[CHM 111] Chapter 5 Questions #5.39

[Andrew J. Pounds]

On 6/8/19 12:53 PM, wrote:
> Hello Dr. Pounds, 
>
> I am a little confused with this problem and how they are getting some of the numbers, the problem says:
>
> A quantity of 2.00 x 10 ^2 mL of  0.862 M HCl is mixed with 2.00 x 10 ^ 2 mL of 0.431 M Ba(OH) 2 in a constant pressure calorimeter of negligible heat capacity. The initial temperature of the HCL  and BaOH2 solution is the same at 20.48°C. For this process H+ (aq) +OH- (aq) ———> H2O (l) the heat neutralization is-56.2 kJ/mol. What is the final temperature of the mixed solution? Assume the specific heat of the solution is the same as that for pure water. 
>
> In order to complete this problem I need to find grams, because they have me everything else. Okay, so I know the first thing I am supposed to do is make a balanced equation and then I am supposed to find the limiting reactant of either BaOH or HCL. I looked for the limiting reactant of HCl, and there appears to be no limiting reactant. After that, I am supposed to calculate the heat. However I am confused, because  when I am finding OH-, how do I know that One mole of OH is equal to -56.2 kJ, because the problem doesn’t say that. 
>
>
> Also, since I am finding grams and they do not give it to me, do I assume that the grams of HCL and BaOH is 1.00 g ? 
>
>
>

You do not need to find the grams because they have given you the
volumes and molarities -- remember molarity multiplied by volume is moles.

Note also that the barium hydroxide is providing TWO MOLES of hydroxide
to the reaction for each mole of barium hydroxide -- so in this case the
molarity of hydroxide is effectively double what is given (rather than
0.431 M, it is 0.862 M).

Now, according to the thermodynamic data given, the ENTHALPY OF
NEUTRALIZATION is for ONE MOLE of H+ reacting with ONE MOLE of OH- --
which is -56.2 kJ (per mole, that is provided in the problem statement).

So - use the molarities and volumes to determine the number of moles
reacted;  then use that value and the molar enthalpy of neutralization
to determine the heat  (q) evolved.

There is no need to assume 1.00 g of HCl and BaOH -- again, all the
information about the moles reacting in this case is tied up in the
molarity and volume.

They tell you to assume that the specific heat for the solution is that
of pure water (4.184 J/deg.g) and you can also therefore assume that the
density of water is 1.00 g/ml.   The solution is made by mixing 200 ml
of HCl with 200 ml of Ba(Cl)2 -- which would be a volume of 400 ml --
and based on the density of water, it should have a mass of 400 g.

Set up $q=mc_s \Delta T$, plug in all your known values and determine
the final temperature.


-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University,  Macon, GA 31207   (478) 301-5627

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