Sanchez’s first goal owed much to fortune rather than brilliance as Iwobi arrived at the back post and saw his effort deflect in off the Chile international.
Livermore was shown a straight red card five minutes before half-time when he blocked Iwobi’s goal-bound effort with his hands.
Sanchez took responsibility for the penalty ahead of recent regular taker Santi Cazorla only to see his low effort saved by Hull goalkeeper Eldin Jakupovic.
It was a decision which also mystified Wenger who revealed it was “not planned for Sanchez to take it” and that he wants “to know what happened and for it not to happen again”.
Walcott notched a second goal shortly before the hour mark when he got on the end of Iwobi’s clever flick with Hull defender Harry Maguire helping the ball into the net with his head.
Snodgrass gave Hull a glimmer of hope when he converted from the spot after Petr Cech had brought down Dieumerci Mbokani, but Arsenal’s response was emphatic.
Sanchez struck after Jakupovic had blocked Walcott’s initial shot, before substitute Switzerland international Xhaka – a £35m summer signing from Borussia Monchengladbach – applied the gloss with the pick of the goals from distance.
Gunners’ season gets going
After three consecutive Premier League wins for the first time since last Christmas, Arsenal’s season now feels properly up and running.
Hull made Arsenal work for this victory but the quality in Arsene Wenger’s side shone through.
Even when he has an off day Sanchez can be a match-winner. These were not particularly classy strikes from the Chilean, but he got the job done.
Wenger, in a season when the pressure is on him to deliver silverware, will likely feel there is plenty to build on from this performance.
Essential Hull hold on to Phelan
Mike Phelan’s side might have been comfortably outclassed by Arsenal here, but the scoreline perhaps flattered the visitors.
Whatever the delay is over caretaker manager Phelan’s future at the KCOM Stadium, Hull’s owners should get it sorted immediately.