[cabaal]

The first figure is an acid catalyzed addition of water to an alkene. The second figure is (part of) the dehydration of an alcohol.

I understand these are the same reactions, just in different directions? But, why in the first figure does the hydrogen pop off? Why doesn't the entire thing pop off as water and go back to the starting alkene?

[MissPhosgene]

Hello,

   Addition of the molecule of water to the alkene in the first figure is reversible. The mechanism is showing the pathway to get to the alcohol in which the molecule of water has to be bound to the alkene.

Best.

[cabaal]

In the first picture, the 3rd step has CH₃-CH₂-H₂O⁺.

In the second picture, the first step has CH₃-CH₂-H₂O⁺.

They are both forward steps in two different types of reactions. Why in the first picture does the hydrogen pop off to make an -OH group, while in the the second picture the entire H₂O pops off? The conditions are the same: they're both CH₃-CH₂-H₂O⁺. That's what I don't understand.

[MissPhosgene]

Since the reactions are the reverse of each other, the second scheme can be seen as an illustration of the reversibility of addition shown in the first scheme.

Reactions are statistical. 

[orgopete]

The difference is that the purpose of the reactions illustrated was to show the steps and not whether and how to make the reactions take place.

First, simply stated, the first reaction is an addition of water and the second is an elimination of water. (Presumably, the alkene formation was the next step.) If water were added in the first reaction and removed in the second, it would shift the equilibrium toward those products. Let's go a little further. Ethylene will not directly give ethanol by the addition of water. The alkene isn't basic enough in the presence of water or the product. If sufficiently strong sulfuric acid is added and limited water, the protonation can take place, but no water is present so bisulfate adds to the carbocation. A second hydrolysis step is necessary to get ethanol. If you add methyl groups to the double bond, then less drastic conditions are needed to protonate the alkene and water can be present to trap the carbocation.

For an elimination, a strong acid like sulfuric acid can also absorb the water and prevent a reverse reaction from taking place. Also heating favors elimination as well as shifting the equilibrium by removing it. So, while the two reactions are reversible and related to one another mechanistically, the products one isolates is determined by the conditions of the reaction.

[cabaal]

So there is no way to know what will really happen without knowing the starting materials?

Alcohol + strong acid: water will pop off, forming alkene.
Alkene + strong acid: hydrogen will pop off, forming an alcohol.

It makes sense I guess, but it's not satisfying. 

[cupid.callin]

I dont think that dehydration of alkene requires a stronf acid. The 2 lone pairs of O serve as very strong bases.
Just like esterification ... strong and weak acidic conditions control reaction to a limit.

Also, you can use something like Anhy. ZnCl2 so that the H2O released donot react back and you get only alkene. Generally all reversible rxns are accompanied with removal of products as soon as they are formed. like in this case. alcohol has higher BP than Alkene,,,, so temp of rxn should be above BP of alkene but below BP of alcohol so alkene evaporate as soon as formed!!!

hope it helps...

and plz giv me a some points . o/o looks lame!!!