Succinic acid, a white solid that melts at 182°C, is heated gently, and a gas is given off. After the gas evolution stops, a white solid remains that melts at a temperature different from 182°C. In terms of the number of atoms contained, how do you thing the size of succinic acid molecules compares with the size of the molecules of the white solid produced by this process?

When succinic acid is heated and gives off a gas, resulting in a white solid that has a different melting point, this indicates that a chemical decomposition has occurred. Succinic acid, which has the formula C4H6O4, can undergo a decomposition reaction when heated, releasing water vapor (gas) and leaving behind a new compound. This reaction is an example of a thermal decomposition reaction, where a compound breaks down into simpler compounds upon heating.

The balanced equation for the decomposition of succinic acid might look something like this: C4H6O4 (succinic acid) → C4H4O3 (anhydride) + H2O (water vapor)

The succinic acid loses two hydrogen and one oxygen atoms as a molecule of water, resulting in the formation of succinic anhydride, which usually has a higher melting point than succinic acid. The molecule size of the remaining white solid (succinic anhydride) is therefore smaller than the original succinic acid molecule because it contains fewer atoms due to the loss of water.

This type of reaction is common in carboxylic acids (which succinic acid is), where heat can cause them to lose water molecules and form anhydrides.

Extra: Succinic acid is a dicarboxylic acid, meaning it contains two carboxyl (-COOH) groups. Upon heating, these -COOH groups can react with each other, eliminating a water molecule and forming a cyclic anhydride. Anhydrides are derived from acids by the removal of water. The resulting anhydride is typically more reactive than the parent acid and has different chemical properties, including a different melting point.

The concept of molecule size change is relevant to many areas of chemistry, especially when considering reactions that involve the making or breaking of chemical bonds. Students should understand that the size of a molecule in terms of the number of atoms can change due to chemical reactions that transform one substance into another. This can affect the physical properties of the substance, including melting and boiling points, solubility, and density. Understanding these changes is crucial in chemical synthesis and material science.