José Rodríguez Alvira
These measures are a variation of measures 1 - 4. The same chords are used but the voice movement is much faster. This sort of diminution and an auxiliary diminished 7th of the V degree in measure 16 help create the climax in measure 18.
In the following example we compare measures 1 - 4 and measures 13 - 16. The lines show how the voice movement is accelerated in measures 13 - 16:
Measures 17 to 25 have some interesting chord progressions.
In measure 20 - 21, Chopin uses a deceptive cadence (V - VI) then adds a Bb to the C major chord. This note creates a C dominant 7th chord but the Bb is used as chromatic appoggiatura and resolves to a II degree chord.
In measure 23 a German augmented 6th chord is used with the augmented 6th in the bass. In many editions it appears spelled as a dominant 7th (C - E - G - Bb) just as the chord used in measure 21.