Is heat absorbed or released when vinegar reacts with baking soda?

When I mixed vinegar and baking soda, I observed a small temperature drop. How can this temperature change be explained?

Neutralization reactions are exothermic in general. But there are several conflicting processes going on in this reaction that contribute to the overall temperature change you observe:

1. The neutralization reaction HC₂H₃O₂ + NaHCO₃ = CO₂ + NaC₂H₃O₂(aq) + H₂O(l) releases heat. This is because there is net bond formation. The products collectively have lower energy than the reactants.
2. Evaporation of the liquid occurs as the carbon dioxide escapes from solution. Evaporation absorbs heat, cooling the liquid.
3. Joule-Thomson cooling of the carbon dioxide as it is released from the solution. Rapidly expanding carbon dioxide cools as it expands; this effect can be visualized as the result of intermolecular forces 'braking' and slowing the molecules as they try to move away from each other during the expansion. Lower molecular speeds mean lower temperatures.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/vinegar-baking-soda-enthalpy.shtml