So focusing again on the reaction as it's written In order to determine the
direction it proceeds, we need to assess a thermal chemical quantity called the
enthalpy. So, enthalpy is a property of a
substance, and we might be able to look up, these are common substances.
Enthalpies will be tabulated for common substances.
So one can look up the enthalpy of rust, of aluminum; and when one does that, one
can determine the change in enthalpy on moving from the left to the right.
In this particular instance, we write from left to right because the enthalpy
change is negative. That means that enthalpy is released,
heat is released. We call that an exothermic reaction.
This in fact is a very exothermic reaction.
The amount of heat that's released is minus 850 kilojoules per mole.
So a joule is the SI unit of energy. And during the course there will be many
opportunities to do unit conversions between units of energy.
But 850 kilojoules would be 850,000 joules.
And so the enthalpy of the reactants is substantially higher than the enthalpy of
the products, and that released enthalpy as the reaction proceeds is available to
do things. So what might that enthalpy do?
Well, it can melt solids. And we can ask, how much enthalpy will it
take to melt the solid? That's actually measured by the enthalpy
of fusion of a substance. So fusion is either transformation from a
liquid to a solid, or the reverse, a solid to a liquid, that would be melting.
And in the case of iron, it take 14 kilo-joules per mole to melt one mole of
iron. it takes 11 kilojoules to melt one mole
of aluminum. If we look in our reaction mixture as
written, it's two and two. So let's call that, two times 14 is 28.
Two times 11 is 22. That adds to a nice round number, 50.
Kilojoules per mole, that leaves us 800 kilojoules per mole of remaining enthaply
that's available to do something, or 800,000 joules.
And what else can it do? Well, it can raise the temperature.
So the next question is, how much does the temperature go up given that amount
of available energy and these substances? To answer that questions, you need to
know another thermochemical quantity. It's called the heat capacity.
So, the heat capacity tells you. How much energy does it take to raise the
temperature of a given substance by one degree?
And so you see that in the units here, the heat capacity for iron, 25 joules per
mole degree Celsius. So, a rise in temperature of one degree
Celsius. For each 25 joules per mole of substance.
And, in the case of alumina, 128 joules per mole will raise it one degree
Celsius. So if I add those together, I've got 153.
Double that, because there's. Actually, we'll take double of that,
because there are 2 moles of iron. So, 50.
Plus 128, 178, why don't we round that and say about 200?
So 200 joules per mole, but there's 800,000 joules per mole still available
to raise the temperature, so that's a pretty simple division, and you'll get
about 4,000 degrees. Now how much is the actual temperature
rise? Turns out it's a bit in excess of 2,500
degrees. And the difference just reflects that we
play it a little loose by assuming all of these values are constants over that
entire temperature range. Turns out aluminum actually begins to
vaporize if you get it hot enough, and that takes energy.
But, that's well above the melting point of iron, which is 1,530 degrees Celsius.
and that's why we observed the iron melting.
Becoming a molten red flowing liquid that fell through the m-, bottom of the flower
pot. And then we saw that pretty golden,
golden red glowing substance lying in the sand as it resolidified.