[kjones]

Dear Smart Chem People,

I am having my kids perform the baking soda and citric acid endothermic reaction.  I would like to compare their experimental heat of reaction to the theoretical heat of reaction.  I can find all the data except for the heat of formation for sodium citrate.  I have looked in the CRC and on the web.  Why is this so hard to find?   Or, does someone know the theorectical heat of reaction?  Formula is

H3C6H5O7 +3 NaHCO3   ---           3 H20 + 3 CO2  + Na3C6H5O7

Info on the lab
The lab is for the students to mixed 10 grams baking soda with 30 ml of 1.5 molar citric acid solution and measure the temp change.  They then assume a specific heat of water for the solution and calculate the heat loss by the water.  Assume all the heat loss by the water goes into the reaction, and change 10 grams to moles of baking soda so we can find the proper heat of reaction for the formula above.

Thanks for your help.  I have looked for the sodium citrate heat of formation in the CRC and on the web.

Kirk Jones

[Albert]

I'm sorry, Mr. Jones, but I could only find the heats of formation for citrate and citric acid:

- citrate: -895,12 [kJ/mol]

- citric acid: -1378,96 [kJ/mol]

[arau]

Please check my logic and math...

I need to calculate the heat of reaction for citric acid and sodium carbonate.  Like Mr. Jones, 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).

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 (endothermic)

Is my math and 30+ year old chemistry right???  If not, please correct!

Thanks!

Arau

[ShadyNarcotics]

Please check my logic and math...

I need to calculate the heat of reaction for citric acid and sodium carbonate.  Like Mr. Jones, 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).

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 (endothermic)

Is my math and 30+ year old chemistry right???  If not, please correct!

Thanks!

Arau

*Ignore me, I am impatient*

Can anyone tell me if =551.8 is correct and if so, how to convert it to KJ/mol

[khaldeman]

For kjones & albert,

You will not form sodium citrate in the reaction since it is in solution. You will form the citrate ion and Na⁺ will be a spectator ion. You can solve it with the info you have by using the net ionic equation.
H₃C₆H₅O₇ + 3HCO₃^1-  C₆H₅O₇^3- + 3H₂O + 3CO₂

[Enthalpy]

Kjones' kids must be grown-up by now.

[Acorn444]

Hi,

First of all, I would like to point out that people may be confused by the response by arau. He does calculate the enthalpy of formation of trisodium citrate, but he goes on to work out the reaction between citric acid and sodium carbonate, NOT with sodium bicarbonate.

Another thing that must be criticised is the use of kcal/gm-mole which is not a commonly used unit. It isn't even technically correct when it should be: kcal/mol. His units are inconsistent as well, changing between kcal/gm-mole and cal/gm-mole

The Next Big Thing: His calculations are incorrect! The actual value of x should be -449.8 kcal/gm-mole.

Using this convertor: http://www.colby.edu/chemistry/PChem/Hartree.html
-449.8 kcal/mol = -1881.963 kJ/mol

The value stated above can be a little discrepant due to rounding errors (many change of units). I calculated the enthalpy of formation using kJ/mol at the very start and worked out -1880.878 kJ/mol.

The Heat/Enthalpy of Formation of (Tri)Sodium Citrate is 1880 kJ/mol (3.s.f.).

Note that for more specific values, it's up to you which exact value you use.