Parejo delivered a corner deep to the back post for Moreno to head across the face of goal where Diakhaby rose highest to nod the ball into the net from six yards.
The Gabonese cuts back before offloading towards Lacazette on the charge, who had an open goal and he slided his left-footed strike into the back of the net.
Lacazette gets the better of his man in the middle and head the ball down towards the bottom corner. Neto pushed the ball against the post, but it strikes the inside of the woodwork and travels backwards over the line.
90th minutes Aubameyang scores to make it 3-1 for the Gunners. Valencia failed to clear their lines after the save from Neto. Kolasinac whips the ball to the back post where the Gabonese was unmarked to slit home.
Arsenal, with one feet in the final, will be hoping to round up with victory when they meet again during the second leg in Spain.
Luka Jovic broke the deadlock for Frankfurt 23 minutes into the game after Chelsea lose the ball in the middle of the pitch. Kostic whipped a cross into the box, which Jovic dives onto the end of to place the ball into the far corner of the net with a header.
On the stroke of half time, the Blues equalized. Pedro with great quick feet in the box after Emerson and Loftus-Cheek linked up well and the ball was laid it off to Pedro and the Spaniard fired the ball into the far corner of the net.
With an away goal advantage, the Blues will be looking to finish up the business at Stamford Bridge during the second leg.