Passage VII

Students dissolved ammonium chloride (NH₄CI) and silver cyanate (AgOCN) together in water. A solid, Product 1 (P1), formed and was isolated. Product 2 (P2) was then recovered from the water in the solution. Samples of the products had the properties listed below:

P1: Melting point > 400°C; hard and brittle; not soluble in water; not flammable; formula unknown

P2: Melting point =132°C; soft and nonbrittle; soluble in water; flammable; formula is CH₄N₂O

Two students discuss the identity of these products.

Student 1

NH₄CI and AgOCN both are salts. Salts are composed of positively charged cations and negatively charged anions that hold together because they have opposite charges. When salts dissolve in water, they dissociate, or break apart, into anions and cations. When NH₄CI and AgOCN are dissolved in water, the following occurs:

NH₄CI + AgOCN → NH₄⁺ + CI^- + Ag⁺ + OCN-

The ions in solution then form new salts, silver chloride (AgCl) and ammonium cyanate (NH₄OCN):

NH₄⁺ + Cl^- + Ag⁺ + OCN^- → AgCl + NH₄OCN

P1 is AgCl: AgCl is insoluble in water, and salts tend to be hard and brittle, to be nonflammable, and to have high melting points. P2 is NH₄OCN, which is a salt composed of an ammonium ion and a cyanate ion:

[NH₄⁺][OCN^-]

This sample of NH₄OCN is soft and nonbrittle because it absorbed water from the air after it was isolated. This also caused the melting point to be lower than expected for a salt.

Student 2

P1 is AgCl, but P2 is not a salt. Ammonium ions do react with cyanate ions to form NH₄OCN, but this compound is unstable. To isolate P2, the solution was heated at 70°C to remove the water. This caused the salt to rearrange and form a more stable covalent compound. In a covalent compound, the atoms are held together by bonds, not by their opposite charges. P2 is urea, which has the following structure:

Covalent compounds are generally not as brittle as salts and tend to have lower melting points. Also, covalent compounds are much more likely to be flammable than are salts. Urea is also a compound that is found in mammalian urine, so it must be soluble in water.