Ask Me A Chemistry Question

Does shaking salad dressing hydrogenate it?

Holly,

Our nutrition teacher says that if you shake a mixture of oil and vinegar, as in salad dressing, the oil gets hydrogenated. This sounds unlikely based on what we learned in class. Don't you need hydrogen gas and a metal catalyst?

A questioning student

Dear questioning,

Oh dearie me. You certainly can't hydrogenate oil by shaking it with water or with vinegar or any acid.

Perhaps your professor mis-spoke, (I never do that!!--Well, OK, maybe I do sometimes...) or perhaps things came out of his mouth backwards and it was misunderstood. Let's try to give him the benefit of the doubt.

If you could hydrogenate oil by shaking it with water or with acid, the ancients would have discovered margarine! It would not be a modern invention of the 20th century. The oil would start to become a solid, and permanently so. It is too silly to imagine.

An acid such as vinegar donates H+, but hydrogen ion is not enough to hydrogenate. Hydrogen ion has NO electrons. You need a source of donate-able electrons, and there just isn't any in the mixtures that you described. Hydrogen gas is very different from hydrogen ion. Hydrogen gas has two electrons, shared by two hydrogen nuclei, and those electrons are needed to become part of the hydrogenated oil.

It's always a good idea to pay attention to where your electrons are, and to make sure you have the right number for whatever task you want to accomplish.

Hydrogenation is an important issue these days, because we have good evidence that when the reaction goes half-way, trans fats can be created, and those are a cardiovascular risk factor in the diet. The double bond normally starts in the cis orientation in an oil, the pi bond breaks, one half of the molecule rotates around the remaining sigma bond, and re-forms the pi bond in the opposite orientation (trans). But this of course can't happen if you just shake oil and water or oil and vinegar, for the above reason. You need the industrial process known as "partial hydrogenation" in order to create trans fats from cis fats.

Volatile gases as per your disscussion in class

Hey Holly!

I think I know why I equate volitility with explosiveness. (am i making up words?)  I know explosive gases have to reach a certain ratio as to oxygen to become explosive.  So volitility is a measure of how quickly they can disperse in the air?  This wouldn't have any relationship with particulates in the air would it? ie grain elevators?

am I even close?  No laughing at my spelling either I'm very sensitive right now.

Dear Mr. Sensitive,

Sorry I am so late getting back to you on this. I am starting to receive a ton of email from people around the world concerning my website, so I just did the sensible thing when confronted with this. I ignored all my email hoping it would all just go away. It did not work and I just ended up feeling like a Bad Person. Now I am atoning slowly by reading through all my emails. But I am delighted answer your question, especially since you are so very entertaining in my class.

But you are quite right, there is an indirect relationship between volatile and explosive, even though the terms do not mean the same thing in chemistry. In chemistry a "volatile" substance simply becomes a gas easily. It does not mean that it explodes easily.  People who are "volatile" are in common parlance said to have explosive tempers. But people don't evaporate readily, (the solid bits make this difficult--ask any crematorium). So to a chemist, people aren't very volatile either.

Substances that are volatile are not very cohesive--they are not very sticky. They are most likely molecular in nature, since molecules are attracted to identical molecules in a pure molecular substance by the weakest of attractions: intermolecular forces.

You don't worry about table salt evaporating on a hot day, for example, because it has actual bonds (ionic ones) that are too hard to break up into separate ions that might fly about the room. Most metals don't evaporate too well since metallic bonding is relatively strong between most metal atoms. Diamond wont evaporate either, since it is actually one giant molecule, all held together by covalent bonds, and a covalent bond is relatively hard to break.

Intermolecular forces on the other hand are terribly weak, and attract molecule to molecule, and include so-called hydrogen "bond"ing (which is not really a bond!), dipole-dipole forces, and London forces. The weak London forces are the only sort of attractions that nonpolar molecules can feel, so nonpolar molecules tend to escape easily from their comrades in a liquid, to fly around the room, and thus they tend to be the most volatile. Size also affects volatility since the larger the molecule, the more surface area it has to stick to another identical molecule by these electrostatic attractions, and the less volatile it will be.

But once you have a gas, the evaporated molecules are not surrounded by their identical kind, they are surrounded by air molecules! And some of these air molecules include the most reactive one: oxygen, O2.

You are simply increasing the area of exposure between your volatile molecule and O2, and the more likely two molecules will collide, the more likely the collision will be productive and you will get a reaction.

Not everything reacts with O2! Water, for example, will not, thank goodness. But O2 has two unpaired electrons, making it very reactive and many things do react with it. So, a volatile substance will only be reactive if it also reacts with O2.

The nonpolar molecules that evaporate most easily are most likely to be hydrocarbons (remember as we learned in class hydrocarbons--molecules made of H and C only--are all nonpolar) or hydrocarbon-like. When something does react with O2 it the reaction will be exothermic--releasing energy in the form of heat and light, which we call a "flame"--and some people might call that an explosion. Or at least "burning."

And yes, the problem with exploding grain elevators is a perfect analogy. You have tiny particles with a lot of surface area exposed to O2.

And you can inventionize any new words that you like as far as I am concerned.

H

PS., I love the book you loaned me, How to Lie with Statistics. It's a classic and everyone should read it! How wise of you to make your children read it so they don't get conned later on in life. Remind me not to accidentally keep your book. I have got to buy my own copy.