Question

Molybdenum is obtained as a by-product of copper mining or is mined directly (primary deposits are in the Rocky Mountains in Colorado). In both cases it is obtained as \(\mathrm{MoS}_{2},\) which is then converted to \(\mathrm{MoO}_{3}\). The \(\mathrm{MoO}_{3}\) can be used directly in the production of stainless steel for high-speed tools (which accounts for about \(85 \%\) of the molybdenum used). Molybdenum can be purified by dissolving \(\mathrm{MoO}_{3}\) in aqueous ammonia and crystallizing ammonium molybdate. Depending on conditions, either \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Mo}_{2} \mathrm{O}_{7}\) or \(\left(\mathrm{NH}_{4}\right)_{6} \mathrm{Mo}_{7} \mathrm{O}_{24} \cdot 4 \mathrm{H}_{2} \mathrm{O}\) is obtained. a. Give names for \(\mathrm{MoS}_{2}\) and \(\mathrm{MoO}_{3}\) b. What is the oxidation state of Mo in each of the compounds mentioned above?

Short answer

a. The names for the given compounds are: 1. MoS2: Molybdenum Sulfide 2. MoO3: Molybdenum Trioxide b. The oxidation states of Mo in the mentioned compounds are: 1. MoS2: +4 2. MoO3: +6 3. (NH4)2Mo2O7: +6 4. (NH4)6Mo7O24·4H2O: +6

Step-by-step solution

Naming the given compounds MoS2 and MoO3.

To name the compounds MoS2 and MoO3, we follow the common rules for naming binary compounds. Binary compounds consist of two elements. The element with the lower group number is named first, and the one with the higher group number is named second with an '-ide' suffix. Mo is Molybdenum and S is Sulfur while O is Oxygen.

Determining the names of the compounds.

Using the aforementioned naming rules: 1. MoS2: Molybdenum Sulfide 2. MoO3: Molybdenum Trioxide

Define oxidation state and its determination.

The oxidation state (or oxidation number) refers to the charge assigned to an atom in a compound due to a loss or gain of electrons. The oxidation state of an element in a compound is determined by the sharing of electrons to satisfy the electrostatic charges in the compound. The total charge of a compound should equal zero.

Determine the oxidation state of Mo in the compounds mentioned.

We will determine the oxidation state of Mo in the following compounds: MoS2, MoO3, (NH4)2Mo2O7, and (NH4)6Mo7O24·4H2O. 1. MoS2: For sulfide(S), the oxidation state is -2. Since there are two sulfide ions, the total charge contributed from sulfide ions would be -4. Therefore, the oxidation state of Mo (Molybdenum) in this compound should counteract the Sulfide ions charge and be +4. 2. MoO3: For oxide (O), the oxidation state is -2. Since there are three oxide ions, the total charge contributed from oxide ions would be -6. Therefore, the oxidation state of Mo (Molybdenum) in this compound should counteract the oxide ions charge and be +6. 3. (NH4)2Mo2O7: In the ammonium ion (NH4)+, the oxidation state of nitrogen (N) is -3, and that of hydrogen (H) is +1. Therefore, the total charge for a single ammonium ion is +1, and since there are two ammonium ions, the total charge contributed by both is +2. Oxygen has an oxidation state of -2, thus total charge due to the seven oxide ions will be -14. So, the oxidation states of the two Mo atoms should counteract both the positive charge from ammonium ions (+2) and the negative charge from the oxide ions (-14) hence 2x(Mo oxidation state) = 12 or Mo oxidation state = +6. 4. (NH4)6Mo7O24·4H2O: The oxidation states for ammonium (NH4+) and oxide(O) are the same as described previously. The total charge contributed by six ammonium ions is +6, and total charge due to twenty-four oxide ions will be -48. Therefore, the oxidation states of the seven Mo atoms should counteract both the positive charge from ammonium ions (+6) and the negative charge from the oxide ions (-48) hence 7x(Mo oxidation state) = 42 or Mo oxidation state = +6. In summary, the oxidation states of Mo in the mentioned compounds are: MoS2 (+4), MoO3 (+6), (NH4)2Mo2O7 (+6), and (NH4)6Mo7O24·4H2O (+6).

Key concepts

Oxidation States

The oxidation state of an element in a compound is a pivotal concept in chemistry. It represents the hypothetical charge that an atom would have if all bonds were 100% ionic. This is handy for understanding electrons' movement during chemical reactions. For Molybdenum, determining its oxidation state helps us understand its role in compounds like MoS _2 and MoO _3.

• In MoS _2: Sulfur has a typical oxidation state of -2. Given there are two sulfur atoms, the total is -4. Molybdenum must balance this with a +4 oxidation state to maintain the compound's electroneutrality.
• In MoO _3: Oxygen generally has an oxidation state of -2. With three oxygens, this totals -6. Thus, Molybdenum must have an oxidation state of +6 to counterbalance.

Being adept at finding oxidation states deepens one’s understanding of redox reactions and elemental reactivity.

Naming Binary Compounds

Understanding how to correctly name binary compounds can make chemistry a lot more approachable. Binary compounds are those with two different elements. The naming process generally follows a set procedure:

• Name the element with the lower group number first. When writing MoS _2, Molybdenum comes before Sulfur because it's from a lower group.
• The second element receives the '-ide' suffix. Thus, MoS _2 becomes Molybdenum Sulfide.
• If the first element can have more than one common oxidation state, like many transition metals, the specific state should be indicated using Roman numerals. However, here, we don’t need them as Molybdenum forms stable compounds with specific states.

Such conventions make the naming coherent and provide insight into compound structure with just a glance.

Transition Metals

Transition metals, like Molybdenum, embody versatility and complexity in chemistry. Found in the d-block of the periodic table, these elements are notorious for having multiple oxidation states, making them central to many technological and biological processes.

• Molybdenum often exhibits oxidation states from +2 to +6, depending on the compound.
• These variable states are pivotal for catalysis. They allow transition metals to facilitate a wide array of reactions, key to industries ranging from petrochemicals to pharmaceuticals.
• The ability to form various oxidation states makes transition metals critical for making alloys like stainless steel, where they enhance properties like strength and corrosion resistance.

By studying transition metals like Molybdenum, we gain invaluable insights into their flexibility and role in sustaining diverse chemical processes.