Final answer:
The order of reactivity for hydrogen halides (HHal) in addition reactions with alkenes is HI > HBr > HCl > HF, with HI being the most reactive due to the weakest bond and HF being the least reactive due to the strongest bond.
Step-by-step explanation:
The order of reactivity for hydrogen halides (HHal), in terms of their addition to a double bond in alkenes, follows the bond dissociation energy trend, which decreases in the following order: HF > HCl > HBr > HI. This means that the strength of the bond between hydrogen and halogen decreases as you go down the group in the periodic table. As such, the most reactive hydrogen halide for addition reactions with alkenes is hydrogen iodide (HI), followed by hydrogen bromide (HBr), then hydrogen chloride (HCl), and the least reactive is hydrogen fluoride (HF).
This reactivity trend is because halogen atoms become larger and the bond between the hydrogen and halogen atoms becomes weaker as you move down the group from fluorine to iodine. Consequently, these bonds are easier to break, enabling the halide ion to attack the double bond more readily in an addition reaction. An example of an addition reaction with hydrogen halides would be the reaction wherein an alkene reacts with HCl to yield a haloalkane:
CH2=CH2 + HCl → CH3CH2Cl