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#1
Old 02-21-2017, 07:53 PM
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Join Date: May 2003
Posts: 31
Enthalpy of dissolved CaCl2 and NaHCO3

The question: What's the enthalpy when aqueous CaCl2 and aqueous NaHCO3 are reacted?

The background:
I'm a high school chemistry teacher and we do a lab where we mix calcium chloride and baking soda. Most things on the internet say this is an exothermic reaction. However, I found that (and Flinn agrees) the dissolving of the CaCl2 is highly exothermic, and the dissolving of the NaHCO3 is endothermic. So I had my students dissolve the CaCl2 first, and feel it get warm. I could also have them dissolve the baking soda in a separate container and feel it get cold.

I tried my own experiment of dissolving both, separately, and leaving them to reach room temperature and then mixing them. To dissolve a good amount of baking soda you have to use a good amount of water (only 4.8g dissolve in 50 mL water at 20 degrees C), so I only barely saw the water change from 19.0 to 17.7 degrees C. So I'm guessing the chemical reaction is endothermic, if both were aqueous. Can anyone confirm that?

Note: I dissolved 3g of baking soda in 50mL of water and ~3g CaCl2 in 5mL of water, to make sure it was in excess, and let them sit until they reached room temperature before putting them together in a cheap styrofoam calorimeter with a Styrofaom lid.
#2
Old 02-23-2017, 11:20 AM
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Join Date: Dec 2012
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Assuming everything stays in solution, the only net reaction I can think of is 2 HCO3-(aq) → CO32-(aq) + CO2(aq) + H2O(l). And I'm fairly sure that the corresponding decomposition of a solid salt (e.g. 2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g) ) has a positive ΔH, i.e. is endothermic.
In your case, you would have to add the enthalpy changes (heats of dissolution for the various species and the heat of vaporization for water) to get from aqueous solution to solids/gases and back. I still believe it would end up endothermic though. (I don't have a table of enthalpies at hand, so you'll have to do some math yourself. Good luck!)
#3
Old 02-25-2017, 05:45 PM
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Join Date: May 2003
Posts: 31
Hmm. The reaction most people agree upon is 2NaHCO3 + CaCl2 --> CaCO3(s) + 2NaCl(aq) + CO2(g) + H2O(l). I just want to start with aqueous reactants rather than solids, since it seems to me that the heat/enthalpy of dissolution for both solids is significant, and one is + and the other is -. So I'd guess the net ionic equation would be 2HCO3 - (aq) + Ca 2+(aq) --> CaCO3(s) + CO2(g) + H2O(l).

I guess I'm not sure why you have Na2CO3(s), or why I'd need the heat of vaporization of water (40.6 kJ/mol).
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