In Harper Lee's To Kill a Mockingbird, what chapter and page number is this quote on—"Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me..."?
"Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me: several times he went so far as to tell me what to do.”
The quote you refer to in Harper Lee's To Kill a Mockingbird...
Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me: several times he went so far as to tell me what to do...
...is found at the beginning of Chapter Twelve (second paragraph of the first page of the chapter). This is the first chapter in Part Two. Jem has endured his punishment of reading to Mrs. Dubose for destroying her flowers (in defense of Atticus), and just learned of her death. He has also learned an important lesson about courage from Atticus as his father tells Jem that Mrs. Dubose was the bravest person he ever knew. This comment he justified by explaining to Jem that while the boy and his sister read daily to Mrs. Dubose, she was overcoming a morphine addiction that held her in its grip because of medication she had taken for the pain of her illness. This is the first impactful example of Jem's on-going "loss of innocence" as he sees the world through new eyes.
The event was a turning point of sorts for Jem. It is important to note that in the novel, Jem and Scout are both growing up, but because Jem is older, the lessons he is learning are much more sophisticated than the ones Scout has had to face.
The second lesson that will change Jem will be the outcome of Tom Robinson's trial, and then the result of Tom's attempted escape. We can infer that more learning will come from the attack made on the children at the end of the novel, but we never hear about it from Jem...only through Scout at the novel's beginning:
When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow. When it healed, and Jem's fears of never being able to play football were assuaged, he was seldom self-conscious about his injury.
Jem is still young enough to care most about his arm. We can assume that he comes to terms with the attack when the effectiveness of his throwing arm is his greatest concern.
The other reason Jem is acting differently is because of his age. Scout notes that he eats as if he has a "tapeworm." He tells his sister to leave him alone so many times that she speaks to Atticus about it. He tells Scout to give her brother some space, and we sense that he is in the stages of becoming a young man, that his moods are changing radically from day to day. (Later Jem will even show Scout the hair emerging on his chest—another sign that he is growing up.)
It is fairly obvious, however, that Scout has little time for her brother's new sense of self, and she is not afraid to knock him down a peg—as she does in Chapter 14 when she gets into a brawl with her brother because of how he was acting:
His maddening superiority was unbearable these days.
With a threat to kill him, Scout throws herself at her brother. Her anger echoes her earlier concerns of how Jem has been trying to tell her what to do.