How many grams of hydrogen are needed to hydrogenate 30.0g of 2 butene?

Question

How many grams of hydrogen are needed to hydrogenate 30.0g of 2 butene?

Answer 1

2 butene only has one double bond. You should know that hydrogenation effectively uses a heterogeneous catalyst such as platinum or nickel to reduce the double bond to a single bond. In other words, in order for the double bond in 2 butene to become a single bond (forming butane) we need 1 hydrogen to go on EACH side of the double bond.

So 2 butene is:   H3C - CH = CH - CH3

which during hydrogenation will become:  H3C - CH2 - CH2 - CH3

so you can see 2 hydrogens were added. Hydrogen itself is diatomic and is found as H2 already. So in order to fully hydrogenate 2 butene we need the same number of moles of hydrogen as 2 butene. That is, 1 mole of every hydrogen (H2) will add 2 hydrogens (again because it is H2) to every mole of 2 butene because a mole a H2 has the same # of molecules as a mole of 2 butene.

ok so we need to figure out how many moles of 2 butene we have (molecular weight 56g):

30.0 g * 1n / (56 g)  = (15/28) n 2 butene

that means we need 15/28 n H2

15/28 n H2  * 2 g /1n = 15/14 grams of H2 or 1.07 grams H2