Thermochemistry

1. Thermochemistry The study of energy and its transformations

2. Definitions - Energy • Chemical systems contain both kinetic energy and potential energy. Energy is the capacity to do work or to produce heat. An example of both is the combustion of gasoline. The gaseous products expand and do work (moving the pistons of an engine) and the reaction also produces heat.

3. Definitions - Energy Kinetic energy is the energy of motion, and it depends upon the mass of the object and its velocity. Since molecules, especially those of gases, are in motion, they possess kinetic energy.

4. Definitions- Energy Potential energy is energy due to position or composition. Chemical energy is potential energy due to composition. For example, gasoline and oxygen have the potential to produce energy if they react.

5. Definitions When examining chemical systems or reactions, we consider the system and its surroundings. • The system is where we put our focus. Typically, it is the reactants and products. • The surroundings include everything else in the universe.

6. Definitions If a reaction results in the evolution of heat, energy flows out of the system and into the surroundings. These reactions are exothermic. The energy lost by the system must be equal to the energy gained by the surroundings.

7. Internal Energy The internal energy (E) of a system is the sum of the kinetic and potential energies of all of the particles of the system. It is generally not possible to determine the internal energy of a system, but we can measure changes in internal energy. Internal energy is changed by the flow of work and/or heat.

8. Internal Energy & the 1st Law The First Law of Thermodynamics states that: The total internal energy of an isolated system is constant. However, it is not possible to completely isolate a system from its surroundings.

9. Internal Energy Since energy may flow to or from the surroundings and the system, we are concerned with energy changes rather than the absolute value of the internal energy. ΔE = Efinal - Einitial

11. Internal Energy Internal Energy is a state function. That is, it depends solely on the present state of the system, and not how it may have gotten to a particular state. A state function is independent of pathway.

12. Internal Energy Internal energy (E) is a state function, and depends only on the state of the system, and not how it got to that state.

13. Chemical Reactions and Energy Bond breaking always requires energy. Bond making always releases energy. In exothermic reactions, more energy is released in forming the products than is used in breaking apart the reactants.

14. Exothermic Reactions • The heat that is released comes from the potential energy stored in the bonds of the reactants.

15. Definitions If a reaction involves the absorption of heat into the system it is an endothermic reaction. More energy is required to break the bonds in the reactants than is released in forming the products.

16. Endothermic Reactions

17. Chemical Reactions & Work In addition to the release or taking in of energy (heat) when bonds are broken and formed, chemical reactions can do expansion work. Expansion work is done when the volume of reactants differs significantly from the volume of reactants.

18. Work Expansion work results when a reaction produces more gaseous products than reactants, and thus pushed back the atmosphere as the reaction proceeds. If the volume of the system contracts during reaction (more gaseous reactants than products), work is done by the surroundings on the system

19. Expansion Work Usually we just consider the volumes of gases in a chemical reaction. 2 H2O(g)  2 H2(g) + O2(g) Since 2 moles of gaseous reactants produce 3 moles of gaseous products, the system expands, and does work in pushing back the atmosphere.

20. Expansion Work

21. Expansion Work work = Force x Distance Pressure = Force/Area, or Force = Pressure(Area) work = Pressure(Area) x Distance work = Pressure(Area) x ∆h work = Pressure (length x width) x ∆h work = P ∆V

22. Expansion Work work = P ∆V If gases are produced by a reaction and the volume expands, the system is doing work on the surroundings. The sign, when considering the system, must be negative. So, work = - P ∆V

23. Energy, Work and Heat As the system loses energy to the surroundings, it can do so by losing heat (q), and/or doing work (w). As a result, ΔE = heat + work ΔE = q + w ΔE = q -PΔV

24. Energy, Work and Heat ΔE = q –PΔV Solving for q, the heat change, q = ΔE + PΔV

25. Energy and Enthalpy Chemical reactions are sometimes carried out in a sealed vessel with a fixed volume, a bomb calorimeter. In this apparatus, the volume of the reaction mixture (system) cannot change. As a result, at constant volume, qv = ΔE

26. Energy and Enthalpy Many chemical reactions are carried out in an open vessel. In this case, the reaction is performed at constant pressure, and the volume of the system is free to change during the course of the reaction. The heat transferred at constant pressure, qp, is defined as: qp = ΔE + PΔV

27. Energy and Enthalpy qp = ΔE + PΔV Since an open vessel is such a common apparatus, the heat transferred at constant pressure is given its own name, the enthalpy change, ΔH. qp = ΔE + PΔV = ΔH

28. Enthalpy Enthalpy is a state function, and is independent of reaction pathway. ΔH = Hfinal-Hinitial ΔH = Hproducts-Hreactants

29. Sign Conventions Our focus will always be on the system. If energy flows out of the system into the surroundings, it will have a negative (-) sign. If energy flows from the surroundings into the system, it will have a positive (+) sign.

30. Energy, Heat & Work

31. Standard Conditions Many reactions are categorized by their standard enthalpy change, ΔHo. The degree sign indicates standard conditions. Standard conditions specify that the reactants and products are in the same molar amounts represented by the coefficients in the balanced chemical reaction.

32. Standard Conditions In a given experiment, the quantities of reactants and enthalpy change will vary, but the standard enthalpy change is reported based on molar quantities. The enthalpy of a reaction will also vary with the physical states of reactants or products as well as temperature and pressure.

33. Standard Conditions A thermodynamic standard state refers to a specific set of conditions. The standard is used so that values of enthalpy changes can be directly compared. The standard state is the most stable form of a substance at 1 atm pressure and 25oC.

34. Standard Conditions • Standard conditions are indicated using a degree symbol ( o ). Standard conditions for thermochemical data differ from the standard conditions used in the gas laws. 1. All gases have a pressure of exactly 1 atm. 2. Pure substances are in the form that they normally exist in at 25oC and 1 atm pressure. 3. All solutions have a concentration of exactly 1M.

35. Standard Conditions For example, since oxygen is a diatomic gas at 25oC, the standard state of oxygen is O2(g) at a pressure of 1 atm.

36. Calorimetry Calorimetry is the science of measuring heat. It typically involves measuring temperature changes as a substance loses or gains heat. Since substances vary in how much their temperature changes as heat is lost or gained, it is important to know the heat capacity (C) of substances involved in the reaction.

37. Heat Capacity (C) The heat capacity of a substance is the amount of heat absorbed, usually in joules, per 1 degree (C or K) increase in temperature. The amount (mass) of the substance also determines the amount of heat lost or gained. C = heat absorbed = _q_ increase in temp. ΔT

38. Heat Capacity The specific heat capacity is for a gram of a substance. It has the units J/oC-g or J/K-g. The molar heat capacity is for a mole of a given substance. It has the units J/oC-mol or J/K-mol.

39. Calorimeter Constant In measuring heat changes during a reaction, any heat absorbed or lost be the calorimeter (the apparatus itself) must be considered. If this amount of heat is significant, the calorimeter constant may be provided or measured. This is the heat capacity of the specific apparatus used, and is expressed in J or kJ per degree change in temperature (K or oC).

40. Coffee Cup Calorimetry A simple device for determining heat changes of aqueous reactions at constant pressure is a coffee cup calorimeter. Since the contents are open to the atmosphere, the pressure, atmospheric pressure, remains constant during the reaction.

41. Coffee Cup Calorimetry The heat change for the reaction, qp, is equal to the enthalpy change for the reaction. If heat is given off, it goes towards warming up the contents of the calorimeter and toward warming up the calorimeter walls, thermometer, stirrer, etc.

42. Coffee Cup Calorimetry qreaction = qcontents + qcal qcontents = (mass of solution) (ΔTsoln)Csoln Csoln is the specific heat capacity of the reaction mixture. If solutions are aqueous and fairly dilute, the specific heat capacity of water, 4.18J/oC-g, may be used.

43. Coffee Cup Calorimetry qreaction = qcontents + qcal qcal = Ccal (ΔT) Ccal is the calorimeter heat capacity. It includes the heat needed to warm up the walls, thermometer and stirrer of the calorimeter, along with any heat loss due to leaks. In many simple calculations, Ccal is assumed to be negligible, and may be ignored.

44. Obtaining ΔH of Reaction The enthalpy change of a reaction, ΔH, can be obtained from qp. First, a sign must be assigned. If the temperature increased during the reaction, the reaction is exothermic, and q is negative. If the temperature decreased during the reaction, the reaction is endothermic, and q is positive.

45. Obtaining ΔH of Reaction The enthalpy change of a reaction, ΔH, can be obtained from qp. Also, qp is for a specific quantity of reactants. Typically, ΔHrxn is for molar quantities of reactants. To calculate ΔHrxn from qp, you must calculate the heat change per mole of reactant.

46. Problem: Calorimetry • A coffee cup calorimeter contains 125. grams of water at 24.2oC. A 10.5 g sample of KBr, also at 24.2oC, is added. After dissolving, the mixture reaches a final temperature of 21.1oC. Calculate ∆Hsoln in joules/gram and kJ/mol. Assume the specific heat of the solution is 4.18 J/g-oC, and no heat is transferred to or from the calorimeter or surroundings.

47. Constant Volume Calorimetry • Certain reactions, notably combustion reactions, do not lend themselves to open vessels. These reactions are usually carried out in a sealed reaction vessel called a bomb calorimeter. The bomb calorimeter is a rigid steel container that is sealed after the reactants have been added. The reaction takes place once an electrical current is sent through an ignition wire to the reaction mixture.

48. Bomb Calorimetry The steel bomb is immersed in an insulated bath containing either water or mineral oil. As the combustion reaction releases heat, the heat is transferred to the bath.

49. Bomb Calorimetry Reaction vessel Ignitionwire O2 inlet gaskets

50. Bomb Calorimetry Once the bomb has been charged with reactants, it is placed in the water or oil bath until it reaches a constant temperature. A current is sent through the ignition wire, and the combustion reaction takes place. The heat given off by the reaction is evident from the increase in temperature of the water/oil bath.