[cseil]

Today I've done my first lab experience within the Physical Chemistry 1 course.
Our aim was to calculate the ΔH of combustion of the salicylic acid, using as a standard the benzoic acid.

We used the Parr Bomb Calorimeter, worked with oxygen at 30 atm and with 400cm3 of water.
I started calculating the thermal capacity Cv with the combustion of the benzoic acid.

I could write the equation dH = dU + d(PV) as dH = dU + RTdn. dH is -3228 kJ/mol. Considering the reaction
C6H5COOH + 7.5 O2 => 7 CO2 + 3 H2O

I have a dn of -0.5. So I calculated dU as dH - RTdn and it is -3226 kJ.
Now dU is CvdT. dT is 3.36K (we calculated it) so Cv is -960 kJ/K.

Now we go to the combustion of salicylic acid.
The reaction is
C7H6O3 + 7O2 => 7CO2 + 3H2O

so dn is 0 and the equation dH = dU + RTdn becomes dU = dH.
dU is CvdT, dT is 2.32K, so dU = dH is -2227kJ/mol.

Is this right? The real value is -3020 kJ/mol. Is it a mistake of calculations or lab data?

[mjc123]

Your calculations appear to assume that you used exactly 1 mol of benzoic acid and salicylic acid. Did you really? If not, you have to correct for the actual number of moles you combusted. Do you have the masses you used?

[cseil]

Oh, I understand.
I actually used 0.33g of benzoic acid. So 2.70x10^-3 moles.

C6H5COOH + 7.5 O2 => 7 CO2 + 3 H2O
The dn now is (2.70x10^-3*7)-(2.70x10^-3*7.5)=-1.35x10^-3.

The fact is that if I put this number into the equation
dU = dH - RTdn it really changes nothing.
dH is -3228kJ, R is 8.314 J / mol * K so I have to convert it into kJ.

The result with the new dn is always -3228 kJ.
So everything continues to be wrong.

(in the equation of salicylic acid the dn is always 0 because of the stoichiometry)

[Borek]

To be honest, I have serious problems following what you are doing, too much chaos, not enough facts.

1. So far you used mass of the benzoic acid, but it is not clear what you calculated from it.

2. You should also use mass of the salicyilc acid, you have not even mentioned it.

[cseil]

I calculated the moles of benzoic acid, then using the reaction I calculated how many moles of O2 have reacted with it and how many moles of CO2 have been produced. The result is 2.70x10^-3 moles of benzoic acid used, 0.0189 moles of CO2, 0.0202 moles of O2. dn is 0.0189 - 0.0202 and it is -1.3x10^-3.

Then I used it in the equation
dU = dH - RTdn to calculate dU. 

But the result is always the same (the difference between dH and the other term is too big to appreciate a change).

I didn't consider the mass of salicylic acid because the reaction says that the moles of CO2 are equals to the moles of O2. So dn will always be 0, isn't it right?

[Borek]

Number of moles is not related to dn only.

If molar heat of combustion is 100 kJ/mole, and you burn 10^-2 moles, how much heat will be produced?

[cseil]

Oh, that's right!!
If ΔcH is -3228kJ/mol, I have to multiply this value for 2.7x10^-3, the number of moles.

Now I got a new value for Cv,
but when I put it into the equation

dH = dU = Cv dT

I get a result for dH. It is J/mol, right?

[Borek]

dH = dU = Cv dT

I get a result for dH. It is J/mol, right?

Why do you guess units instead of calculating them?

[cseil]

If dH = dU I obtain a value in J.
When I divide it by the number of moles of salicylic acid I obtain an absurd value (-10000 or so kJ) when I have to obtain a value ~ -3020kJ.

I don't understand if the problem is my calculation or my lab data.

[Borek]

You have to show your calculations in an ambiguous way, so far you want us to guess what you did. Won't work.

J is a correct unit for change in enthalpy, when it is not yet related to the amount of substance.

[cseil]

Ok, I'll do a recap of what I've done till now.
I'll start considering the benzoic acid.

The combustion of C6H5COOH follows the reaction
C6H5COOH + 7.5 CO2 => 7O2 + 3H2O

dn is -0.5 moles, because I have to consider only the species at the gaseous state.

I used 0.33g of benzoic acid, 2.7x10^-3 moles.
So dn in my reaction is -0.5*2.7x10-3 = -1.35x10-3 mol.

I have to calculate the heat capacity of my system.
I consider the equation

dH = dU + RTdn
dU = dH - RTdn

dH is -3228kJ/mol, so considering 2.7x10-3 moles, dH in my reaction is -8.715kJ/mol.

dU = -8711 J.

dU = CvdT
Cv = dU/dT

With dT = 3.36K (experimental), Cv = -2592 J/K.

Now I can start talking about the salicylic acid.
I have to find the dH value.

The combustion follows the reaction

C7H6O3 + 7O2 => 7CO2 + 3H2O

so dn is 0 for this reaction.
dH = dU.
and
dU = CvdT

dT is 2.32K (experimental) so dU = -6013 J.

I used 0.2g of salicylic acid, 1.45x10-3 moles.
So dH is -6013J /1.45x10-3 mol => -4147 kJ/mol.

The result is -3020 kJ/mol.
This is too much error..

I hope this is clear now,
what do you think I've done wrong?

[mjc123]

Your calculation looks mainly correct; I get dH for benzoic acid = -8.7315 kJ (not kJ/mol), looks like you made a typo. But this does not make a very big difference to the final answer, which is still way off. I'd suggest you check whether you made a mistake in transcribing your masses or temperatures. If not, it looks like something was wrong in the experimental procedure.