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handout03 - 2.12 States of Matter Matter can be transformed...

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Unformatted text preview: 2.12 States of Matter Matter can be transformed by heat such that it changes its phase . This phenomenon can be explained by considering the relative sizes of the kinetic vs. the potential energy contributions to the Hamiltonian: H ( ~ x , ~ p ) = V ( ~ x ) + K ( ~ p ) . (2.60) fluid gas solid melting boiling freezing condensing phase changes solid: V ( ~ x ) dominates the particles remain close to a grid-like arrangement (thermal motion around a fixed location) fluid: K ( ~ p ) is big enough so that particles can leave the grid 223 positions but they still interact strongly (E.g. solvation shells) fi gas: K ( ~ p ) dominates the particles move freely. The interparticle distance is larger than in solid or fluid phases. QUESTION: How much energy does it take to heat up a material? ANSWER: To good approximation, this is a linear relationship, when no phase change is involved : Q = C p T , (2.61) where Q is the heat required, C p is heat capacity of the material at constant pressure, T is the change in temperature. 224 material C p h Joule mol K i ice 38.08 water 75.33 steam 37.47 iron 25.10 paraffin wax 900.00 diamond 6.12 NOTE: It depends on how we heat up the material: constant pressure: use C p constant volume: use C v NOTE: There is an energy associated with changing the phase of a material without changing its temperature. This energy is called latent heat . 225 EXAMPLE: We heat one mole of water to 120 C, starting from- 5 C. We keep track of how much heat is required at each stage. 20 40 60 80 100 120 10 20 30 40 50 60 Temperature [C] Heat [kJ] Heating One Mole of Water at Constant Pressure heating melting vaporising transition L h Joule mol i water: solid to fluid 6012 water: fluid to gas 45000 NOTE: The latent heat of the fluid to gas phase transition is particularly large. 226 EXAMPLE: Fhn (Alps) or Chinook (North America) winds: wet winds blown over a mountain range will heat the air up (latent heat of condensation). On the windward side: lots of rain On the lee side: warm, dry wind (often strong, gusty) B: wet wind dry, warm air condensation heats the air dry, cold air A: dry wind In Canada: Southern Alberta climate (Lethbridge) especially affected by Chinook winds....
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handout03 - 2.12 States of Matter Matter can be transformed...

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