Question

Manganese silicide has the empirical formula MnSi and melts at \(1280^{\circ} \mathrm{C}\). It is insoluble in water but does dissolve in aqueous HF. (a) What type of compound do you expect MnSi to be: metallic, molecular, covalent-network, or ionic? (b) Write a likely balanced chemical equation for the reaction of \(\mathrm{MnSi}\) with concentrated aqueous \(\mathrm{HF}\).

Short answer

MnSi is a molecular compound. The balanced chemical equation for the reaction of MnSi with concentrated aqueous HF is: MnSi (s) + 6 HF (aq) → Mn²⁺ (aq) + 2 F⁻ (aq) + SiF₄ (g)

Step-by-step solution

Identifying the type of compound MnSi

First, we need to identify the type of compound MnSi is. We're given that it has an empirical formula MnSi, melts at 1280°C, is insoluble in water, and does dissolve in aqueous HF. Mn (Manganese) is a transition metal while Si (Silicon) is a non-metal. Based on this, we can eliminate metallic (as it would involve metal atoms only) and ionic (which requires a metal and a non-metal with a significant electronegativity difference). Between molecular and covalent-network, covalent-network compounds are usually comprised of non-metallic elements. Since Mn is a transition metal and generally forms metallic bonds with other elements, the compound MnSi should be categorized as molecular.

Writing a balanced chemical equation for the reaction of MnSi with concentrated aqueous HF

Next, we need to write a balanced chemical equation for the reaction of MnSi with concentrated aqueous HF. MnSi is able to dissolve in aqueous HF, so we can assume that it reacts with HF to form products. Since Mn is a metal and can form positive ions, it is likely to form Mn(II) ions (Mn²⁺). Silicon will form SiF₄ upon reaction with HF: MnSi (s) + 6 HF (aq) → Mn²⁺ (aq) + 2 F⁻ (aq) + SiF₄ (g) Here, we have a balanced chemical equation: 1 MnSi reacts with 6 HF molecules to form 1 Mn²⁺ ion, 2 F⁻ ions, and 1 SiF₄ molecule.

Key concepts

Empirical Formula

The empirical formula represents the simplest whole-number ratio of the elements within a compound. It's like a recipe that gives the most basic version without the actual quantity. For instance, the empirical formula of manganese silicide is MnSi, indicating that for every atom of manganese (Mn), there's one atom of silicon (Si). However, it does not tell us how many of these pairs are in a single molecule.

Understanding the empirical formula is crucial because it is the starting point to determine the molecular formula, which shows the actual number of atoms in a molecule. When students tackle exercises involving empirical formulas, they need to be adept at recognizing how to simplify the ratios derived from the atomic or molar masses to their simplest form. By mastering this concept, students ensure that their basic understanding of chemical composition is solid, forming an essential foundation for more complex chemistry topics.

Covalent-Network Compounds

Covalent-network compounds are structures where atoms are bonded covalently in a continuous network extending throughout the material. Unlike discrete molecules, they form large 3D structures, such as diamonds or quartz. These compounds are characterized by high melting points and hardness and are generally poor conductors of electricity.

Manganese silicide, MnSi, based on its properties—high melting point and insolubility in water—could be mistaken for a covalent-network compound. However, the presence of manganese, a metal, suggests that the bonding in MnSi does not fit the typical covalent-network compounds, which are usually made of non-metal elements. As such, in the exercise improvement advice, it would be beneficial to highlight the distinctions between the different compound types more clearly, to avoid confusion. By exploring why MnSi does not qualify as a covalent-network compound, students gain a deeper understanding of the properties associated with different types of chemical bonding.

Balanced Chemical Equation

A balanced chemical equation is like a detailed accounting equation, but for chemistry. It ensures that the number of atoms for each element is the same on both sides of the reaction, adhering to the Law of Conservation of Mass. In the exercise given, MnSi reacts with aqueous HF to form manganese(II) ions, fluoride ions, and silicon tetrafluoride gas. The equation is balanced by making sure the same number of each type of atom appears on both the reactant and product side.

In the context of the exercise, optimizing the students' comprehension of how to balance equations is vital. By practicing with the coefficients in the equation—like putting a '6' in front of HF to ensure there are enough fluorine atoms to balance the products—students enhance their skills in stoichiometry. Learning how to properly balance chemical equations is an essential skill in chemistry that has wide applications, such as predicting the amounts of substances consumed and produced in reactions and understanding reaction stoichiometry.