[arau]

Please check my logic and math...

I need to calculate the heat of reaction for citric acid and sodium carbonate.

I couldn't find the heat of formation for sodium citrate.  I did, however, find the heat of neutralization for citric acid with NaOH.  (see: http://www.ecama.org/level_2/frames/safety_fr.html, click on citric acid; the math: 265 cal/gm = 0.265 * 192 = 50.9 kcal/gm-mole, exothermic).

Knowing the heat of formation of NaOH (-112.2 kcal/gm-mole), water (-68.3 kcal/gm-mole) and citric acid (-369 kcal/gm-mole), I back calculated the heat of formation of trisodium citrate as -551.8 cal/gm-mole:

citric acid + 3NaOH ---> Na3 citrate + 3H2O
-50.9=(x+(3*-68.3))-(-369+(3*-112.2))
x=-551.8

I plugged this value into the stoichiometry of the citric acid/Na2CO3 reaction (knowing the heat of formation for CO2 is -94 kcal/gm-mole and Na2CO3 is -275 kcal/gm-mole):

2 Citric Acid + 3 Na2CO3 ---> 2Na3citrate + 3 CO2 + 3 H2O
({2*-551.8} + {3*-94} + {3*-68.3}) - ({2*-369} + {3*-275})=-27.1 (exothermic??)

I am sure that effervescent reactions are endothermic, but I can't figure out where my chemistry and/or math is wrong.  Could the heat of neurtralizatoin for citric acid really be expressed in kcal/gm-mole rather than kcal/gm?  Any and all help is appreciated!

Thanks!

Arau

[Albert]

I didn't check your calculations. However, heat of formation of citric acid is -439.3 kcal/mol, citrate = -213.94 kcal/mol.
Instead of sodium carbonate, you can use carbonate ion whose heat of formation is - 161.83 kcal/mol.

I got these data from the NIST website and my physical chemistry book.

[arau]

Thanks!

So, solving for: 2 Citric ion⁽³⁺⁾ + 3 Carbonate ion ^(2-)  --> 2 citrate ion ^(3-) + 3 CO₂ + 3 H₂0

is the same as solving reaction with the sodium ions included (as first stated)?

Arau

[Albert]

I would say it's the same.

2 Citric ion⁽³⁺⁾ + 3 Carbonate ion ^(2-)  --> 2 citrate ion ^(3-) + 3 CO₂ + 3 H₂0

You meant citric acid, didn't you?

[arau]

Good point.  Yes, I meant citric acid rather than citric ion.

Thanks for your help.  Truly appreaciated!

Arau