Boiling points and Intermolecular Forces Copy this before class do: pg 105 # 44-50, on the back of the worksheet. For this—go into atmosphere has pressre, what boiling is, and intermolecular force, evaporation For this—go into atmosphere has pressre, what boiling is, and intermolecular force Goal; through slide 10

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For homework: read p. 400-401 in textbook. The homework assignment was: For homework: read p. 400-401 in textbook. Explain how molecules of water evaporate off of a sidewalk at 39 degrees F. Also, you must learn to “see” with your imagination—to see what your eyes cannot. There is an energy system here. The particles in the puddle are surrounded by molecules that have a temperature of 39 deg F. Remember this: You have learned enough to figure this out. The facts we will use: 1. the law of conservation of energy 2. heat energy transfers—warm to cold. 3. temperature is a measure of average KE

Answer: It goes down. Why? Question: Ok, sticking to water, how do you suppose water can evaporate from a sidewalk at 40 degrees F? Answer: Some molecules are moving fast enough to escape. It has a vapor pressure. Question: What happens to the temperature of the liquid on the sidewalk? Answer: It goes down. Why? Question: What happens next to the temperature? Answer: It goes up because it is now cooler than the environment and absorbs heat. What happens next? The process repeats!! This process is called evaporative cooling

It can be understood based on a simple average. But, a look at the molecules reveals that the temperature is an average of: 10, 20, 50, 70, 100 = 250/5 = 50° But if the 100 speed particle leaves the liquid, the new average is: 10 + 20 + 50 + 70 = 150/4 is 37.5° So the liquid is now colder than the environment so it absorbs energy and that produces more fast moving molecules—which leave and the process repeats.

More molecules are escaping at higher temps. What then, is the relationship between the temperature and the vapor pressure of a liquid? Easy. As the temperature of a liquid ___________, the vapor pressure _________________. Why would this be? More molecules are escaping at higher temps.

They evaporate at different rates!! Have you noticed how nail polish remover seems to evaporate really easily? So does gasoline. Paint thinner? If you pour the same amount of water on a floor as gasoline the gasoline evaporates faster! Why? They evaporate at different rates!!

Factors that affect evaporation: Evaporation is the vaporization of a liquid which occurs below the boiling point. It is the process by which molecules in the liquid state enter the gaseous state. So what affects evaporation? Factors that affect evaporation: The intermolecular force between the molecules. The temperature. The amount of surface area. Water evaporates from a puddle faster than from a bottle. Wind—can sweep the molecules of vapor away.

Factors effecting Vapor pressure copy Factors effecting Vapor pressure (They are slightly different because now we are talking about a closed container) The intermolecular force—the less it is the easier it is to vaporize the liquid. The temperature Surface area-has no effect on vapor pressure. What’s dissolved in it—more on this later.

So now we know—different substances have different vapor pressures So now we know—different substances have different vapor pressures. For example at 40°C which has the highest and the lowest vapor pressure? Can you think why? Due to im force!! So if you were asked for the substance with the least intermolecular force, what would you pick?

So then, what is a boiling point? The boiling point of a liquid is the temperature at which the vapor pressure is equal to the external pressure—which is usually the atmospheric pressure. What happens during boiling? The liquid has the ability to form bubbles of vapor, which then rise to the surface and release the gas. As you know, these bubbles of separated molecules form rather than the temperature rising. It is evaporation with bubbles. It is the maximum temperature at which evaporation occurs. So what makes this happen? Look again at the graph. “opposing” pressure forces!!

So what’s the difference between boiling point and evaporation? Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules located near the liquid's edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Why substances boil…… Notice the line here is at one atmosphere Please be careful!! This happens at different temperatures! Under normal conditions, the temperature rises until the external pressure from the atmosphere is overcome by the vapor pressure of the liquid—then the substance boils.

Water can be boiled using ice!!! How do you explain this? Water can be boiled using ice!!! Hyperlink for boiling water using ice Alternate website for boil water without heat.

How did that work? This is similar to why water boils at a lower temperature on Mount Everest. You have lowered the pressure that is preventing the boiling!!

A closer look at the situation.

A new boiling point is established We have lowered the pressure, so water boils more easily. The vapor pressure necessary to boil is lower. So: New pressure line Means that: A new boiling point is established 70 What happens if the pressure above the liquid is increased?

So how does the ice trick work So how does the ice trick work? In the ice trick, we lowered the pressure by cooling the molecules above the liquid!! So a new lower boiling point was established.

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Hyperlink for boiling water using ice Hyperlink for soda pop that instantly freezes. Hyperlink for a seed crystal in a supersaturated solution Hyperlink for liquid nitrogen exploding a soda bottle like a rocket. For this—go into atmosphere has pressre, what boiling is, and intermolecular force Note: go back to this site to check out the magic trick—the vanishing water trick—try to find the answer!

Animation of vapor pressure equilibrium

For homework: read p. 104 in review book on Vapor Pressure. Do # 44, 48, 49. Also—answer this:

Copy before class Read page 104-105 in handout. Do 44-50 page 105.