Alkenes react by addition with H2 to give an alkane. For this reaction to proceed at a reasonable rate, a metal catalyst must be present to activate the H2 toward reaction.
Alkenes react by addition with halogens (Br2, Cl2) to give dihaloalkanes with the halogen atoms on adjacent carbon atoms. A single product is formed because the two parts of the reagent are the same.
Alkenes react by addition with HX (HCl or HBr) to give monohaloalkanes. As this reagent has two different parts, two different haloalkanes form except if the groups at one doubly-bonded carbon are the same as the groups at the other one..
Alkenes react by addition with concentrated sulfuric acid (H2SO4). Adding water to the sulfate product formed gives the alcohol. The H+ activates the double bond toward reaction. This reagent is asymmetric; therefore two alcohols form if the alkene is asymmetric.