Defender, David Luiz was sent-off for bundling over Tammy Abraham. The resultant penalty was converted by Jorginho.
The Chelsea forward had pounced on a terrible backpass by Shkodran Mustafi and rounded keeper Bernd Leno when he was shoved over by Brazil defender Luiz.
Chelsea were dominant for long periods but failed to kill the game off and were punished when N’Golo Kante slipped inside the centre circle allowing 18-year-old Martinelli to gallop free and produce an excellent finish, the Brazilian’s 10th league and cup goal of the season.
It was Arsenal’s first shot on target and came after a Chelsea corner which was cleared by Mustafi, the German atoning for his earlier error.
Chelsea thought they had won it when skipper Cesar Azpilicueta scored from six yards six minutes from time but spirited Arsenal earned a point through captain Hector Bellerin – making his first appearances since 5 December, BBC reports.