In chapter 5 of To Kill a Mockingbird, why does Jem wait until Atticus is out of earshot to yell at him?
Jem waits to yell at his father for the same reason most of us do not yell directly at our parents; he doesn't want to get in trouble and he has a healthy respect for Atticus.
Although Atticus might not punish Jem for saying that he does not want to be a lawyer anymore, one cannot be sure. Jem makes this comment out of anger and that is what Atticus would probably have a problem with. Atticus is smart enough to understand that Jem probably wouldn't mean what he was saying, but he might be tempted to teach him a lesson about being impulsive and disrespectful.
Jem also respects Atticus, even if he is mad at him. So, he stops short of saying this to his father's face because he runs the risk of actually hurting his father's feelings, which Jem does not really want to do. He is just angry at the moment and this sentiment comes out as a result.
Jem is not stupid. He respects his father with a very healthy fear. He fears that yelling at Atticus would get him into trouble. Besides, what Jem says could be rather hurtful to Atticus. He tells Atticus that he doesn't think he wants to be a lawyer anymore. This is probably not true, but Jem says it to demonstrate his frustration with Atticus' involvement in the kids' efforts to discover Boo Radley. I think Jem believes that his dad doesn't understand their curiosity and his job of being a lawyer is part of what contributes to Atticus being so boring.