CH%20301%20Chapter%206%20notes%20part%202v2

CH%20301%20Chapter%206%20notes%20part%202v2 - CH301 Notes...

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CH301 Notes Chapter 6 part 2 Enthalpy Chemistry is done in open beakers at atmospheric pressure. Pressure changes are small – we assume constant pressure. Enthalpy change H: the amount of heat transferred in to or out of a system undergoing a chemical or physical change at constant pressure . Also in use: “heat change” or “heat of (process)” We CANNOT measure absolute values of H, so: H = H final – H initial or H = H products – H reactants H determines change in heat content at constant pressure . Shows if process (physical (e.g. condensation) or a chemical reaction (e.g. combustion)) produces or consumes heat. If H < 0 the process is EXOTHERMIC. If H > 0 the process is ENDOTHERMIC. Enthalpy of Physical Processes (Includes CHANGES OF STATE: ) Melting (Fusion) / Freezing; Boiling (Vaporization)/ Condensing; Sublimation/ Deposition H = H final - H initial For example, H fusion = H liquid - H solid Why not: U of Physical Processes? All occur at constant pressure : quote H not U. NOTE: H for a given process usually varies only slightly with temperature. e.g., H vap (enthalpy change of vaporization) of water at 100ºC and 25ºC are similar H values for physical processes -Table 6.3, page 217 UNITS: kJ/mol (check your units!) ALL VALUES SHOWN ARE POSITIVE: They are for the ENDOTHERMIC processes. The H for an EXOTHERMIC process is just the negative of the reverse process. e.g., H condensation = - H vaporization For heating a substance , use one of the heat capacity/specific heat equations, e.g. q = mC s T where C s is in J/g°C or J/gK* For a phase change use: q = n H where H vap or H fus is in J/mol* *See earlier: units on given values may vary between g and mol, be able to convert. *Remember a change of 1°C and 1K are identical. Heating / Cooling Curves Must break calculation into steps of changes of state, temperature: During change of state, TEMPERATURE is CONSTANT. The SLOPES of temperature vs. heat supplied varies with the heat capacities of each state of matter. The energy required to melt a substance is much less than that required to vaporize a substance. Why?? Example 1: H fus of benzene is 10.59 kJ/mol. I have 47g of liquid benzene at its melting point, 278.6K. If I remove 2000J of heat energy from this sample, how much liquid benzene is left?
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Example 2: What will be the final temperature of 20L of water at room temperature (25°C) if 200g of ice at –1 ° C is dropped into it? Example:3 Heat of Fusion vs Heat of Vaporization Compare the energy given out to your arm if its exposed to: A. 5g of boiling water. B. 5g of steam Assume water ends up at body temperature, about 38ºC H fus water = 6.01kJ/mol H vap water = 40.7kJ/mol C s water = 4.184 J/gºC Relationship of H and U The total amount of heat energy that system can provide to surroundings : H= U + P V H = change in enthalpy of system U
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This note was uploaded on 09/09/2009 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas.

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CH%20301%20Chapter%206%20notes%20part%202v2 - CH301 Notes...

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