Enthalpy

1. Enthalpy By: Sarah Foley, Catherine Chang, Yebin Chae

2. What is a State Function? • Key concept: A state function is a property whose value does not depend on the path taken to reach that specific value. • Example: Density – A substance's density is not affected by how the substance is obtained. If one has a certain amount of H2O, it does not matter whether that H2O is obtained from a tap, well, or bottle, because as long as all three have the same states, they will have the same density.

3. State functions (continued) • Note: When deciding whether a certain property is a state function or not, keep this rule in mind: is this property or value affected by the path or way taken to establish it? If the answer is no, then it is a state function, but if the answer is yes, then it is not a state function.

4. State Function Practice Problem • PRACTICE PROBLEM: Is Dr. Williams going from the 1st floor of Chemhall, to the 9th floor of Chemhall, the same thing as going from the 1st floor of Chem hall, to the 3rd floor, to the5th floor, to the 9th floorof Chemhall?

5. Answer to state function practice problem • Yes, it is because the question describes a state function. Your position is dependent only on the final and initial position, which are respectively 9th floor of Chem hall and 1st floor of Chem hall, and not on the path or way taken to get there.

6. What Is Enthalpy? • The heat content of a system at constant pressure. • Includes the internal energy and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. • Key Terms: • Heat(q) - energy that transfers from one objectto another due to their temperature differences • System- the part of the universe on which one focuses his or her attention on. • Internal Energy – energy required to create a system

7. Enthalpy’s relation to state functions • Enthalpy itself is a state function • It only depends on the initial and final conditions and not on the path taken to establish these conditions. • In essence, you can take the integral of state functions using only 2 values: the final and initial value.

8. Additional concepts and terms • Enthalpy is an extensive property. • This means that, for homogeneous systems, the enthalpy is proportional to the size of the system. • Key terms: • Extensive property- a property that depends on the amount of matter in a sample • Homogenous – components are easily distributed and not easily distinguished.

9. Enthalpy Formula • Formula: Hsys = Esys + PV • where: • H is the enthalpy of the system • E is the internal energy of the system • P is the pressure of the system • V is the volume of the system

10. Enthalpy Representation • Represented by H • The actual change in enthalpy is symbolized with ΔH(Delta H) • We will come back to change in enthalpy (enthalpy change)

11. Enthalpy Measurement • The unit of measurement in the International System of Units (SI) for enthalpy is the joule • However, other historical, conventional units are still in use, such as the small and the large calorie.

12. Enthalpy Change • Key Concept: Enthalpy Change is the heat released or absorbed by a reaction at constant pressure and is represented by ΔH (delta H). • Actual enthalpy of a system cannot be measured directly. • Enthalpy Change is used. • If the initialand final states have enthalpies Hiand Hf, respectively then the enthalpy changebecomes • ΔH = Hf - Hi

13. Enthalpy (continued) • Note:  The term "enthalpy change" only applies to reactions done at constant pressure. That is actually how most lab reactions are done - in tubes or flasks open to the atmosphere, so that the pressure is constant at atmospheric pressure.  

14. Enthalpy Change Measurement • Enthalpy change is defined by the following equation: • where: • q= heat flow • Delta H=enthalpy change • m=mass Note: Heat and Enthalpy Change are essentially interchangeable q = ΔH = m x C x ΔT • C=specific heat • Delta T=change in temperature

15. Diagraming Enthalpy Changes • Horizontal lines correspond to different absolute values of enthalpy, H. • A horizontal line drawn higher in the diagram represents a larger value of H. • The vertical distances between the lines show the changes in enthalpy. • The total decrease in energy is the same regardless of which path is taken, so the total energy evolved in the two-step path has to be the same as in the one-step reaction.

16. Enthalpy usage in real life • Example 1: Refrigerators • Change in enthalpy is useful for measuring processes in refrigerators. Refrigerators chill food by evaporating refrigerants such as freon. • Freon goes through a series of coils in the refrigerator and is eventually vaporized. • The cold gas is sent through another set of coils and absorbs heat resulting in a lower temperature inside the refrigerator, allowing the food to be chilled!

17. Enthalpy Usage in Real life • Example 2: Hand Warmers • Chemical hand warmers are heated by the reaction of iron to oxygen, or exothermic oxidation. When you crush the pack, the reactants inside are exposed to air and begin heating up. When one shakesthe pack several times until it starts heating your hands, this is an example of change in enthalpy.

18. Standard State • Key Concepts: Standard State is a common reference point that one can use to compare thermodynamic properties. • The quantity of heat absorbed or released during a reaction varies with the temperature • → so standard state has been adopted by scientists as 25 ºC 25 ºC= 1 atm = 101.3 kPa

19. Standard enthalpy Change • Also known as standard enthalpy of formation or the molar heat of formation • It is the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states as 25 ºC

20. Standard enthalpy Change (Continued) • The standard enthalpy change of formation = the sum of the standard enthalpies of formation of the products - the sum of the standard enthalpies of formation of the reactants. • IMPORTANT: when a pure element is in its reference form, it is zero. Sample table:

21. Enthalpy calculation example 1 • How many J would it take to raise the temperature of 200 grams of water from 5 degrees C to 85 degrees C? (Specific heat of water is 4.18 J/g degrees C)

22. Answer to example 1 Q = (200 g)(4.18)(80 degrees C) = 66880 J

23. Enthalpy calculation example 2 • What would the specific heat capacity of a substance be if 750 J caused 100 grams of it to go from 90 degrees C to 135 degrees C?

24. Answer to question 2 750 J = (100 g)(X)(45 degrees C) X = 0.167 J/g degrees C

25. SignConvention • To avoid confusion when working with numbers, a universal convention was made for when one uses a positive sign or a negative sign. • Sign Convention for heat, q • Heat is transferred into the system q > 0 • Heat is transferred out of the system q < 0

26. Sign convention of enthalpy change • When energy flows into the system as a result of heat or work, the sign is positive (the system gains energy; an endothermic reaction) • When energy flows out of a system, the sign is negative (the system loses energy; an exothermic reaction)

27. Works Cited • Nguyen, Jonathan, and Garrett Larimer. "Standard Enthalpy of Formation." Chemwiki. University of California, n.d. Web. 31 Mar. 2014. • Billings, Allison, Rachel Morris, Ryan Starr, and AngadOberoi. "State Functions." Chemwiki. University of California, n.d. Web. 31 Mar. 2014. • Hurley, Katherine, and Jennifer Shamieh. "Enthalpy." Chemwiki. University of California, n.d. Web. 30 Mar. 2014. • Mombourquette, Michael J. "Energy, Enthalpy and Thermochemistry." Energy Enthalpy and Thermodynamics. N.p., 5 Sept. 2012. Web. 30 Mar. 2014. • "Enthalpy." Enthalpy. N.p., n.d. Web. 29 Mar. 2014.