080103__CHE_101_Full_Class_Notes_5

080103__CHE_101_Full_Class_Notes_5 - 3.2 ENERGY BALANCES ON...

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3.2 ENERGY BALANCES ON NON-REACTIVE SYSTEMS We will now investigate methods to estimate specific internal energy and enthalpy changes when tables of those properties are not available. We will focus on on-reactive systems including situations where there is temperature change and pressure change as well as phase changes. Mixing will also be studied. 3.2.1 STATE PROPERTIES / HYPOTHETICAL PROCESS PATHS We will now examine methods to estimate U and H what tables of these properties are un- available. What we will do is use a known reference value for enthalpy and then, with a hypothetical process path, determine the change in enthalpy or internal energy. EXAMPLE Calculate H for ice being turned into steam (from -5°C to 300°C and 5 atm) First we will learn how to calculate each of the enthalpy changes. Then we will use them in energy/ mass balance situations. Changes in pressure at constant temperature For solids and liquids (incompressible), ΔU ~= 0, ΔV hat ~= 0 So H = U +Δ(P V ) = U +ΔP V + PΔ V = ΔP V (usually small, only significant for large changes in pressure). Ideal Gasses 1
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H = U +Δ(P V ) = ΔU hat +Δ(nRT) ; By definition and Ideal gases do not interact differently due to pressure so H = U = 0 Real gasses however, you need to consider what impact pressure has on U and therefore H. All the above is saying is that for the most part we will consider U and H to not be functions of pressure. Changes in temperature DEFINITIONS Sensible Heat : Heat that must be transferred to raise or lower the temperature of a substance or a mixture of substances. Heat Capacity at constant volume (C v ) What happens if both T and V are changed for an ideal gas? How can we calculate Uhat?? 2
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Heat capacity at constant pressure (C p ) What if P and T are changed for an ideal gas? Liquid? Side note:
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080103__CHE_101_Full_Class_Notes_5 - 3.2 ENERGY BALANCES ON...

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