I'm thinking you are referring to Book V, since several of the books contain a "chapter 5", and it is written differently than the other books.
Book V is characterized by quick movements and dramatic situations. Most of the important action takes place within a few days, whereas the events in the other books are slow-moving, taking months to unfold all of the events. Hardy develops a melodramatic writing style, which is reflected in the characters, their actions, and their dialogue. Clym raves in his anger, losing all sense of perspective. Thomasin decides to bring her baby with her while walking through a storm. The passionate prose reflects the wildness of the characters.
Another difference in Book V is it doesn't reveal the truth about what happens to Eustacia -- was her death an accident, or did she jump? We know Eustacia has suicidal tendencies, but we aren't told the whole truth as we are in the rest of the novel. Critics suggest one reason for this is to show how unreliable the narrator is, while others say it's done on purpose so the reader can better understand the characters. Since the reader doesn't know if Eustacia committed suicide or not, we are forced to make our own decision based on what we already know about Eustacia, giving her a reality of her own.