Hi. It's Mr. Andersen and this is chemistry essentials video 50. It's on energy changing
processes. This is a picture right here of Seattle. And this is a picture of where I
live. This is Bozeman, Montana. And both of these have about the same latitude. In other
words they're each about the same distance from the equator. But if we look at their
average high temperatures throughout the year we'll find that Bozeman's is going to fluctuate.
And so in January it's going to be relatively low. And it's going to get pretty warm in
the summer. But if you look at Seattle it's going to be more temperate. In other words
it's high is not as high and it's low is not as low. Now what is causing that is the ocean.
The Pacific Ocean is during the winter, it's actually releasing heat into the surroundings.
And during the summer it's consuming that heat. And water has a really high specific
heat. And so it's good at moderating temperature in a city and also inside your body. And so
in a chemical system energy can move into a system or it can move out. Or we can see
energy changes. And so if we're heating or cooling a system or if we're transitioning
from one phase to another or if we're going through a chemical reaction we can have energy
go in or energy come out. And so if we're heating we're obviously adding energy to a
system. If we're cooling we're removing energy from that system. And we can measure the amount
of energy that's consumed or released in different materials using a term called specific heat
capacity. If we look at phase transitions, in other words going from solid to a liquid,
or from a liquid to a gas, we're adding energy to a system. And likewise when we remove that
energy we can condense or we can freeze that material. Now we measure that in different
materials using something called the enthalpy of fusion or the enthalpy of vaporization,
depending on which phase we're actually switching between. And then in a chemical reaction we
can add energy to the system and that would be an endothermic reaction. Or we can release
energy from that chemical reaction and that would be exothermic. We measure the amount
of energy in different materials or different reactions using the enthalpy of reaction.
And so let's start with a little simulation here. So what we have is some iron and we're
heating it up. And so when we heat it up we're adding energy to the iron. That's kinetic
energy of those molecules. And a lot of that energy is eventually going to be released
to the environment. Likewise when we cool something down like water, then we're removing
energy from it. It has a lower amount of energy. If I take a warmer object like iron and put
it in that colder water you can see the energy is now flowing out from the iron into the
water. And we'll talk more about this process in the next video on calorimetry and how we
can measure that. But know this. If we heat something, that means that we're adding energy
to it. And so if I have two beakers and they have the same amount of water in each one
and one is 1 degree and one is 25 degrees, which one has more energy? Well it's going
to be the one that's warmer. We've added more energy to that. And so we're going to represent
that as these little cubes. And so what were to happen if we were to put this warm object
next to this cold object? Over time we're going to see energy flowing from the warm
to the cold. And so we're going to start to approach thermodynamic equilibrium. And so
I have a time-lapse here. And this is simply a glass of ice that's going to melt over time.
And so what happens as it melts is it's adding energy to that ice. It's speeding up the molecules.
And it's going to change from one phase to another. And then likewise. As we cool it
down we're going to remove energy from that system. And so if we let it run like this
we can see over about an hour and a half that's changing from ice to water. And so what's
happening is energy from the surroundings is flowing into that system. And so if we
were to look at the energy around the glass it's actually decreasing. Now if we were to
run that backwards and freeze that again, what would be happening? Well we're removing
energy from that system and that energy is going to move to the surroundings. A freezer
does this. And if you ever put your hand behind a freezer what you'll find is there's heat
there. And that's because we're removing heat from that system. And so a way that we measure
the different amounts of energy added or released as we heat something up is using a term called
specific heat capacity. And it's going to be different in each of the different materials
that we have. So you can see here in gold it's going to be a relatively low number.
And we measure that in joules. That's the amount of energy or the heat divided by the
grams degrees kelvin. And so the matter or the amount we have there is important. And
the temperature that we're increasing is going to be important as well. So as we heat this
up we'll find that the one that has the highest specific heat capacity, that is water, its
temperature is going to change the least amount. And so we can measure how different things
are heated or cooled at different rates using specific heat capacity. And so as we move
phases from solid to liquid to gas, obviously we're increasing the temperature of that.
But as we switch between one phase and another phase we find something kind of interesting.
If we look at the heating curve of water what we'll find is as we add heat to it we're going
to see an increase in the temperature. In other words the ice is going to be cold. And
as we add temperature to it it's going to increase. But what's interesting is it will
flat line for awhile. And why is it doing that? Well it's making this transition from
either ice to water or water to water vapor. And so during that solid liquid transition
as we switch from one phase to another we're actually adding energy to it but we're not
increasing the temperature for awhile. Likewise as we move from water to vapor the same thing
is going to occur. And that's because we require a certain amount of energy to switch from
one phase to another. And so if we look at that same kind of a diagram to go from solid
to liquid we have to add energy. And that's called the molar enthalpy of fusion, to switch
from one to another. It's going to be different in any material or substance that we have.
You can see in water and iron it's going to be a different amount. The molar enthalpy
is simply measured in the kilojoules per mole. So it's going to depend on the amount of material
that we have. Likewise as we move from liquid to a gas we're going to add that molar enthalpy
of vaporization. And so that's going to be a positive value. We have to put energy into
that system to get that phase change to occur. Now likewise if we go in the other direction,
as we go from a gas to a liquid, then we're going to have a negative molar enthalpy of
vaporization. It's the same thing but in this case it's actually releasing energy to the
surroundings. And likewise as we go from a liquid to a solid we're going to see a release
of energy as well. The values are exactly the same. It's just going to be negative in
value. We also have chemical reactions remember. And those can either be exothermic or endothermic.
Those can be releasing energy or they can be consuming energy. And so, for example the
thermite reaction, when we're taking iron oxide and aluminum and then we're reacting
that together, if we look at the energy diagram we find that the reactants have a higher amount
of energy than the products do. And so over time what's happening is we're seeing an enthalpy
of reaction. We had a higher amount of energy in the reactants than the products. And so
where did that energy go? It came out of the system. It was released out of the system.
And so how do we figure that out? Well there's an equation. We just look for enthalpy of
reaction. We look at the amount of energy in the products and we subtract the amount
of energy in the reactants. And so since the product right here is a lower value than the
reactants we'd find that our enthalpy of reaction is going to be a negative value. Again, what
does that mean? It's releasing energy into the surroundings. If we were to look at an
endothermic reaction, like this ice pack reaction when we're breaking down ammonia nitrate,
what's happen here is the reactants have a lower amount of energy than the products.
And so if we calculate that same thing that enthalpy of reaction, we find now that since
the products are greater than the reactants we now have a positive enthalpy of reaction.
So where is that energy going? That energy is being consumed by the system. And so what
it's doing is it's really cooling down the surroundings. And that's why it gets very
cold around an ice pack like that. And so did you learn to relate energy change to the
specific heat capacity? Again that's just heating or cooling. The enthalpy of fusion
and vaporization again, that's going to be phase changes. And the enthalpy of reaction?
I hope you did. And I hope that was helpful.