For the reaction, 2‌NO+Br2⟶2‌NOBr the following mechanism has been given NO+Br2‌
‌ Fast ‌
⟶
NOBr2 . . . (i) NOBr2+NO‌
‌ Slow ‌
⟶
2‌NOBr . . . (ii) As the second step is the rate determining step, ‌ Rate ‌=k′[NOBr2][NO] . . . (iii) From (i) step k′′=‌
[NOBr2]
[NO][Br2]
or [NOBr2]=k′′[NO][Br2] . . . (iv) By substituting this value in rate law step (iii), we get ‌ Rate ‌=k′k′′[NO]2[Br2] or Rate =k′′[NO]2[Br2]