The climax of a story is the point in the narrative where the conflict is at its peak. The tension has been stretched to the breaking point. The climax is often a turning point in a story, and readers should see that the story moves from rising actions to falling actions.
In Chains, I would say that the climax is a sequence of events that happens in rapid succession beginning in chapter 43. Chapter 42 sees Isabel being asked to carry a note to Captain Morse. Before being able to deliver the note, Madam Lockton confronts Isabel about the interaction and demands that Isabel hand over the note. Isabel defies Madam Lockton's order and throws the note into the fire. Madam Lockton is enraged by Isabel's actions and threatens to sell her and Ruth.
This is a critical moment, because both readers and Isabel have been assuming that Madam Lockton already sold Ruth; however, it turns out that Lockton wasn't able to sell her. Madam Lockton sent her south to the Lockton's estate in Charleston. Madam Lockton again threatens to sell Ruth, and she locks Isabel in the potato bin.
If you have to pick a singular moment for the climax, I would use the confrontation just described; however, I think the climax continues into the next couple of chapters. Isabel breaks out of the potato bin, gets into Lockton's office, and forges her freedom papers. Next, Isabel manages to break Curzon out of prison. I would say that rescue is part of the climax, and the falling action and conclusion happen once Curzon is free. The two characters leave New York, and Isabel plans to seek out her sister.