature of 1.50 x 10 3 g of water in the calorimeter rises from 25.00 o C to 27.32 o C. The heat capacity of the bomb is 837 J/K and the specific heat of water is 4.20 J/gK. Calculate (a) the heat evolved per gram of sucrose and (b) the heat evolved per mole of sucrose.

31. Homework After reading sections 5.5 and 5.6, you should be able to do the following… p. 244 (31-40)

32. 5.7 Hess’s Law sometimes products immediately undergo other reactions and therefore calorimetry cannot be used Hess’s Law states that if a reaction is the sum of two or more other reactions, ΔH for the overall process is the sum of the ΔH values of those reactions

33. Energy Level Diagrams can visualize Hess’s Law by using diagrams to show the formation of different steps in a reaction as well as the enthalpy changes involved See figure 5.16 p. 234

34. Practice Problem What is the enthalpy change for the formation of ethane, C 2 H 6, from elemental carbon and hydrogen? 2C(s) + 3H 2 (g)  C 2 H 6 (g)

35. Practice Problem Use Hess’s Law to calculate enthalpy change for the formation of CS 2 (l) from C(s) and S(s) from the following enthalpy values. C(s) + O 2 (g)  CO 2 (g) ΔH = -393.5 kJ S(s) + O 2 (g)  SO 2 (g) ΔH = -296.8 kJ CS 2 (g) + 3O 2 (g)  CO 2 (g) + 2SO 2 (g) ΔH = -1103.9 kJ C(s) + 2S(s)  CS 2 (g) ΔH = ?

36. Standard Enthalpies Standard molar enthalpy of formation, ΔH o f, is the heat change for the formation of 1 mol of a compound directly from its component elements in their standard states. See appendix L

37. Standard Enthalpies Standard enthalpy of formation for an element in its standard state is zero. Most values are negative can compare thermal stability; more exothermic is more stable

38. Enthalpy Change for a Reaction ΔH o rxn = Σ[ΔH o f (products)] - Σ[ΔH o f (reactants)] Find enthalpies in a table and then plug the values into the equation above

39. Practice Problem Calculate the standard enthalpy of combustion for benzene, C 6 H 6. C 6 H 6 (l) + 7½O 2 (g)  6CO 2 (g) + 3H 2 O(l) ΔH o f [C 6 H 6 (l)] = +48.95 kJ/mol

40. 5.8 Favored Reactions product-favored reactions go from left  right most reactions that are exothermic (have negative values of enthalpy) are product-favored most reactions that are endothermic (have positive enthalpy change) are reactant-favored

41. Practice Problem Calculate ΔH o rxn for each of the following reactions and decide if the reaction may be product- or reactant-favored. 2HBr(g)  H 2 (g) + Br 2 (g) C(diamond)  C(graphite)

42. Homework After reading sections 5.7 and 5.8, you should be able to do the following problems… p. 245 (43-54) skip diagrams on 43 + 44