Problem 13 - Chemical Principles: The Quest for Insight

Question

Name the compounds (a) \(\mathrm{Ag}_{2} \mathrm{~S}\); (b) \(\mathrm{ZnC}_{2}\); (c) \(\mathrm{CF}_{3} ;\) (d) \(\mathrm{Mg}(\mathrm{OH})_{2} ;\) (c) \(\mathrm{NiSO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} ;\) (f) \(\mathrm{PCl}_{5} ;\) (g) \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{PO}_{4}\right)_{3}\); (h) \(\mathrm{As}_{2} \mathrm{O}_{3}\); (i) \(\mathrm{MnCl}_{2}\).

Step-by-Step Solution

Answer

(a) Silver sulfide, (b) Zinc dicarbide, (c) Carbon trifluoride, (d) Magnesium hydroxide, (c) Nickel (II) sulfate hexahydrate, (f) Phosphorus pentachloride, (g) Chromium (III) hydrogen phosphate, (h) Diarsenic trioxide, (i) Manganese (II) chloride.

Step 1 - Naming Simple Binary Ionic Compounds

To name simple binary ionic compounds, identify the cation (metal) and the anion (non-metal) and use the full name of the cation followed by the base name of the anion with the suffix '-ide'. Compounds (a) and (i) fall into this category. For (a) \(\mathrm{Ag}_{2} \mathrm{~S}\), we have silver (Ag) as the cation and sulfur (S) as the anion, so the name is silver sulfide. Similarly, for (i) \(\mathrm{MnCl}_{2}\), manganese (Mn) is the cation and chlorine (Cl) the anion, resulting in the name manganese (II) chloride.

Step 2 - Naming Ionic Compounds with a Polyatomic Ion

To name ionic compounds containing a polyatomic ion, name the metal cation followed by the name of the polyatomic ion. Compound (d) is \(\mathrm{Mg}(\mathrm{OH})_{2}\), with magnesium (Mg) as the cation and hydroxide (OH) as the polyatomic anion, so its name is magnesium hydroxide. Compound (c), \(\mathrm{NiSO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\), contains nickel (II) as the cation and sulfate (SO4) as the polyatomic anion, and is named nickel (II) sulfate hexahydrate.

Step 3 - Naming Molecular Compounds

For molecular compounds, use the appropriate Greek prefixes to indicate the number of atoms of each element. Compounds (b), (c), (f), and (h) are molecular compounds. (b) \(\mathrm{ZnC}_{2}\) is named zinc dicarbide because we use 'di-' for the two carbon atoms. Compound (c) \(\mathrm{CF}_{3}\) is called carbon trifluoride. Compound (f), \(\mathrm{PCl}_{5}\), is phosphorus pentachloride. And (h) \(\mathrm{As}_{2} \mathrm{O}_{3}\) is diarsenic trioxide.

Step 4 - Naming Compounds with Transition Metals

Transition metals can have more than one possible oxidation state. Indicate the oxidation state of the metal with Roman numerals in parentheses. This applies to compounds (a), (i), and also (g). Compound (g), \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{PO}_{4}\right)_{3}\), is a complex ionic compound with chromium, which can have various oxidation states. Here it acts as a cation with hydrogen phosphate as the polyatomic ion, resulting in the name chromium (III) hydrogen phosphate.

Key Concepts

Binary Ionic Compounds

Understanding the naming convention for binary ionic compounds is essential when starting with chemical nomenclature. These compounds are composed of two different elements - typically a metal and a non-metal.

The key to naming them is to identify the cation, which is the metal, and the anion, which is the non-metal. Take the full name of the metal and directly append it with the base name of the non-metal, altering the non-metal’s ending to '-ide'. For example, let's look at the compound \(\mathrm{Ag}_{2} \mathrm{~S}\). Here, \(\mathrm{Ag}\) is silver, and \(\mathrm{S}\) is sulfur, hence the name silver sulfide. Similarly, for \(\mathrm{MnCl}_{2}\), manganese is the cation, and chlorine is the anion, resulting in manganese (II) chloride, indicating manganese has an oxidation state of +2.

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more atoms covalently bonded, or of a metal complex that acts as a single unit in chemical reactions. Naming ionic compounds containing polyatomic ions requires a slightly different approach.

First, name the metal cation as you would in a simple ionic compound. Then, add the name of the polyatomic ion. For example, \(\mathrm{Mg}(\mathrm{OH})_{2}\) comprises magnesium (Mg) as the cation and hydroxide (OH) as the anion, leading to the name magnesium hydroxide. Also, compound \(\mathrm{NiSO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) includes nickel with a +2 oxidation state and the sulfate ion, \(\mathrm{SO4}\), named nickel (II) sulfate hexahydrate. Hexahydrate signifies the presence of six water molecules associated with the compound.

Molecular Compounds

Molecular compounds are formed when two or more nonmetals share electrons and form a covalent bond. Unlike naming ionic compounds, the nomenclature for molecular compounds involves using Greek prefixes to indicate the number of atoms present.

For example, the compound \(\mathrm{CF}_{3}\) consists of one carbon atom and three fluorine atoms, hence named carbon trifluoride. Similarly, \(\mathrm{PCl}_{5}\) is named phosphorus pentachloride, where 'penta-' signifies the presence of five chlorine atoms. Greek prefixes are integral to differentiating molecular compounds since they precisely convey the number of each atom contained within the compound.

Transition Metals Oxidation States

Transition metals are known for having various possible oxidation states, making their naming more complex. When naming compounds with transition metals, it is critical to indicate the metal's oxidation state using Roman numerals in parentheses.

For instance, with \(\mathrm{MnCl}_{2}\), manganese can exist in multiple oxidation states, but the presence of two chloride ions implies it is in the +2 oxidation state, leading to the name manganese (II) chloride. Similarly, chromium in \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{PO}_{4}\right)_{3}\) acts as a cation with an oxidation state of +3, as it is paired with three hydrogen phosphate ions, resulting in the name chromium (III) hydrogen phosphate. Identifying the correct oxidation state is crucial for the accurate naming of transition metal compounds.