Does the addition reaction of ethene and water need a catalyst?
Aren't alkenes so reactive that a cataylst isn't needed?
You are confusing two types of reactivity. There is kinetic reactivity (lability), which describes how fast something reacts. There is also thermodynamic reactivity, in which physics predicts a reaction will occur to a great extent. The difference can be shown in the following examples.
If you add an indicator to a solution and then add an acid, the acid will QUICKLY change the colour of the indicator (assuming you swirl the solution). Acid-base reactions are generally quite fast, even though they don't proceed to completion. So this is something that is fast kinetically but not favoured thermodynamically.
Now, let's look at the hydrogenation of ethene to form ethane. Thermodynamically, this reaction should proceed almost to completion. However it's not very fast. Catalysts and high temperatures are used if you need an acceptable reaction rate for some reasons, such as the hydration of ethene to form ethanol. Just because an alkene is reactive doesn't mean it'll react fast.
Another example. Lithium is the strongest reductant on your standard electrode potential table. It isn't, though, the fastest thing to react with water. Sodium will fizz, potassium will fizz violently and produce hydrogen gas so vigorously the hydrogen gas burns, while lithium does like nothing.
You'll learn more about the differences between reaction rate and extent of reaction.
What is it that causes salicylic acid to have higher rf values than aspirin, considering salicylic acid is more polar?
Thanks
Rf values in what solvent and stationary phase?