[nickthegolfer]

I'm embarrassed to be turning to you guys once again, but I'm pulling out all stops here.  Grades are due, the teacher presumably is paid a salary based on how many students fail, and I don't know how they can reference evolution in a chem book.  But I digress.  I need to finish a thermo-chem review, problem is I only comprehend the specific heat part.  If someone could throw me a bone here, well, if you ever need a kidney donor...
Anyways, here's the problem...

1.

1. A piece of metal weighing 500.0 g is put into a boiling water bath. After 10 minutes, the metal is

immediately placed in 250.0 g of water at 40.0 °C. The maximum temperature that the system

reaches is 50.0 °C. What is the specific heat of the metal?



2. When 1 mole of hydrogen gas reacts with 1 mole of iodine gas to produce hydrogen iodide gas, 51.8

kJ of energy are absorbed.


a) Write a balanced thermochemical equation.


b) How much heat is involved in the decomposition of 5.843 g of HI into H2?


c) Is the heat in the decomposition absorbed or evolved? Justify your answer.



3. Use the equations:


2 C2H2(g) + 5O2(g) ? 4CO2(g) + 2H2O(g) ?H = -2512 kJ


N2(g) + ½ O2(g) ? N2O ?H = 104 kJ


To obtain ?H for the reaction:


C2H2(g) + 5N2O(g) ? 2CO2(g) + H2O(g) + 5N2(g)

...and here's what I've got so far:

(1)
1.833

(2)
(a)H2(g)+I2(g) --> 2HI(g) ?H=51.8 KJ
(b)...would you just use stoichiometry here, or is there something I'm missing?
(c) dear lord.  I suffered through AP Bio as a freshman.  I know evolution backwards and forwards, and I've yet to get to apply that knowledge.  What is heat evolution and heat absorption? (not in the glossary of the Chem book, and a human can probably explain better than google can)

(3)
if this were multiple choice I'm sure I'd be okay.  Sadly, I can't rely on deus ex machina in this situation.  Anyone willing to assist me?

Thanks for anything you can give me

[Dan]

(b)...would you just use stoichiometry here, or is there something I'm missing?
(c) dear lord.  I suffered through AP Bio as a freshman.  I know evolution backwards and forwards, and I've yet to get to apply that knowledge.  What is heat evolution and heat absorption? (not in the glossary of the Chem book, and a human can probably explain better than google can)

(3)
if this were multiple choice I'm sure I'd be okay.  Sadly, I can't rely on deus ex machina in this situation.  Anyone willing to assist me?

Thanks for anything you can give me

2(b) Yes, just convert to moles and use stoichiometry.
(c) evolution = given off or released ie. exothermic reaction
absorbtion = taken in ie. endothermic reaction
hint: what does the sign (positive or negative) of delta H tell you about this?

3. You need to construct a thermodynamic cycle (Hess cycle)

Hava a look at this picture http://upload.wikimedia.org/wikipedia/en/0/04/Hess%27_cycle2.png

DH_fR and DH_fP can be extracted from the given information. It doesn't matter which whay round the cycle you go, the energy is the same. So, reactants -> elements -> products is the same as reactants -> products.

You could have a look at this wikipedia article http://en.wikipedia.org/wiki/Hess's_law but I don't think this is a very good article to learn from, it has a heavily mathematical approach with almost no diagrams.