José Rodríguez Alvira
Measures 7 and 8 are a variation of measures 3 and 4. The melody is transported an ascending minor 3rd. The harmony in the first 2 beats is exactly the same (a minor 3rd higher), but from then on Chopin moves chromatically the dominant seventh chords. Each seventh chord can be analyzed as the German sixth of the next chord:
The last chord will be used as a German Sixth chord. It will resolve into a Bb tonic in 2nd inversion.
The series of dominant chords moving chromatically can also be explained using the tritone substitution concept from jazz theory. Tritone substitution is the substitution of a dominant chord by another dominant chord that shares the same tritone and that is at the distance of tritone from the first chord. For example, G7 can be substituted by Db7 because they both share the same tritone (B - F and F - Cb):
If you use tritone substitution on every other chord on a cycle of fifths progression you get a series of chromatically descending chords just like in Chopin's prelude: