Thermochemistry The study of energy

1. Thermochemistry The study of energy Chapter 6

2. Kinetic Energy Ek The energy of an object in motion Ek= ½ mv2 Mass need to be in kg A car moving at 40 mph has a greater kinetic energy than a car moving at 20 mph

3. Types of kinetic energy • Work: energy used to cause an object with mass to move • Heat: energy used to cause the temperature of an object to increase

4. Potential Energy • Stored energy in an object by virtue of its position. • Units of energy: Joule J, 1 Calorie Cal = 4.184 J

5. 5. 1 The Nature of Energy • Energy is capacity to do work or to produce heat. • Thermochemistry is the study of heat change in chemical reactions.

6. Temperature (K) = Thermal Energy (J) Energy Changes in Chemical Reactions Heatis the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy Said another way temperature is a measure of random motion (KE) of particles. 6.2

7. Physics vs. Chemistry Chemical energy: is the energy stored within the bonds of chemical substances (potential) Thermal energy: is the energy associated with the random motion of atoms and molecules (Kinetic molecular theory) (kinetic) In this chapter we will discuss the transfer of these types of energy.

8. Energy Systems System: portion we single out to study typically the chemical Ex: reactants products. Surroundings: everything else Ex: Reaction vessel, environment

10. Types of Systems

11. C B A

12. Exothermic A reaction that results in the evolution of heat. Thus heat flows out of the system Exo = out = heat loss from system = energy loss = -heat

13. 2H2(g) + O2(g) 2H2O (l) + energy H2O (g) H2O (l) + energy An Exothermic Reaction

15. Endothermic A reaction that absorbs heat from their surroundings. Thus heat flows into a system. Endo = In to = heat gain to system = energy gain = + heat

16. energy + 2HgO (s) 2Hg (l) + O2(g) energy + H2O (s) H2O (l) An Endothermic Reaction

18. Work, work, work • Recall that energy Can be transferred in the form of motion and heat Work = force x distance Units f = Newton, N d = meter, m W = N*m or Joule, J

19. Force is any kind of push or pull exerted on an object. • Ex: gravity, mechanical, electrostatic • Distance is how far the object moved as a result of the force applied.

20. Work in Biology • Think about water moving from the ground up the trunk of a tree. What part of the system if any undergoes a change in potential energy? Is work done in the process?

21. Guided Questions What is changing location ? Does this change involve potential energy?

22. Water moves up the trunk against the force of gravity. Thus the potential energy of the water does change • Work is movement of a mass over a distance against an opposing force.

24. Homework Chang: pg 255 1, 9, 10 ****BL:Pg 188 • 1, 2, 3, 6, 9, 11,

25. 5.2 First law of thermodynamics Aka: The law of conservation of energy Energy is neither created nor destroyed, thus energy is converted from one form to another. Thus the energy of the universe is constant Universe = System + Surroundings

26. Chemical energy lost by combustion = Energy gained by the surroundings system surroundings

27. Internal Energy • The energy (E) of a system can be defined as the sum of the kinetic and potential energies of all of the particles in a system. • Internal energy can be changed by a flow of HEAT, WORK, or BOTH

28. Change in a system energy E = Efinal - Einitial E > 0 = system gained energy E < 0 = system lost energy to the surroundings Enthalpy can not be negative, think about it like a bank account (EX pg 235)

29. Relating heat to work E = q+ w E = change in system’s internal energy q = heat w = work

30. Signs signs every where are signs

31. Write a figure description for this figure so that a student reading this text would understand. Identify key terms in your description.

32. Work (w) W > 0 = work is done ON the system ( + W) W < 0 = work is done BY the system on the surroundings. (-W)

33. Prove it…. • Write an example that illustrates work being done by a system Then write a sentence illustrating work being done on a system

34. Heat (q) q > 0 = heat is added to the system. (+ q endothermic) q < 0 = heat is released from the system (-q exothermic)

35. Prove it …. • Write an example that illustrates heat being added to a system Then write a sentence illustrating heat being released form a system

36. Example: Calculate E for a system undergoing an endothermic process in which 15.6 kJ of heat flows and where 1.4 kJ of work is done by the system.

37. Answer q = + 15.6 (endothermic) W = - 1.4 kJ (work is done by the system) E = q+ w E = 15.6 + (- 1.4) = 14.2

38. State Function A Property of a function that is determined by specifying its condition or its present state The value of a state function depends only on the present state of the system - not how it arrived there Energy IS A STATE FUNCTION Enthalpy IS A STATE FUNCTION Temperature IS A STATE FUNCTION Heat IS NOT A STATE FUNCTION Work IS NOT A STATE FUNCTION

39. Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.

40. Example It does not mater whether I heated the water (C  B) or cooled that water (A  B) to get it to the temperature. Internal energy at point B would be the same regardless. A B C

41. Change in energy ΔE • ΔE is a state function because it is independent of pathway. (all we care about is energy at Ei and energy at Ef. Heat (q) and work (w) are not state functions!!!!!

42. Example

43. 5.2 Homework Chang: pg 255 11,12,17,18 ***BL: Pg 189 17, 19, 21, 22, 25, 26

44. 5.3 Enthalpy Enthalpy (H): Accounts for the heat flow in chemical changes occurring at constant pressure when no forms of work other than P, V work *** state function**** Results from a change in P (atm) or V (L) of the system.

45. What is Pressure volume work Build up of gas that causes a piston to lift against the force of gravity. Work = -P ΔV

47. Enthalpy : H = E + PVE = internal energy Surroundings Surroundings HEAT HEAT System System ΔH >0 Endothermic ΔH <0 Exothermic

48. Don’t forget you can manipulate these equations Enthalpy = H = E + PV E = HPV H = E + PV

49. Example: • Calculate the work associated with the expansion of gas from 46 L to 64 L at a constant external pressure of 15 atm. W = -PΔV

50. Answer w = -PΔV P = 15 atm V = 64L – 46L = 18L w = -15atm (18L) = -270 L*atm To convert to J 101.3 J/1 L*atm