[Fudgey123]

Hi guys,

I'm new here and just would like to ask for some help with this.

So, I conducted an experiment on the structures, bonding and properties of different substances, and one particular test I need to evaluate is the ethanol-hexane miscibility test.

So my experiment had shown that ethanol and hexane ARE miscible, but I'm having some trouble explaining why this outcome occurred.
I know that Ethanol is polar and hexane is non-polar, so how would I go about writing up the outcome of my result? 

Thanks,

[Corribus]

First, despite being a useful concept in chemistry, there's no real scientific scale for "polarity", and certainly reducing it to two mutually exclusive categories is going to lead you to a lot of bad predictions.

On to explaining the outcome: I'd start with what determines miscibility. What do you know about the thermodynamics of mixing?

[Fudgey123]

Corribus, thank you for replying.

So, just to say. The answer I need is to be a fairly short one on my report, it asks me to use knowledge of molecular forces of attraction and interaction. So I should explain the compatibility/incompatibility of the interactions between ethanol and hexane.

These two are miscible because they mixed together to form a homogeneous solution. The molecules that make up ethanol are held together by strong hydrogen bonds and from what I can remember hexane has non-polar covalent bonds?

Ethanol and hexane have different bonds holding their molecules together, this would mean breaking the bonds of these substances apart would require a lot of energy, and so during the breaking process a lot of energy will be used to break these bonds. Because ethanol and hexane are miscible, this would mean that they have compatible interactions in a way? Because to be miscible substances must have similar intermolecular bonds, would this be right?

I'm currently an undergraduate student and I'm a little rough on Chemistry as I have been away from it for 2 years (due to taking 2 gap years before commencing on my university course), it's a module on my Forensic Science course.

[Corribus]

The molecules that make up ethanol are held together by strong hydrogen bonds and from what I can remember hexane has non-polar covalent bonds?

The carbon and hydrogen atoms in hexane are held together by covalent bonds (both polar and nonpolar) but hexane molecules are held together by van der Waals forces. Ethanol molecules, too, are held together by van der Waals forces but also, as you note, hydrogen bonds.

Ethanol and hexane have different bonds holding their molecules together, this would mean breaking the bonds of these substances apart would require a lot of energy, and so during the breaking process a lot of energy will be used to break these bonds. Because ethanol and hexane are miscible, this would mean that they have compatible interactions in a way? Because to be miscible substances must have similar intermolecular bonds, would this be right?

As in any chemical process, the favorability of mixing is determined by a combination of entropy and ethalpy. (ΔG = ΔH - TΔS). Mixing usually results in an increase in entropy, which you should know from every day experience. If you take blue marbles and red marbles and toss them together in a jar - a scenario in which there is essentially no enthalpic considerations - it is certainly favorable from them to mix.... and it will take some energy to separate them.

Therefore, whether two solvents will mix or not is usually determined by whether it is enthalpically favorable for them to do so.  In brief terms, because mixing entails no (covalent) bond breaking, enthalpy considerations for mixing boil down basically to the sum total strength of electrostatic interactions in the pure substances versus those in the mixture. To mix, you have to disrupt some interactions in the pure substances, and these are replaced by new interactions in the mixture. Whichever wins out will determine miscibility. That is, if the intermolecular interactions in the mixture are stronger than those in the pure substances, mixing happens because the enthalpy change is negative. If not, not. (Not completely true: more accurately, two solvents will mix if their enthalpy change in the negative is enough to overcome the generally positive contributions due to entropy, which are not insubstantial. Mixing can be result in an "unfavorable" change in enthalpy but still happen because the entropy change is positive enough to dominate. Hint! Hint!)

Now let's consider your mixture and some other similar mixtures.  By consulting a solubility chart, e.g., here, we can see that, as your experiment shows, ethanol and hexane are expected to be miscible. What about hexane and methanol? Hexane and water? Hexane and propanol? More generally, do you see more immiscible mixtures or miscible mixtures? Start by thinking about that and we'll see where we go.

(If it helps, you can think of the problem of solubility of a solid in a liquid, which is governed by many of the same principles. )
(Also, one thing to keep in mind, is that the enthalpy of mixing, and therefore miscibility, is dependent on the mole fractions of the two substances.)

[Babcock_Hall]

Corribus,

Although I mostly agree with your excellent post, there is one type of exception.  IIRC the enthalpy of mixing toluene and water at room temperature is close to zero.  The process of mixing the two is unfavorable entropically.

[Corribus]

Water offers many exceptions to expectations. I apologize to the opening poster if my post seemed to imply that entropy changes during mixing are always positive. In many solutions that deviate strongly from ideality - i.e., those in which mixing is not random - this is certainly not the case, and water, due to the strength of interactions it forms, frequently falls into this category under many conditions. The behavior of solutions is difficult to generalize; as soon as you do, someone points out an example to the contrary. There are so many factors that become important - even if you predict a trend will go in a certain way, often it doesn't. The trick then is to try to figure out why.

For the question asked here, it is probably acceptable to simply understand (and state in your answer) the factors that are involved.

EDIT: By the way, after I wrote that earlier post, I looked around to see if I could find thermodynamic data for the ethanol-hexane and other alcohol-hexane systems. There is certainly plenty of it, and (having glanced about a half dozen papers or so) the results weren't entirely within my expectations. For example, the excess entropies deviate strongly from zero (Smith and Robinson, J. Chem. Eng. Data 1970, 15(3), 391), implying that these systems do not mix randomly and are very far from ideal, like the other one you (Babcock Hall) pointed out. TS^E of ethanol-hexane mixture is negative at all mole ratios assayed. Likely this has to do with the fact that alcohols have hydrogen bonding, too. I thought these interactions in alcohols might be weak enough to be close to the regular solution regime, but apparently not. So, your counterexample is quite pertinent to the discussion, and comparing ethanol-hexane, methanol-hexane, and etc. miscibility is not as straightforward as my previous post may have implied.