One thing that will greatly help you while reading Dune is to first read the appendices in the back of the book. Being familiar with the content of the appendices will go a long way to helping you contextualize the events of Dune and the universe it takes place in, particularly when Herbert name-drops unexplained content like the Orange Catholic Bible.
I would also recommend that you stick closely to your instructor's guidelines for a reflection, reaction and prediction, rather than just my advice. Without knowing your instructor's guidelines, my recommendations may actually stray from what they expect.
A reflection and a reaction are very close in meaning and content, and you'll probably have a lot of overlap between them. One will probably end up being much shorter, depending on which you prefer. The content can be summarized by thinking about how we use these words; when we encounter something, we react to it first, then we reflect upon it later. The reaction should cover your immediate feelings and opinions about what you've read, and the reflection should focus more upon what you've learned or what you take away from it. For example, your reaction to the description of Paul's test with the Reverend Mother may find the situation repulsive, but on reflection this is part of a rite of passage and it reveals much about his character without us having to be banally told "Paul is strong-willed and defiant."
A prediction is going to look more at your ability to understand the characters, their motivations, and how they fit or don't fit with the common plots and arcs of a story. For example, the Reverend Mother has essentially fulfilled her role in the story, and I would interpret her departure to be the last we see of her. Pick one or two elements that are unresolved and say how you expect them to be advanced, if at all, in the next chapter.