Question

The early Romans discovered lead in mines for other valuable metals such as silver. They discovered rich fields of galena or lead sulfide in Derbyshire, which is located in what is now known as the United Kingdom, and began to mine it for use in their aqueducts and water pipes. What are the chemical formulas and correct IUPAC names of the two lead sulfides that could exist? Which of these compounds is given the common name galena?

Short answer

The chemical formulas for lead(II) sulfide and lead(IV) sulfide are PbS and PbS2, respectively. The IUPAC names are lead(II) sulfide and lead(IV) sulfide. Galena is the common name for lead(II) sulfide, which has the formula PbS.

Step-by-step solution

Identify the Possible Lead Sulfides

To find the two possible lead sulfides, we must recognize that lead can exhibit two oxidation states: +2 and +4. Therefore, the two possible lead sulfides can be formed by the combination of lead(II) and lead(IV) with sulfur, which has an oxidation state of -2.

Write the Chemical Formulas

Using the oxidation states, the chemical formulas for the lead sulfides can be determined. For lead(II) sulfide, the formula is PbS. For lead(IV) sulfide, the formula is PbS2. These are formed by balancing the charges of lead and sulfur ions in each compound.

Determine the IUPAC Names

The IUPAC names for the compounds are determined based on the oxidation state of the lead. The IUPAC name for PbS is lead(II) sulfide and for PbS2 is lead(IV) sulfide.

Identify Galena

Galena is the mineral form of lead(II) sulfide, which is PbS. It is the more common form of lead sulfide found in nature and the one that was historically mined by the Romans.

Key concepts

Chemical Formulas

Understanding chemical formulas is essential for identifying and working with different compounds. Chemical formulas are a way of representing the elements in a compound and the ratio of atoms of each element. They are composed of the element's symbols from the periodic table followed by subscript numbers indicating the number of atoms of each element present in the compound. For instance, the formula for water, H₂O, tells us that each molecule of water is composed of two hydrogen atoms and one oxygen atom.

In the context of lead sulfides, two chemical formulas exist due to lead's ability to form compounds in multiple oxidation states. The formula PbS represents lead(II) sulfide, where lead is in the +2 oxidation state and is chemically bonded with a single sulfur atom. Alternatively, the formula PbS₂ represents lead(IV) sulfide, where lead is in the +4 oxidation state and is bonded with two sulfur atoms. These formulas show the simplest ratio of ions that balance the overall charge of the compounds, fulfilling the rule that the total charge in an ionic compound must be zero.

IUPAC Naming

The International Union of Pure and Applied Chemistry (IUPAC) has standardized a nomenclature system to name chemical compounds systematically. The IUPAC naming system uses prefixes, suffixes and infixes to convey the composition and structure of compounds. In the case of the lead sulfides, we apply the oxidation state of the metal in the name to differentiate between the two compounds.

For PbS, the IUPAC name is lead(II) sulfide, indicating that the lead is in the +2 oxidation state. In PbS₂, the IUPAC name is lead(IV) sulfide, indicating that lead is in the +4 oxidation state and is bound to two sulfur atoms. This systematic approach to naming ensures that chemists around the world can communicate clearly and precisely about specific compounds, avoiding confusion with different naming conventions that may have been used previously.

Oxidation States

Oxidation states, also known as oxidation numbers, are a concept in chemistry that reflects the degree of oxidation of an atom in a compound. In simple terms, the oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were entirely ionic. Understanding oxidation states helps in predicting the combining power of an element, especially in ionic compounds where charges must balance.

Lead, with the symbol Pb, is a metal capable of exhibiting different oxidation states, the most common being +2 and +4. In PbS, lead is in the +2 oxidation state, indicating that it could lose two electrons to form an ionic bond with sulfur. Conversely, in PbS₂, lead is in the +4 oxidation state, meaning it could hypothetically lose four electrons when forming bonds with two sulfur atoms. Recognizing these states is essential for writing correct chemical formulas and IUPAC names and for comprehending redox reactions where oxidation states of elements are altered.