Melting point question

[sillyjoe]

You know the general rule that an impurity in a compound will lower the melting point? Is this only when you are looking at the compound with the higher MP's perspective?

For example, if you had 100 g of a compound A that has a MP of 100 degrees but it contains an impurity of 1 g of a compound B with a melting point of 200 degrees. Will the MP be less than 100 degrees? Or do you look at it from the perspective of compound B and say the MP of the combination will be less than 200 degrees?

---

Members don't see this ad.

 

[TexasSurgeon]

You know the general rule that an impurity in a compound will lower the melting point? Is this only when you are looking at the compound with the higher MP's perspective?

For example, if you had 100 g of a compound A that has a MP of 100 degrees but it contains an impurity of 1 g of a compound B with a melting point of 200 degrees. Will the MP be less than 100 degrees? Or do you look at it from the perspective of compound B and say the MP of the combination will be less than 200 degrees?

No you're looking it from the perspective of compound A.

So if you have water which melts at 0 C, and you add sodium chloride, the melting point of water will be reduced.

You're looking at it from the MP of the solvent (basically whichever quantity is in excess). I hope I'm correct in this


The water example can be a bad example.

If you have a solid gold bar (24 karat) which melts at 1000 C, and then another gold bar with say an impurity (say 20 karat), then the 20K gold bar will melt at a lower temperature

 

[el_duderino]

Well, salt melts at a much higher temperature than ice....

 

[sillyjoe]

No you're looking it from the perspective of compound A.

So if you have water which melts at 0 C, and you add sodium chloride, the melting point of water will be reduced.

You're looking at it from the MP of the solvent (basically whichever quantity is in excess). I hope I'm correct in this

What if you have two solids though that you crystalized from an experiment. Compound A and Compound B.

You have 100 grams of compound A and 1 gram of compound B.

Compound A has a MP of 100 degrees
Compound B has a MP of 200 degrees

Will the MP be greater than or less than 100 degrees in this scenario?

 

[el_duderino]

Less.

 

[sillyjoe]

Less.

What is the reasoning behind that? It disrupts the intermolecular forces?

 

[el_duderino]

What is the reasoning behind that? It disrupts the intermolecular forces?

I've been pondering that. I would say that's it, but I can't quite put my finger 100% on why that is. I almost think it would have to depend on the compounds. In general I suspect that any impurities will decrease intramolecular forces, but my gut says that won't always be the case. For these purposes, and as a general rule, it should work.

 

[TexasSurgeon]

What if you have two solids though that you crystalized from an experiment. Compound A and Compound B.

You have 100 grams of compound A and 1 gram of compound B.

Compound A has a MP of 100 degrees
Compound B has a MP of 200 degrees

Will the MP be greater than or less than 100 degrees in this scenario?

It would still be less than 100, because introducing compound B will disrupt the intermolecular ordering of the compound. You're basically introducing a foreign substance in a pure substance.

If you had a significant quantity of compound B, say 100 grams mixed with 100 g of compound A, the MP would be exactly half of the two, so 150 degrees

 

[Concrete Jungle]

If compound B is soluble within the compound A at the molecular level, compound B in effect disrupts the crystal lattice of compound A. This weakens the lattice and less energy is required to overcome and melt it. Think about inserting a random object into a crystal lattice as it is forming, it is like a chink in the armor.

If you have a mixture of two components, the compound with the larger mass will be depressed regardless of the melting point of the lesser mass component.

 

[Chrisz]

You know the general rule that an impurity in a compound will lower the melting point? Is this only when you are looking at the compound with the higher MP's perspective?

For example, if you had 100 g of a compound A that has a MP of 100 degrees but it contains an impurity of 1 g of a compound B with a melting point of 200 degrees. Will the MP be less than 100 degrees? Or do you look at it from the perspective of compound B and say the MP of the combination will be less than 200 degrees?

The amount that is less is the impurity. Impurity will decrease the MP of a substance. The reasoning is that it disturbs the crystal lattice structure of the substance (lower intermolecular force), so easier to melt the substance at a lower temp. different combinations of two compounds has different MP. Starting with a pure compound, it has its highest MP. Adding another compound will lower MP until the lowest point possible, called euteric point, usually around 50:50 ratio. After this point, adding more of second compound will increase the MP of the mixture. As the percentage of the second compound approaches 100% second compond, the MP approaches that of the second compound. Below is a graph I searched over the internet.

 

[sillyjoe]

The amount that is less is the impurity. Impurity will decrease the MP of a substance. The reasoning is that it disturbs the crystal lattice structure of the substance (lower intermolecular force), so easier to melt the substance at a lower temp. different combinations of two compounds has different MP. Starting with a pure compound, it has its highest MP. Adding another compound will lower MP until the lowest point possible, called euteric point, usually around 50:50 ratio. After this point, adding more of second compound will increase the MP of the mixture. As the percentage of the second compound approaches 100% second compond, the MP approaches that of the second compound. Below is a graph I searched over the internet.

This is great. Did you learn about this in a class? I have never heard of the eutectic point.

 

[Chrisz]

This is great. Did you learn about this in a class? I have never heard of the eutectic point.

Yes, actually in the orgo lab section. We even had midterm and final for the lab section when I took orgo

 

[sillyjoe]

Yes, actually in the orgo lab section. We even had midterm and final for the lab section when I took orgo

It's really interesting, thanks again.

 

[gettheleadout]

It would still be less than 100, because introducing compound B will disrupt the intermolecular ordering of the compound. You're basically introducing a foreign substance in a pure substance.

If you had a significant quantity of compound B, say 100 grams mixed with 100 g of compound A, the MP would be exactly half of the two, so 150 degrees

As @Chrisz explained above, this is not correct. At very small ratios of impurity : compound, the presence of the impurity disrupts the structure of the crystalline solid much like other types of point defects, but as you increase the amount of impurities the number of these defects increases and the way they interact with their surrounding compound molecules, along with the effects on crystal lattice stability, depends on the properties of the two. This is why the eutectic point is unique to a compound-impurity pair.

I was able to find this document (PDF warning) explaining this with an example and a more readable plot.

 

[TexasSurgeon]

As @Chrisz explained above, this is not correct. At very small ratios of impurity : compound, the presence of the impurity disrupts the structure of the crystalline solid much like other types of point defects, but as you increase the amount of impurities the number of these defects increases and the way they interact with their surrounding compound molecules, along with the effects on crystal lattice stability, depends on the properties of the two. This is why the eutectic point is unique to a compound-impurity pair.

I was able to find this document (PDF warning) explaining this with an example and a more readable plot.

I'm so sorry! I totally forgot about the eutecic point. Yes this is accurate.

 

[justadream]

What about if you have relatively the same amount of the two substances (let's just say 50/50 ratio) but the two compounds actually attract/repel (pick one: I'm kind of confused so consider both) each other?

Would the MP of the combination be HIGHER than the MP of either of the individual components? Is this possible?

I ask this because in TBR GC's vapor pressure section, it talks about how if you combine 2 things that attract each other, the total VP will be decrease. However, if you combine two things that repel each other, the total VP will increase.