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London Dispersion Forces 2 Electrons  -  + None on Evaporation This Side

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London Dispersion Forces 2 Electrons  -  + None on Evaporation This Side Still Helium London Dispersion Forces 2 electrons - + None on Evaporation this side. • ~A short lived induced dipole dipole on this side. attraction between atoms that don’t normally This side is This side is • Evaporation is a change in phase from have a dipole moment. now negative. now positive. liquid to gas, but is not the same as vaporization! • An orbital is an area of probability of an Put this atom near another atom... The positive side will • Vaporization requires you to heat the electron. - + - + attract the electrons, substance to its boiling point. • The electron does not have a uniform motion, increasing the • Evaporation can happen at much lower at least we don’t think it does. We don’t chance of poles temperatures. know what the motion of an electron is forming again. • Volatility- A measure of how easily a liquid It not only forces another atom to have poles, but the evaporates. “new” atom forces the original to keeps its poles. London Forces Is the vapor above the boiling Probability • This force is random and short lived, as the point? electrons do constantly move, and will eventually end the dipole moment. o • Flip a coin 2 times, should you get 1 heads • It is also fairly weak. • No it is not! (water vapor is not +100 C) and 1 tails? • You can tell it is really weak in helium because it • It is possible to get matter in a phase that its • Not really. There is a chance you will but, stays a gas until -268.9o C. temperature does not agree with. but it is only the highest probability. • Larger atoms or molecules (with more electrons) • It is like a solution (dissolved water in air) • You have a 50% chance of getting 1 heads 1 have stronger London forces. tails, 25% chance of getting 2 heads and • It is also possible to get liquids above or 25% chance of getting 2 tails. • With more electrons it is easier for the atom or below their freezing points. (supercooled or molecule to have its electrons unbalanced and stay superheated liquids) • Apply that to our atom… that way for an extended period of time. • Iodine (I2) is a solid at room temperature. Helium Evaporation works like this Positive nucleus with 2 Liquids have molecules moving around in them electrons in a 1 s orbital. Temperature is the average kinetic It is neutral because the Evaporation and Vaporization energy (which depends on the speed) negative electrons cancel of these molecules. If they are out the positive charges. Some are moving moving fast enough, at just Imagine a line cutting the orbital in half. faster than others! the right angle, What is the probability the electron is on some will escape either side? Molecules are the surface of the 50/50, but just like the coin flip should we held in by always expect to find 1 electron on either liquid intermolecular and turn into a gas. side? No. However, if we don’t… forces. These evaporated! 1 Why do fans/wind feel cool? Why are they a “gas” After a liquid evaporates • The majority of the water vapor from your sweat is directly around you. • Intermolecular forces determine whether • The remaining liquid is cooler. • A fan or wind pushes air from somewhere else something is a solid liquid or gas. • This is because the molecules with the most over to you, and the air that was around you • In order to have intermolecular forces you kinetic energy (heat) escaped. somewhere else. need to have multiple particles. • The water vapor that evaporated can’t condense • Water has a “cooling” effect because it evaporates. • The ones that escaped aren’t next to any back on you. other particles. • Sweat cools your body by evaporation. • This only works if it isn’t extremely humid out. • Since they have almost no intermoleluar • Provided it is not humid out. • If it is extremely humid the air from somewhere forces they have to be a gas. • Humidity is a measure of the amount of water else contains a lot of water vapor that will vapor present in the air. condense on you. Where did they go? • They are in the air around the liquid. Muggy (humid) weather Vaporization or Boiling • They are called vapors, anything that naturally is a solid or liquid under standard conditions that is currently a gas at standard conditions. • In humid weather, the water vapor in the air is • Evaporation occurs at the surface of a liquid. • If enough of them get together they will closer to its vapor pressure. • As you continually heat a liquid, the particles condense and reform a liquid. • Less net water can evaporate, and cool you inside move faster. • As more of the molecules evaporate and fill the off. • Eventually the particles move so quickly, that air around the liquid, the chance that some of they break free of all intermolecular forces and • The rate of evaporation hasn’t changed, but form gas pockets inside of liquid. them may condense increases. more water vapor is condensing than • These are always less dense than the liquid so • Provided the vapors can’t escape, the liquid will normally. reach a state where the rate of condensation and the float to the surface and escape. evaporation equal each other. • This is vaporization or boiling. Vapor Pressure Increasing Evaporation Evaporation and Vaporization • Vapor Pressure of a substance is the pressure of • Intermolecular forces play a big part. the vapor required for the rate of evaporation and Evaporation occurs at the surface condensation to be the same. • Low molecular forces mean the substance will easily evaporate. These substances are volatile. • At this pressure the substance will reach a dynamic equilibrium. • Evaporation occurs at the surface of a liquid so increasing the surface area will increase the rate If I get the • Dynamic means changing, equilibrium means of evaporation. substance hot staying the same. • Allow evaporated vapors to escape so it can’t enough • At a molecular level, constantly molecules are reach vapor pressure. evaporating and condensing. However, since these cancel out, there is not net change. • Heat the substance to increase to kinetic energy. 2 Evaporation and Vaporization This is vaporization or boiling I can force gas bubbles to form in the middle of the liquid 3.
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