Why not peroxide effect observed in case of addition of HCl or HI with alkene but it observed with HBr only?

The peroxide effect is not noted in the case of HCI or Hl. This is due to the fact that HCI bond is too strong to be broken to produce free radicals (430.5 kJ mol-1) than the HBr bond (363.7 kJ mol-1). Hl has a lower bond energy (296.8 kJ mol-1) and in case of the addition of HI although H-I bond is easily broken by homolysis, but the iodine atoms so produced immediately react to form iodine molecules rather than attacking the double bond of carbon atom in alkene. 

           °CH3 + HI -->. CH4 +  I°

             I° + I°---> I2

Which of the following is a strongest acid among oxy acids of chlorine and why? "HClO, HClO2, HClO3, HClO4"

According to Bronsted Lowery concept, a strong acid has weak conjugate base and vice versa. 

The given oxy acids of chlorine HCIO, HCIO₂, HClO₃ and HClO₄ and their conjugate bases are CIO-, CIO₂^-, ClO₃^-, ClO₄^- respectively. These anions are stabilised to greater extent, due to the presence of π-bond and lone pair, if number of π-bond increase extent of stabilisation increase. In case of ClO^- there is no π-bond hence if form less stable conjugate base hence act as weak acid while in case of ClO₄^- it has three pπ-dπ bond which stablised greater extent and form more stable conjugate base hence it conjugate acid (HClO₄) is strongest acid. Oxy acids of chlorine and their number of pπ-dπ bonds given as:

HOCI (0~Pπ-dπ)

HOCIO (1~Pπ-dπ)

HOClO₂ (2~Pπ-dπ)

HOCIO₃ (3~Pπ-dπ)

Thus overall order of acidic strength of oxy acids of chlorine is as:

                  "HClO < HClO₂ < HClO₃ < HClO₄" 

Related Questions:

Although electron affinity of chlorine is higher than fluorine, yet fluorine act as better oxidizing agent than chlorine? Explain.

Why chlorine trifluoride (ClF3) exists but fluorine trichloride (FCl3) does not?

Why Chlorine (Cl2) bleaches a substance permanently but sulphuredioxide (SO2) does it temporarily?

What is the correct order of electronegativity of Cl atoms in HClO, HClO2, HClO3 and HClO4 ?

How does the thermal stability of oxy acids of chlorine increase with an increase in the oxidation state of chlorine atom ?

Which of the following is a strongest acid among oxy acids of chlorine and why? "HClO, HClO2, HClO3, HClO4"

Which will have very fast tendency to dimerized ClO2 or NO and wh