A gas evolution equation describes a chemical reaction that produces carbon dioxide gas (CO2) as a result of the transformation of certain reactants. In this context, the term "medium product" refers to an intermediate product that forms before it fully converts into the final product. Specifically, in gas evolution reactions involving acids and bicarbonate ions, the medium product is carbonic acid (H2CO3).
When hydrogen ions (H+) react with bicarbonate ions (HCO3-), they form carbonic acid. However, carbonic acid is unstable and cannot exist in isolation; it quickly decomposes by losing a water molecule (H2O). The reaction can be summarized as follows:
H+ + HCO3- → H2CO3 (medium product) → CO2 (gas) + H2O (liquid)
Similarly, when H+ reacts with carbonate ions (CO32-), carbonic acid is again formed as the medium product, which subsequently decomposes to yield carbon dioxide gas and water:
H+ + CO32- → H2CO3 (medium product) → CO2 (gas) + H2O (liquid)
In essence, the gas evolution equation illustrates how carbonic acid, while initially formed, ultimately leads to the production of carbon dioxide gas and water through the loss of a water molecule. This process highlights the dynamic nature of chemical reactions, where intermediate products play a crucial role in the formation of final products.