Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: kensher on July 22, 2016, 11:53:12 AM
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Problem
My text book (and Wikipedia) says that 1-propanol can be used for mixing water and gasoline. They are normally seperated because water is polar and gasoline is non-polar. My textbook used hexane (C6H14) to represent gasoline
My thoughts
I have tried to make a visual illustration of this process inspired by the explaination in the textbook (see attachment). However, there is some things that dosen't make sense to me:
1) How can 1-propanol seperate the water molecules from each other? Isn't the bond between two H2O molecules much stronger than the hydrogen bond between H2O and C3H7OH?
2) How does it look like on the molecular level when water, gasoline and 1-propanol is mixed togheter?
Thanks in advance
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Quite good scheme. Water can form more hydrogen bonds (with hydrogens and oxygens). And remember - solubility of water in this mixture is limited.
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Quite good scheme. Water can form more hydrogen bonds (with hydrogens and oxygens). And remember - solubility of water in this mixture is limited.
Thanks for your answer.
So the alcohol dosen't seperate water molecules, it just join them? How will this look?
Why is the solubility of water limited in this mixture?
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Like in sketch no. 3 - up to 4 alcohol molecules surrounding 1 water molecule.
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Like in sketch no. 3 - up to 4 alcohol molecules surrounding 1 water molecule.
Ok. But the water molecules isn't free, so how do they make new bonds with 1-propanol?
I attached a picture which illustrates water (how I understand them at least). The molecule number 3 is full and therefore can't make new hydrogen bonds with 1-propanol. But aren't all the molecules (or at least almost all) of them full?
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Ok. But the water molecules isn't free, so how do they make new bonds with 1-propanol?
I attached a picture which illustrates water (how I understand them at least). The molecule number 3 is full and therefore can't make new hydrogen bonds with 1-propanol. But aren't all the molecules (or at least almost all) of them full?
First of all, be reminded that hydrogen bond isn't really a type of chemical bond, but some sort of enhanced electrostatic attraction. The water molecules can 'free up' some of their hydrogen bonds to make new hydrogen bonds with 1-propanol, but remain stable(do not lose too much attraction with other water molecules and escape into air) due to the much higher strength of hydrogen bonds compared with van der Waals' forces.
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Ok. But the water molecules isn't free, so how do they make new bonds with 1-propanol?
I attached a picture which illustrates water (how I understand them at least). The molecule number 3 is full and therefore can't make new hydrogen bonds with 1-propanol. But aren't all the molecules (or at least almost all) of them full?
First of all, be reminded that hydrogen bond isn't really a type of chemical bond, but some sort of enhanced electrostatic attraction. The water molecules can 'free up' some of their hydrogen bonds to make new hydrogen bonds with 1-propanol, but remain stable(do not lose too much attraction with other water molecules and escape into air) due to the much higher strength of hydrogen bonds compared with van der Waals' forces.
Thanks for your reply! ☺
I know that water break apart and reform all the time since they are a liquid. Is it when they break apart that they form bonds with 1-propanol? That would make sense to me -- that they broke apart and sometimes made hydrogen bonds with 1-propanol, but more often with other water molecules.
What dosen't make sense to me, is that they would free up bonds with each other to make a weaker bond with 1-propanol, if they weren't going to break apart anyway.
Hope my question made at least a little sense.
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What dosen't make sense to me, is that they would free up bonds with each other to make a weaker bond with 1-propanol, if they weren't going to break apart anyway.
Hope my question made at least a little sense.
Why do you think that the hydrogen bonds between water and 1-propanol are weaker? Both type of hydrogen bonds involves the attraction of -OH groups, and the electronegativity difference between O and H atoms are the same in water molecules and 1-propanol molecules. The strength of those hydrogen bonds should be the same.
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Great point. ☺
Guess I mixed the inter-molecular bonds with inter-atomic bonds and confused myself.
But still, why does they break a bond with each other (water molecules) to form just an equally strong bond with 1-propanol, when they just could stay with each other?
Thanks in advance. Really appreciate it.
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Hydrogen bonds are relatively weak. Then their formation is an equilibrium process for homogenic systems (you mension gasoline).
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Some elements:
At +100°C=373K, water molecules separate to form vapour, a gas. +25°C=298K is only a bit less, say 3/4 of that. Since temperature is a statistical value, at 298K sometimes enough energy is available to separate two water molecules, and often too little. That's why water is liquid rather than solid, simplifying very little: enough bonds are broken, and the molecules can move against an other.
In this huge mess, a water molecule separated from an other and which meets a propanol molecule can bond with the alcohol function. It wouldn't even need the bond to be equally strong. If the bond were weaker, it would only last for shorter, hence be observed less often statistically, but still exist. The proportion is computed by a thermodynamic distribution, by the way.
I suppose - but am not sure - that water, hexane and propanol don't build a mixture but rather a microemulsion. In an emulsion, water and the hydrophobic liquid are separated, one liquid making droplets in the other which is continuous, and the tenside makes the interface, one end (the alcohol) at water and the other (the hydrocabon end) at the hydrophobic liquid. It's just like oil and vinegar. Or like fat dirt in cleaning water, with soap making the interface of the emulsion. Propanol is much shorter than soap molecules, hexane is short too, so they should make tiny droplets, hence the "micro"-emulsion.
https://en.wikipedia.org/wiki/Emulsion
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Some elements:
At +100°C=373K, water molecules separate to form vapour, a gas. +25°C=298K is only a bit less, say 3/4 of that. Since temperature is a statistical value, at 298K sometimes enough energy is available to separate two water molecules, and often too little. That's why water is liquid rather than solid, simplifying very little: enough bonds are broken, and the molecules can move against an other.
In this huge mess, a water molecule separated from an other and which meets a propanol molecule can bond with the alcohol function. It wouldn't even need the bond to be equally strong. If the bond were weaker, it would only last for shorter, hence be observed less often statistically, but still exist. The proportion is computed by a thermodynamic distribution, by the way.
I suppose - but am not sure - that water, hexane and propanol don't build a mixture but rather a microemulsion. In an emulsion, water and the hydrophobic liquid are separated, one liquid making droplets in the other which is continuous, and the tenside makes the interface, one end (the alcohol) at water and the other (the hydrocabon end) at the hydrophobic liquid. It's just like oil and vinegar. Or like fat dirt in cleaning water, with soap making the interface of the emulsion. Propanol is much shorter than soap molecules, hexane is short too, so they should make tiny droplets, hence the "micro"-emulsion.
https://en.wikipedia.org/wiki/Emulsion
Thanks! This answered all of my questions.
Feeling a deep relief right now :)
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Btw!
It should have been 2-propanol and not 1-propanol. Sorry about that.