Phase Changes and Heat

1. Phase Changes and Heat

2. Phase Changes

3. THE NATURE OF ENERGY • Energy - the ability to do work or produce heat • 2 types: potential and kinetic • Potential – stored energy • Kinetic – energy of motion

4. Phase Change Terminology • Solid (s)– volume and shape are constant • Has low KE, high PE • Liquid (l)– volume is constant, but shape is not defined • As temperature increases, KE increases and PE decreases • Gas (g)– no defined volume or shape • Has high KE, low PE

5. Lets Draw Phase Change Diagrams –Open to page 4 in your packet to draw

6. Another View of Phase Change Diagram

7. Heating Curve

8. Cooling Curve Gas Boiling Condensation Liquid Temp (C) Melting Freezing Solid High Kinetic Energy Time (min)

9. In Words… • Melting point – temperature at which a solid changes to a liquid • Freezing point – temperature at which a liquid changes to a solid • Boiling point – temperature at which a liquid changes to a gas • Condensation point – temperature at which a gas changes to a liquid • Sublimation – temperature at which a solid changes to a gas • Think of dry ice

10. Phase Changes and PE • Heat energy during the melting phase is being used to overcome attractive forces between molecules • Solid phase: molecules stay in place and vibrate • Liquid phase: molecules can flow past each other • With a phase change, there is a change in potential energy (energy of position of molecules next to each other)

11. Phase Change and PE • Melting point and freezing point are at the same temperature • Melting: PE increases • Freezing: PE decreases • Boiling point and condensation point are the same temperature • Boiling: PE increases • Condensation: PE decreases

12. e d Temp. c b a Time Practice • Identify each phase and energy type (KE/PE) for sections a, c, and e:

13. Heat

14. TEMPERATURE AND HEAT • Temperature • Measure of the average kinetic energy of the particles in a substance • Heat • The transfer of energy from one substance to another due to temperature differences • Always from high E to low E

15. We can measure heat… q = m x ∆T x c • q = heat • m = mass • ∆T = change in temperature • Tfinal – T initial • c = specific heat capacity (specific heat) • The quantity of heat required to raise temp. of an object by 1oC

16. Using the formula • We need 3 out of the 4 so we can solve for the 4th q = m x ∆T x c • If have q, m, and c – what solving for? • If have m, c, and ∆T – what solving for?

17. Value of “q” • What is “q” – HEAT • q can be positive or negative • If q is positive – energy is added • Therefore reaction is endothermic • If q is negative – energy is removed/leaving • Therefore reaction is exothermic

18. You said what?? • Exothermic process: a change (ie. a chemical reaction) that releases heat. • Ie: Burning fossil fuels • Think “exit” • Endothermic process: a change (ie. a chemical reaction) that absorbs heat. • Photosynthesis is an endothermic reaction (requires energy input from sun)

19. Practice Problem –#3 on Worksheet #1 in your packet • How much heat is required to raise the temperature of 854 g of water from 23.5oC to 85.0oC? (specific heat of water is 4.184J/goC)

20. Heat Transfer

21. Heat Transfer • Heat will transfer from one object to something else • This is a transfer of energy from a place of high E to a place of low E (high temp to low temp) • The transfer of E will change the temperatures(equalizing of temperature)

22. reaction reaction Heat Transfer in Water Exothermic reaction, heat given off & temperature of water rises Endothermic reaction, heat taken in & temperature of water drops

23. reaction But what about temperature? Exothermic reaction: -temperature of water will increase -temperature of object will decrease UNTIL temperature of water and object are equal!! Therefore Tfinal for water and object are the same

24. reaction Endothermic reaction: -temperature of water will decrease -temperature of object will increase UNTIL temperature of water and object are equal!! Therefore Tfinal for water and object are the same

25. Therefore… • We can say that: q lost = q gained • But wait… one is going to be negative • Therefore we must: • - q lost = q gained

26. But what does q equal? • q = m x ∆T x c • Therefore we can say that: • - q lost = q gained • -qmetal = qwater (Lab #17) • -(m x ∆T x c) = m x ∆T x c

27. Example Problem • Worksheet 2 problem #1 • A 11.2 g piece of metal with an initial temperature of 99.9oC is added to 120.3 g of water with an initial temperature of 25.0oC. The final temperature is 28.0oC. Calculate the specific heat of the metal. Cp(water) = 4.184 J/goC

28. Heat of Fusion

29. Heat of Fusion/Vaporization • How much heat is needed to change a solid to a liquid, or a liquid to a gas? • The amount of heat needed depends on: • How much substance you have (mass) • The substance itself – but we cannot use C because we are talking about phase changes… So what do we use???

30. Formulas • Enthalpy change (ΔH): Heat released or absorbed during a chemical reaction • q = m Hfus • q = m Hvap

31. Heat of Fusion • Heat absorbed by one mole of substance in melting from solid to liquid. • How much heat is needed to melt 25.0 g of ice? (Hfus ice is 334 J/g) q = m Hfus q = (25.0 g)(334 J/g) q =8350 J

32. Heat of Vaporization • Liquids absorb heat at boiling point and become vapors. • How much heat is needed to evaporate 25.0 g of water? q = m Hvap q = (25.0 g)(2260 J/g) q = 56500 J

33. Heat of Solution

34. Heat of Solution • Heat of solution • The heat produced by a chemical reaction, or the heat required for a chemical reaction to occur • Heat is absorbed from the atmosphere or released into the atmosphere • q = m Hsolution

35. Heat of Solution • Endothermic reaction • Requires heat energy from the environment to get reaction to run • Heat is transferred from the environment to the reaction • Is a positive value • Exothermic reaction • Produces heat • Heat is transferred from the reaction to the environment • Is a negative number

36. Worksheet #3 and #4 • Enthalpy of Fusion Problems • q = m Hfusion • -mHfusion=m CT of water • Enthalpy of Solution Problems • q = mHsolution • -mHsol substance =m CT of water