Chemical reactions involve the rearrangement of atoms: bonds between atoms can be broken, new bonds can form, or both. For a bond to break, energy is required. And, when a bond forms, energy is released. So, if you add up all the energy consumed to break bonds and produced when bonds form, you get the total change in energy for the reaction. But, any energy consumed needs to come from somewhere, and any energy produced needs to go somewhere. In a reaction happening in water, if energy is produced (an exothermic reaction) it increases the temperature of the water. If energy is consumed (an endothermic reaction) it is taken from the water, making the water colder (an endothermic reaction).

So, to determine whether a reaction is endothermic or exothermic, use a thermometer to measure the temperature! For baking soda and vinegar, the reaction will become cold, because this reaction consumes energy. Try it!

To simply answer the question as written: the reaction of baking soda with vinegar is endothermic (i.e., the reaction takes in thermal energy from the surroundings). However, a deeper look at the thermodynamics is informative though.

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The following text in this answer is for more advanced students who already have deeper knowledge of thermodynamics. However I recommend to any reader to go through the answer and find out the conclusion of our scientist who wrote it; READ THE END OF ANSWER #3, the deep research of our scientists is an excellent source of knowledge of the science behind this question.
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The reaction of baking soda and vinegar (the former being sodium bicarbonate and the latter a solution of acetic acid and water) is spontaneous, meaning the products (here, sodium acetate, water, and CO₂) are lower in energy than the reactants (sodium bicarbonate and acetic acid), meaning the forward reaction is favored. Since this reaction involves gaining thermal energy from the surroundings, one can conclude that there must be an additional contribution to the energy change.

When temperature and pressure can be considered constant and other sources of work are not acting on the system (the conditions for an open lab, where this reaction is probably taking place) the Gibbs free energy G is used to describe the state of the system, and the change in energy is given by dG. A spontaneous reaction thus requires that dG < 0 is satisfied (total energy decreases).

G is given by G = H - T*S, where H is the enthalpy (a measure of thermal energy), T is the temperature, and S is the entropy (a measure of the number of arrangements, often called disorder); and the change in energy by dG = dH - T*dS (there is no dT because temperature is constant under these conditions). This reveals that there are two contributions to the energy change of this reaction, one from thermal energy and one from the number of arrangements.

As stated above, this reaction is endothermic, meaning dH > 0. If this were the only factor then dG would also be >0 and the reaction would not be spontaneous. Thus, the magnitude of T*dS must be larger than the magnitude of dH, and that allows this reaction to take place without requiring additional stimulus despite taking in energy from the surroundings.

Some other discussions of endo/exothermic reactions here; this particular reaction on ScienceLine 1 , 2; and here.

[Getting farther afield now] The enthalpy of the reaction dH is due to breaking and forming chemical bonds.

Breaking bonds always takes energy, and forming bonds always releases energy. In this reaction, bonds of the baking soda and acetic acid are broken; bonds are formed to make sodium acetate, water, and CO₂.

This paragraph continues in the following answer,

The entropy change can be thought of as adding or removing possible arrangements. Considering the reaction of sodium bicarbonate with acetic acid accounts only for the chemical reaction that occurs when baking powder and vinegar are mixed though. Vinegar is a only ~5% acetic acid and 95% water, meaning that this reaction occurs in a solution. dH and dS of the overall reaction must include contributions due to the enthalpy and entropy of dissolution because intermolecular bonds between the water and other molecules break and form, and the reactants and products are distributed throughout the solution to different degrees.

(I could not find a good reference, but I think the reaction between the sodium bicarbonate and acetic acid is exothermic and it is the dissolution component that makes the overall reaction of baking soda+vinegar endothermic).

When you mix baking soda and vinegar together, you will notice that the mixture drops in temperature. This is because the reaction is endothermic and requires the absorption of heat in order to create the products of the reaction which are sodium acetate, water, and carbon dioxide.

Best,

Since endothermic reactions require heat to be put into the reaction, the temperature of the container in which the reaction takes place will decrease. Exothermic reactions emit energy as heat and the container temperature will increase. If you place vinegar and baking soda into a glass, as the reaction starts and bubbles begin forming, you can place your hand on the glass and feel the temperature decrease. You can also use a thermometer to track the temperature change before, during, and after the reaction.

Mixing baking soda and vinegar together creates an endothermic reaction, which means it absorbs heat and becomes cold. This is because when the two chemicals are mixed together, their atoms have to rearrange to create the final product. Rearranging atoms like this takes energy in the form of heat, so the mixture becomes slightly colder.

Does the glass get hotter or colder when you mix the 2 in a glass? Wear glasses and do the experiment over a sink. ... It looks like you need a thermometer.

I didn't feel much temperature change, but the mixture foamed up over the glass.

Here's the answer:
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Exothermic. Generally-speaking, if a chemical reaction happens spontaneously, it is exothermic.