The Kinetic Molecular Theory of Gas (part 1)

this is the first lesson our unit about gases we're going to start off this unit with a quick review about

kinetic

energy phase changes and states of matter that's information may be familiar to you but even if it is I think it'll still be a useful review then we're going to talk about a

theory

with a really scary name the

kinetic

molecular

theory

of gases which is a lot scarier than it sounds and it gives us some rules for how we can think about gases and finally we'll look

at a few unique properties of gases that are very different from those of solids or liquids so let's take a look at

kinetic

energy first of all

kinetic

energy is a type of energy that anything has if it's moving so a Mack truck if it's barreling down the highway it has

kinetic

energy and even a tiny little atom moving around has

kinetic

energy think about

kinetic

energy is the faster something's moving the more

kinetic

energy it has so kids sprinting down the street has more

kinetic

energy than that same kid just walking down the street likewise an atom that's moving really fast has more

kinetic

energy than that same atom that's moving very slowly the idea of

kinetic

energy is

part

icularly important when we start talking about phases of matter let's take a quick review brushing up on what we probably already know about phases of matter I have here indicated some certain representations of phases of matter I have a container and some

part

icles in it

I've got a representation of a solid a liquid and a gas let's start with a solid a solid here like all the other phases of matter is made up of

part

icles that's what these little red circles represent

part

icles can be either atoms or they can be molecules that are formed by the atoms coming together either way all things are all matter and all things are made up of

part

icles so in a solid the

part

icles as you can see are packed together really tightly they have very little

kinetic

energy they're moving around a little bit but for the most

part

they're locked in place and they're locked to their neighbors the

part

icles in liquid have more

kinetic

energy they're moving around and they're freer to move they're still locked in with their neighbors to some degree but they're swimming around they're swimming in closed proximity to the

part

icles nearby gas is on the other hand have a ton of

kinetic

energy in fact to make this picture even more

accurate what I'm going to do is I'm going to add a few arrows these arrows are going to represent the fact that these gas

part

icles are in constant motion they're moving around all over the place they're banging against each other and they're banging against the sides of the container they're in in fact these gas

part

icles are moving so quickly that a room temperature they have about an average speed of thousand miles an hour that's just how fast they're zipping

around here in this container so solid liquid and gas have increasing amounts of

kinetic

energy gases have the most

kinetic

energy they're flying around in there and they're not connected at all to their neighbors now this is a very superficial representation of what a gas looks like but often we're going to want to look at problems and think about gas conceptually in a way that will make it necessary to have some some deeper understanding about what a gases and how these gas

part

icles behave we obviously can't see gas so in the problems that we're going to do later on we have to have a way to think about it a way to conceptualize it so it's useful to set up a series of rules for how we expect gases to behave well make these rules or assumptions and then we can keep them in mind when we have to solve problems or do calculations out this list of rules is what's called the

kinetic

molecular

theory

of gases oftentimes it's just referred to as the

kinetic

theory

of gases and as I said it's a list of rules expectations assumptions of how we expect gases to behave now if a gas follows every single one of these rules we call it an ideal gas but in the real world it's very hard to come up with an example of anything that always follows all the rules we might want to think that there exists some things like a perfect student ideal student or an ideal kid but rarely that's the case almost always we find a few exceptions to the rules

and so because of that it's helpful to think of an ideal gas but in the real world none of these gases that we're going to talk about ever follow all the rules all the time at the end of this unit we'll look at some

part

icularly bad offenders gasses which break rules more more often than others do and we'll look at certain situations that cause gases to break the rules for the most

part

though when all the gases that we're going to look at now we're going to assume that

their ideal gases we're going to assume that they follow all the rules all the time and for the most

part

most of these gases only break the rules in little bits once in a while so we can safely assume that all the gases were deal dealing with follow these rules of the

kinetic

molecular

theory

so let's take a look at what some of the rules of this are and once again sometimes it's just referred to as a

kinetic

theory

I want to write down here

kinetic

molecular

theory

of gases so in

no

part

icular order let's take a look at some of the assumptions that we make about gases so here's the first one gases consist of very small

part

icles that are far a

part

relative to their size this is something that's very difficult to depict visually and I certainly didn't do a good job of it in the phase and the phase diagrams that I just showed you gas

part

icles in the in the picture that I drew you look like they're the size of marbles in a glass jar this isn't true

at all gas

part

icles are so tiny that instead of thinking of them as marbles and glass jar if the gas

part

icles are the size of marbles our container would be like the size of a football stadium so these guys are absolutely tiny and there's a ton of empty space between them that's the first thing that we want to keep in mind when we're dealing with gases here is a second thing and this is very important gas

part

icles are in constant random motion we hinted about this earlier with

those arrows that I drew the gas is moving around the moving

part

icles constantly collide with each other and with the walls of the container so all the time whenever I have gas in any sort of container or even if it's just in a room these guys are zipping around they're bouncing against the walls and they're bouncing against each other now let's think about those balances a little bit more there are a variety of ways for things to bounce into each other and here we say that

collisions between gas

part

icles and container walls are elastic collisions what's an elastic collision let's think about two balls of slime these two balls of slime on either sides of me come together and what's going to happen they're just going to hit each other in the girl this is what we call an inelastic collision that means that the

kinetic

energy that both of these guys had got wasted in the collision these guys were both moving they came together and they just kind of

went lap and all the

kinetic

energy to speed the motion that they had disappears in the collision this is like what happens if you chuck an egg against the side of a wall it hits the wall and then it just drips down but the motion that it had the

kinetic

energy disappears that's an inelastic collision the collision between gas

part

icles and container walls on the other hand are elastic collisions a good way to think about an elastic collision is think about what happens when to red or pink

round rubber balls hit each other they hit and they bounce right off or one of those pink rubber balls hits the side of a wall BAM it bounces right back the

kinetic

energy isn't wasted in the collision this ball hits here and it has the same amount of

kinetic

energy afterwards that it had when it started that's the exact kind of collision that gas

part

icles get into they bang into each other and they just fly right a

part

or they hit the side of a container wall and they just bounce right

off it so whenever you think about gas

part

icles colliding you always want to keep in the in in mind the idea of elastic collisions additionally we can say that there are no forces of attraction or repulsion between gas

part

icles some

part

icles like water molecules kind of like each other and so almost like weak little magnets they tend to attract other

part

icles to

part

icles that have the same charge don't like each other they're sort of afraid of each other and so they're going to

repel they don't want to get anywhere near each other what this loss is is saying when we say there are no forces of attraction or repulsion between gas

part

icles what we mean is that the gases are flying around and the not going to start clumping together because they're attracted to each other that doesn't happen there's not that attraction likewise let's assume that two gas

part

icles are passing each other they're just going to fly right by if they repelled each other

soon they'd hit out that way when they get close that doesn't happen they don't repel each other and they don't attract each other either lastly here's what I think is probably the most important thing to keep in mind when we're talking about the

kinetic

theory

of gases and that's what the average

kinetic

energy of gas

part

icles depends on the temperature of the gas the hotter it is the faster they move so remember that hotter for gas movement equals faster this is

tremendously important the hotter it is the faster these gas

part

icles move around so in this

kinetic

theory

of gases we've looked at a few of the rules that we always want to keep in mind whenever we're dealing with gases and again we're going to assume that all the gases we're dealing with follow follow these five rules that we just talked about