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Unit 10: Thermodynamics
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Review: A 15 gram piece of metal was cooled in a freezer to oC and added to a room temperature (22oC) water bath containing 75 grams of water. The temperature of the water decreased to 20oC Identify the metal by solving for it’s specific heat capacity.
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What kinds of processes can have an enthalpy?
1. Chemical reactions: called “reaction enthalpy” or “enthalpy of reaction” (∆Hrxn or simply ∆H) 1a. Combustion reactions: called “enthalpy of combustion” (∆Hcomb or simply ∆H) 2. Dissolving: called “enthalpy of solution” (∆Hsoln)
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What kinds of processes can have an enthalpy?
3. Phase changes (melting, evaporating, etc.) 3a. Melting: called “enthalpy of fusion” or “heat of fusion” (∆Hfus) H2O (s) H2O (l) ∆Hfus = + 6 kJ/mol Why don’t you need to list the “enthalpy of freezing” for water?
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What kinds of processes can have an enthalpy?
3. Phase changes (melting, evaporating, etc.) 3b. Evaporating: called “enthalpy of vaporization” or “heat of vaporization” (∆Hvap) H2O (l) H2O (g) ∆Hvap = kJ/mol So for condensation: H2O (g) H2O (l) ∆H = ???
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Example 1: How much heat must be transferred to convert 10 grams of methanol vapor into liquid methanol at it’s boiling point? The ∆Hvap or methanol is 35.3 kJ/mol.
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Phase Changes: Predicting ∆H sign:
Just like chemical reactions and dissolving, phase changes also involve heat transfers Predicting ∆H sign: Melting: H2O(s) H2O(l) ∆H = Condensing: CH4(g) CH4(l) ∆H = Subliming: CO2(g) CO2(s) ∆H =
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Phase Change Diagram for Water
As E is added to ice the T increases, during the phase change T stays constant, then once it’s all melted the T increases, during the phase change T stays constant, then once it’s all vaporized the vapor’s T increases.
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Phase Change Diagram for Water
Example 2: Calculate the thermal energy that must be transferred to convert 200 g of ice at -50oC to water at 75oC.
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Practice: Try # 1c, 3 and 7 NOW!!!
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Practice: Try # 1c, 3 and 7 NOW!!!
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Practice: Try # 1c, 3 and 7 NOW!!!
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Practice: Try # 1c, 3 and 7 NOW!!!
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Practice: Try # 1c, 3 and 7 NOW!!!
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Practice: Try # 1c, 3 and 7 NOW!!!
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