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Name: Barbara
Status: student
Grade: 9-12
Location: CA
Date: July 2008


Question:
When baking soda and calcium chloride are mixed with water, they undergo a chemical change to produce water, salt, calcium carbonate and carbon dioxide. What function does the water added to the reactants have? Why is it not included in the reactants side of the equation?



Replies:
The reaction between baking soda and calcium chloride is:

2 NaHCO3 + CaCl2 ===> CaCO3 + CO2 + 2 NaCl + H2O

Notice that in the balanced equation water appears a reaction product. So it plays a role in the reaction as a reaction product. More water added acts as a solvent for the reaction dissolving the NaHCO3 and CaCl2 reactants as well as the product NaCl -- and to a lesser extent some CO2. The product CaCO3, calcium carbonate has very limited solubility in water, unless the pH is very low. It is customary to assume that reactions like the one you describe is dissolved in water, unless otherwise specifically noted. The reaction will in fact run in the "solid" state. However, the rate of reaction is very slow due to the difficulty in "mixing" the solid reactants. I would expect a "dry" mixture reactants would form a "clump" as some water is produced in the reaction.

Vince Calder


Barbara,

You apparently balanced the equation:

2 NaHCO3 + CaCl2 = CaCO3 + CO2 + 2 NaCl + H2O.

The water on the product side is formed during the reaction. We could perform this reaction with dry chemicals; it would take a very long time to go from reactants to products. In order to speed up the reaction we dissolve the reactants in water. Both reactants are ionic, meaning that they dissociate (break apart) in water. Then the kinetic energy of the water molecules bumps the ions from the reactants into each other. Hydrogen ions bump into oxygen ions and covalently bond into water. In similar ways, carbon and oxygen combine to form carbon dioxide, a gas that bubbles out of the reaction, and the Calcium carbonate forms -- only a little dissolves, the rest is a solid. Finally the sodium ions and chloride ions remain in solution, but they can be captured/collected by gently boiling the solution. Water evaporates, leaving solid (crystalline) sodium chloride.

Warren Young



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