Need a little push in the right direction

[Dao Tuat]

I have two questions.

First:

a coffe-cup clorimeter, 125 mL of a 2.75 M solution of silver nitrate at 25.00 degrees C is comined with 350 mL of a 4.00 M strontium chloride solution, aslo at 25.00 C. The temperature of the final solution is 32.15 degrees C. If the density of the final solution is 1.25 g/mL and the heat cpacity of the final solution is 4.33 J/C*g, calculate delta H in kJ/mol of silver chloride formed.

I'm not sure if I'm answering this correctly. Heres what I did:

I found that there is .34375 moles of AgNO3 and 1.4 moles of SrCl2

Using density = M/V, I found the mass to be 608g

I then calculated the heat absorbed to be 18.8 KJ

With this, I determined that the amount of moles burned = 349

I assumed a constant pressure, so that delta H = delta E = 18.8/349, thus ending up with .0539 kJ/mol

Does this seem right at all?



Second:

I have no idea how to do this problem, or even attempt to do it:

I'm asked to calculate the enthalpy change for CaC2 + 2 H2O = Ca(OH)2 + C2H2, given:
Ca + 2C = CaC2 delta H =-62.8 kJ
Ca+(1/2)O2 =CaO delta H =-635.5 kJ
CaO + H2) = Ca(OH)2 delta H =-653.1kJ
C2H2 + (5/2)O2 = 2 CO2 + H2O delta H = -1300kJ
C + O2 = CO2 delta H =-393.51kJ


Can anyone help me out here? I don't know anyone who has taken a chemistry class and my book isn't that good (I got the cheapest one I could find in english). An explanation or any good websites discussing problems such as these would be highly appreciated.


Thanks

 

[lizzyb]

Okay, the idea here is manipulate the given equations
into the form you want, noting that you may
switch the direction of the equations provided you
change the sign on delta H, and you can adjust
the stochiotic (sp?) values provided you multiply
or divide delta H by that same amount. Here goes:

Want: CaC2 + 2 H2O = Ca(OH)2 + C2H2

Have:

(1) Ca + 2C = CaC2 delta H =-62.8 kJ
(2) Ca+(1/2)O2 =CaO delta H =-635.5 kJ
(3) CaO + H2O = Ca(OH)2 delta H =-653.1kJ (<-- CHANGED to H2O)
(4) C2H2 + (5/2)O2 = 2 CO2 + H2O delta H = -1300kJ
(5) C + O2 = CO2 delta H =-393.51kJ

CaC2 = Ca + 2C                 delta H = 62.8 kj (using (1))
CaO + H20 = Ca(OH)2            delta H = -653.1 kj (using (3))
2 CO2 + H20 = C2H2 + (5/2)O2   delta H = 1300 kj (using (4))

Lets add these together and see what we get and work from
there:

CaC2 + CaO + 2 H2O + 2 CO2 = Ca + 2C + Ca(OH)2 + C2H2 + (5/2)O2

                               delta H = 62.8 + -653.1 + 1300
                                       = 709.7 kj

so next you may want to get rid of CaO on the LHS, so you might
add

   Ca + (1/2) O2 = CaO         delta H = -635.5 kj

so we have:

CaC2 + CaO + 2 H2O + 2 CO2 + Ca + (1/2) O2 =

Ca + 2C + Ca(OH)2 + C2H2 + (5/2)O2    delta H = 709.7 - 635.5 kj
                                              = 74.2

But some of the elements cancel (Ca on the left and
Ca on the right, etc), so we have

CaC2 + CaO + 2 H2O + 2 CO2 = 2C + Ca(OH)2 + C2H2 + 2 O2

                                      delta H = 74.2

Note what happened to O2, we had (1/2) on the left and
(5/2) on the right, (5/2) - (1/2) = 2.

Anyway you sort of work it like that until you get
the right equation.

 

[Dao Tuat]

Okay, that makes sense. Thanks a lot!

How does the first answer look? I feel like I did it wrong. Is it safe to assume constant pressure, or should I be trying to solve this some other way?

Thanks again for your help.