this is in sonata rondo form. in a sonata-rondo, the piece follows the thematic outline of a rondo (abacaba), and the tonal outline of a sonata (i v i or i iii i, etc.). beethoven uses mozart's favourite rite rondo form for this movement (abacba). the absence of the a theme in between the c and second b is a surprise and adds interest by reducing the repetition of the a theme.