When do shifts of the tone and mood of The Hobbit occur?
One of the important shifts in tone occur in Chapter 14, which is where the narrator moves away from Bilbo and the dwarves and records what happened to Smaug and how he was killed by Bard, and then how Bard was appointed leader and King. What is significant about this chapter is that it takes a much more serious tone in what, up until this stage, has been on the whole a rather light-hearted adventure. It is in this chapter that the narrator chooses to stand back and explore what is happening elsewhere in Middle Earth and also the various political and ethnic groupings that all want Smaug's treasure now he is dead. This is where Tolkien sets up the war that will dominate the final chapters of the story, which is of course a massively serious event. Note for example how Bard begins to think of Smaug's treasure, now that its owner is dead:
Then even as he was speaking, the thought came into his heart of the fabled treasure of the Mountain lying without guard or owner, and he fell suddenly silent. He thought of the Master's words, and of Dale rebuilt, and filled with golden bells, if he could but find the men.
This represents a massive shift in mood and tone, not least because of the way in which the focus shifts from Bilbo and his companions to the humans and to other events, but also because it sets up the war and introduces the way that greed is such a powerful instinct not just in Thorin and the dwarves, but also in the humans and other races as well.