[memcop]

I don't get it. Just started an ochem class and we just went over the mixed melting points graph but only its general outline; no formulas, no guidelines, no specifics. How the heck do you predict the melting point of a mixture ~50:50 of two known compounds? Thanks.

[Borek]

I don't think you are expected to predict them exactly, more like just what to expect in general. Higher than each pure substance? Lower? Somewhere in the middle?

[memcop]

I don't think you are expected to predict them exactly, more like just what to expect in general. Higher than each pure substance? Lower? Somewhere in the middle?

That makes more sense, I think it would be a bit lower than both right?

[Arkcon]

That makes more sense, I think it would be a bit lower than both right?

Why?  What general chemistry concept are you working with?

[memcop]

That makes more sense, I think it would be a bit lower than both right?

Why?  What general chemistry concept are you working with?

Organic chemistry lab. Kind of like a crash course since there is no lecture involved, all I have gathered is that the melting point of an impure compound should be lower than the pure one. Sorry if this comes across as dumb. I just really have had no lecture about melting points other than that information.

[discodermolide]

Depression of melting point is all you need to know. Other than that there is not much to say about melting points.

[Arkcon]

Depression of melting point is all you need to know. Other than that there is not much to say about melting points.

Which is something ou learn in general chemistry, under colligative properties.  See, whether you add sodium chloride to water ice, or mixed two organic crystalline compounds, the result is the same.

[orgopete]

I deal with this in my manuscript. You may find books refer to ionic bonds being strong because of their high melting points. Although that can seem true, you have to be careful as to what it means. If you add NaCl to ice, some ice melts as the ions are dissolved. The temperature also falls. This should only tell you about the strength of the matrix, not the strength of an individual bond. You can drive your car over a frozen lake. That does not mean the hydrogen bonds are strong bonds, it means the matrix of hydrogen bonds are stronger than individual bonds themselves. If you built a bridge out of toothpicks, you would not be able to predict it strength by measuring the strength of an individual toothpick.

When you melt an organic compound, you are breaking the matrix of weak bonds. This may include some hydrogen bonds. No covalent bonds are broken during a mp determination (usually). If you introduce another compound to the matrix of a pure compound, will the resulting matrix be weaker, stronger, or just the same? Although this is not explicitly explained, anyone could have reached this same conclusion.

[zsinger]

And remember….colligative phenomena are NOT chemical phenomena…..rather physical ones.  The properties are directly related to particle number rather than particle nature.  (This can be further argued with activities, which is the basis of colligative properties including Van't Hoff Factor, etc.)