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PROFESSOR CIMA: Let's do a phase diagram with the compound.
What does a phase diagram with a compound look like?
So here's a phase diagram, B and A. And I'll continue with B melting at a
lower temperature than A. So that's liquid up there.
There's their melting points. And I'll have something like-- here's some
alpha phase. And here's some beta phase.
And there's a number of combinations of these things.
But let's say it forms a compound. That was supposed to be halfway.
But let's say this compound AB. There's a specific interaction between them.
Well, I'm gonna have a liquid. Liquid looks like this.
This is liquid plus alpha. This is compound.
I'll call that gamma just to shorten, I think. This is a two-phase region here, gamma plus
alpha. Here I have a liquid plus gamma.
And I'll just draw a eutectic in here. How's that?
A eutectic -- this one. So this little region here is beta plus liquid.
And down here we have beta plus gamma. So that's a typical--
So, you can see the solubility in these two end members is small.
But in this case, it's not necessarily because of a difference in size.
They could be the same size. But if there's this difference in electronegativity,
they'll want to transfer electrons and form a compound.
Now, this is called the eutectic. The eutectic in this case is the liquid is
going to beta plus gamma. So if I'm at this composition, the reaction
that happens at this temperature is forming two solids, beta plus
gamma. In this case, when we perform this compound--
so let's say I'm cooling this-- I'm creating this compound from a mixture
of liquid plus alpha goes to gamma.
And in this case I take one phase and create two.
In this case I take two phases and create one.
That's called a peritectic. Just the lingo.
So let's look at an example of that, copper and gold.
Whoops. Well, this one's not exactly like that.
This is an interesting one. Copper and gold are completely miscible at
high temperatures but start to form compounds.
Now why might that be? Well copper is FCC.
Let's just do it here. Copper, FCC.
Gold, FCC. 1.281, 1.46, 1.9, and 2.54.
See what's happening here? 25% difference in electronegativity.
This is 304.6. This is 324.4.
This is 7.11. And this is 10.2.
And you get 42.8 here and 31.8. That's 25% difference.
So the thing that went up here is the difference in
electronegativity is now large. Now notice it doesn't happen 'til you get
to low temperatures. Let's do one last one.
I got lots of compounds here. Here's a compound.
Here's a compound. Now they're happening at high temperatures.
So this is gold and vanadium, forms these compounds.
Let's look at this one. Well, first off, vanadium is BCC.
So we don't expect them to dissolve in one another.
It's got a 1.36 difference in size. So that's only 6.8% different.
This was 25%. This one ends up being 1.63.
From 2.54 to 1.63, that is a big difference, a delta of 0.9, right, in
electronegativity. Because vanadium's on the left-hand side of
the chart, and gold's on the right-hand, or the transition series.
So this is huge. And the internal pressure ends up being, well,
41% different-- I won't write it down--
41% difference. And so, again, why we're forming compounds
now is because of the specific interaction between the two end members.
So just because we're looking at phase diagrams doesn't mean you forget all
the chemistry that you learned. That's the point of today's lecture. 'Cause
the chemistry's embedded in the phase diagram.