Tuesday, 8 December 2009

Change of state; latent heat

Three state of matter: solid, liquid and gas. (Ionized gas or plasma is the fourth state.) Heating process: Heat until solid reaches melting point (MP). It absorbs latent heat of fusion and melt (fusion). Heat that liquid until it reaches the boiling point (BP). It absorbs latent heat of vaporization and vaporizes. Cooling process: Cool down some gas until it reaches boiling point. It releases latent heat of vaporization and condenses. Cool down that liquid until it reaches the melting (freezing) point. It releases latent heat of fusion and freeze(solidification). Cooling curve: when we cool down some liquid/gas, the behavior of temperature is: cool down in a concave curve, stays constantly in the BP/MP, after it is totally condensed/frozen, it cools down in a curve again. *In the experiment about cooling down octadecan-1-ol, we can data-logger to record the cooling curve. Latent heat is the energy released or absorbed when a substance changes its state without changing it’s temperature. The unit of latent heat is Joule(J). Latent heat of vaporization is energy released or absorbed when a substance changes its state from liquid to gas (or gas to liquid) without changing it’s temperature.*(1) Latent heat of fusion is energy released or absorbed when a substance changes its state from liquid to solid (or solid to liquid) without changing it’s temperature.*(2) Specific latent heat is the energy required to change the state of 1kg of substance without changing its temperature. The uinit is Jkg^-1 Specific latent heat of vaporization the energy required to change the state of 1kg of substance between gas and liquid without changing its temperature. *(3) Specific latent heat of fusion the energy required to change the state of 1kg of substance between solid and liquid without changing its temperature.*(4)We use the symbol (1) L_v, (2)L_f , (3)l_v , (4)l_F to represent those physical quantities. (Note: L_v means that the "v" is written in the right-bottom corner of L, etc.) We have l=Q/m=L/m=Pt/m. This formula is the way to calculate the specific latent heat through a graph. Important specific latent heat: vaporization 2.26*10^6 Jkg^-1 fusion: 3.34*10^5 Jkg^-1 *experiment of measuring specific latent heat of fusion Experimental set-up: electrical supply -> Joulemeter->immersion heater fully inserted into the crushed melting ice(to ensure that it’s 0˚C) , in a filter funnel. Under the filter funnel there’s a beaker measuring amount of water being melted. Control set-up: immersion heater without electrical supply, ceteris paribus. By l_f=Q/m , Specific latent heat of fusion = energy transferred/mass of ice melted. Note that “mass of water melted” is equal to “water melted in the experimental set-up minus water melted in the control set-up” to ensure that the “mass of water melted in the formula” is about “ice melted by the energy from the heater”. Energy transferred can be obtained from the difference between initial and final reading of Joulemeter. Assumption: 1) No energy was absorbed by the ice from the surrounding air. 2) All energy from the heater is absorbed by the ice. *experiment about measuring specific latent heat of vaporization Electrical supply->kilowatt-hour meter->immersion heater fully immersed into a beaker of water on a triple beam balance. By l_v, Specific latent heat of vaporization = energy transferred/amount of water vapourized. Assumption: 1) All energy from the heater is transferred to the water. 2) No energy lost from heater/water to the surroundings. 3) No water is split out before vaporized and condensed back to the beaker. Evaporation is the process that changes liquid below BP (boiling only occurs at BP) to gas on the surface of the liquid (Boiling occurs through out the liquid). Evaporation in terms of molecular motion: In the liquid molecules moving in wide range of speeds and hence containing a wide range of kinetic energy. When they collide each other, some gain energy and some lose energy. When some of them obtain enough KE, they escape from the liquid and become vapour. As those escaped molecules contain more kinetic energy than the liquid, the average kinetic energy of the remaining liquid decreases and therefore evaporation gives a cooling effect. Condensation: warm air can hold more water (vapour) than cold air. When warm air is cooled suddenly, some of the vapour condenses. Since they releases latent heat of vaporization during condensation, it rises the temperature on the object which it condenses. Missing table: factors affecting vaporization notes available. Sorry for putting so much thing into 2 pages. I'll try to put them in a better way after try to add some extra concepts.

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