In the first movement, the string quartet plays an arpeggiation of the tonic triad which is answered by the arpeggiation of the dominant seventh chord.  
The composer of this piece had projected an additional slow movement, a larghetto in D major, of which only sixteen measures survive.  The second movement features a dialogue between the first violins and the low strings to the accompaniment of sixteenth notes in the middle instruments. In the first movement, the string quartet plays an arpeggiation of the tonic triad which is answered by the arpeggiation of the dominant seventh chord.  The composer halted his work on Don Giovanni to write this piece, K. 525.  FTP, identify this serenade by Mozart that shares its name with a Stephen Sondheim musical.
The composer of this piece had projected an additional slow movement, a larghetto in D major, of which only sixteen measures survive.  
Eine Kleine Nachtmusik or A Little Night Music