Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: nickthegolfer on January 21, 2007, 07:03:34 PM
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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 :)
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(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
DHfR and DHfP 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.