I can think of a few literary devices present in these chapters, and maybe others can think of more.
Chapter 30 of The Adventures of Huckleberry Finn offers both a denouement to the plot line following the King and the Duke as well as a little bit of comic relief. As they make their escape, the two continue to fight, convinced that the other was trying to steal the gold.
In Chapter 31, the climax of the novel occurs. Huckleberry Finn comes to the realization that Jim is human, not just property. Chapter 31 also offers a great example of irony: Huckleberry Finn becomes aware of Jim's humanity at the exact moment that he is captured and re-enslaved.