lecturenotes_sept30th2010

lecturenotes_sept30th2010 - Lecture notes for Lecture#2...

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Lecture notes for Lecture #2, September 30 th , 2010 Definitions We should now be very familiar with the following expressions and their meanings: ∆E thermal = C∆T The change in thermal energy that a substance undergoes due to a transfer of energy (to the substance or from the substance) in the form of heat. ∆E bond = ∆m∆H The change in the substance s bond energy as a phase transition occurs. ∆E total = ∑∆E i A statement of the law of conservation of energy; that the change in the total energy of a system is equal to the sum of the changes in all of the energy systems that makeup or interact within the overall system, which may be open or closed to the environment. ∆E total = ∑∆E i =0 This condition describes a closed system. ∆E total = ∑∆E i = Q + W, This condition describes an open system where energy in the form of heat and/or work is transferred outside the system to the environment. Heat Capacity . This is a property of the substance. It can be calculated according to C = m x c, where c is the specific heat. Specific Heat . T he specific heat of a substance is a “look up” value. Common specific heats, or at least the ones you will need for this course are given on page 5 (pink page) of your textbook. It is also commonly referred to as the amount of energy transferred in the form of heat required to raise the temperature of a gram of a particular substance, one degree centigrade. Enthalphy Also called heat of fusion or heat of vaporization. It is a “look up” v alue. Enthalpies for common substances are given on page 5 (pink page of your textbook). Units ∆E total , ∆E bond , ∆E thermal , all represent energy changes, and so we expect the units for these expressions to be in Joules (which is the unit of energy). Heat capacity is given by m x c , where m is the mass, and c is the specific heat. So the units are [energy/temperature] Specific heat is given in units of [energy/mass-temp] Enthalpy , or heats of fusion (or vaporization) has units of [energy/mass] Understanding Heat Capacity We saw in class that the heat capacity of a substance has a graphical interpretation. In fact, when we closely examine Temperature versus Added Energy graphs, also known as
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Heating Curves, we can directly see the effect of heat capacity; it is related to the inverse slope of the parts of the curve that represent thermal energy changes. In the interval between A and B we see that the slope of the line connecting A and B is steeper than the slope of the line connecting C and D. Why is that the case? Recall that for solid H2O (ice), the look-up value for specific heat is 2.05 KJ/kg-K. Therefore, for 1.0 kg of solid H2O we calculate a total heat capacity as follows: Heat Capacity (ice) = mass (ice) X specific heat (ice) = 1.0 (kg) X 2.05 KJ/kg-K = 2.05 KJ/K Similarly, recall that for liquid H2O (water), the look-up value for specific heat is 4.18 KJ/kg-K. Therefore, for 1.0 kg of liquid H2O, we calculate a total heat capacity as
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lecturenotes_sept30th2010 - Lecture notes for Lecture#2...

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