Exam prep questions.....

A couple questions have come in on email:

How do we do a problem like this? i got it from the spring 2008 exam 3a.
You have determined that ΔHºreaction for the following reaction is –311.2kJ/mol.
3 Ca(OH)2(s) + 2 H3PO4(s) -> Ca3(PO4)2(s) + 6 H2O(l)
What is ΔHºreaction for the reaction:
2 Ca3(PO4)2(s) + 12 H2O(l)-> 6 Ca(OH)2(s) + 4 H3PO4(s)
a. 622.4 kJ/mol
b. 311.2 kJ/mol
c. 155.6 kJ/mol
d. -311.2 kJ/mol
e. -622.4 kJ/mol

This question is all about how the value of ΔHºreaction changes as a reaction is modified. The "new" reaction is the reverse of the original reaction, so the "new" process must be endothermic rather than exothermic (change the sign of ΔHºreaction). The "new" reaction is also balanced as a multiple of the first, so if 2x as much reaction occurs, then 2x the heat is required. The correct answer is "a".

I was doing some practice exams from previous years and are we
going to have to know how to do a qualitatively correct reaction coordinate
diagram for a stepwise process?

Yes, I expect you to be able to draw these. "Qualitatively correct" means I'm not looking for perfectly measured jumps and drops, but if a step in a reaction is 20kJ/mol exothermic it should look significantly larger than a step that's 2 kJ/mol exothermic, and both of them should be steps down. Your text doesn't always exactly call them reaction coordinate diagrams, but the figures on p. 242, 260, and 264 are all reaction coordinate diagrams. Although they're formatted a little differently than the way I typically draw them, these diagrams are conveying the same type of information. For those of you who will be taking Chem 210, some of the concepts we talk about in the second semester will be significantly easier to visualize if you start thinking about reactions as they appear/occur in a reaction coordinate diagram.

Keep the questions coming, I will post answers to the blog as soon as I can.