9

Citric acid and baking soda: Producing cold

A small amount of water (about 50 g) is put in a paper cup. Then some 15 g of citric acid (H3C6H5O7) are dissolved in it. It is observed that the temperature of the solution decreases from 22°C to about 15°C (Fig. 1). Then, baking soda (NaHCO3) is added eight times (2 g each time). There is a strong reaction, gas is produced, and the temperature of the solution decreases again every single time. In between, the temperature increases slowly.

Interpretation

The dissolution of citric acid in water is like a chemical reaction: We go from a solid substance to an aqueous form (dissolved in water). Dissolved citric acid contains more entropy than the solid form. This entropy is taken from the water, making the solution colder.

The reaction of citric acid with baking soda clearly releases energy which must produce entropy. Still, the temperature of the solution continues to fall. We conclude that the product of this reaction (sodium citrate, carbon dioxide, and water) take much more entropy than can be produced in the reaction running by itself. The water of the solution must provide the entropy. Therefore, its temperature drops.

Each time the temperature decreases, the form of the data taken indicates that the reaction proceeds quickly at first and then slows down. This is what has been observed before in reactions. In between adding the substances and letting them react, the temperature increases because of emtropy flowing in from the warmer environment.