How did Haydn’s approach to themes differ from Mozart’s approach in sonata form?

In the mature Classical style of Joseph Haydn and Wolfgang Amadeus Mozart, the primary role of musical themes was to function as elements of tonality, rather than simply serving as attractive melodies. While both composers utilized sonata form as a principle of organization based on key contrast, their approaches to thematic material differed in notable ways.1

Haydn often favored a "monistic" thematic approach, frequently using the same basic theme or material in both the tonic and the contrasting key areas. His works sometimes demonstrate a subtle interplay between thematic design and tonal schemes, such as in his Symphony No. 99, where the first theme is restated in the dominant key, initially mimicking a second subject before a distinct motive appears.12

In contrast, Mozart typically preferred strongly differentiated themes. His handling of thematic material often reached high levels of complexity; for instance, in his Symphony No. 35, he introduced a second theme that functions as a new perspective on the tonic, while the first theme persists as a counterpoint. Additionally, Mozart frequently reshaped his second subjects significantly when they reappeared in the recapitulation.12