Unit 10: Thermodynamics

Review: A 15 gram piece of metal was cooled in a freezer to -25oC 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.

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)

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?

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 = + 40.65 kJ/mol So for condensation: H2O (g)  H2O (l) ∆H = ???

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.

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 =

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.

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.

Practice: Try # 1c, 3 and 7 NOW!!!

Practice: Try # 1c, 3 and 7 NOW!!!

Practice: Try # 1c, 3 and 7 NOW!!!

Practice: Try # 1c, 3 and 7 NOW!!!

Practice: Try # 1c, 3 and 7 NOW!!!

Practice: Try # 1c, 3 and 7 NOW!!!