Question

Other nonmetals form compounds that are structurally analogous to those of carbon, but these inorganic compounds are usually more reactive. Predict any missing products and write balanced equations for each reaction: (a) the decomposition and chlorination of diborane to boron trichloride; (b) the combustion of pentaborane \(\left(\mathrm{B}_{5}\mathrm{H}_{9}\right)\) in \(\mathrm{O}_{2} ;\) (c) the hydrolysis of trisilane \(\left(\mathrm{Si}_{3} \mathrm{H}_{8}\right)\) to silica \(\left(\mathrm{SiO}_{2}\right)\) and \(\mathrm{H}_{2} ;\) (d) the complete halogenation of disilane with \(\mathrm{Cl}_{2} ;\) (e) the thermal decomposition of \(\mathrm{H}_{2} \mathrm{~S}_{5}\) to hydrogen sulfide and sulfur molecules; (f) the hydrolysis of \(\mathrm{PCl}_{5}\).

Short answer

Balanced equations: (a) \[\text{B}_{2}\text{H}_{6} \rightarrow 2\text{B} + 3\text{H}_{2}\] \ and \[2\text{B} + 3\text{Cl}_{2} \rightarrow 2\text{BCl}_{3}\]; (b) \[\text{B}_{5}\text{H}_{9} + 12\text{O}_{2} \rightarrow 5\text{B}_{2}\text{O}_{3} + 9\text{H}_{2}\text{O}\]; (c) \[\text{Si}_{3}\text{H}_{8} + 6\text{H}_{2}\text{O} \rightarrow 3\text{SiO}_{2} + 10\text{H}_{2}\]; (d) \[\text{Si}_{2}\text{H}_{6} + 6\text{Cl}_{2} \rightarrow 2\text{SiCl}_{3} + 6\text{HCl}\]; (e) \[\text{H}_{2}\text{S}_{5} \rightarrow 5 \text{S} + 2 \text{H}_{2}\text{S}\]; (f) \[\text{PCl}_{5} + 4\text{H}_{2}\text{O} \rightarrow \text{H}_{3}\text{PO}_{4} + 5\text{HCl}\].

Step-by-step solution

Decomposition and Chlorination of Diborane (a)

The decomposition of diborane \(\text{B}_{2}\text{H}_{6}\) produces elemental boron and hydrogen gas: \[\text{B}_{2}\text{H}_{6} \rightarrow 2\text{B} + 3\text{H}_{2}\] Then, the boron reacts with chlorine to form boron trichloride: \[2\text{B} + 3\text{Cl}_{2} \rightarrow 2\text{BCl}_{3}\]

Combustion of Pentaborane (b)

Balance the combustion equation of pentaborane \(\text{B}_{5}\text{H}_{9}\) in oxygen: \[\text{B}_{5}\text{H}_{9} + 12\text{O}_{2} \rightarrow 5\text{B}_{2}\text{O}_{3} + 9\text{H}_{2}\text{O}\]

Hydrolysis of Trisilane (c)

The hydrolysis of trisilane \(\text{Si}_{3}\text{H}_{8}\) forms silica \(\text{SiO}_{2}\) and hydrogen gas: \[\text{Si}_{3}\text{H}_{8} + 6\text{H}_{2}\text{O} \rightarrow 3\text{SiO}_{2} + 10\text{H}_{2}\]

Complete Halogenation of Disilane (d)

Balancing the complete halogenation of disilane \(\text{Si}_{2}\text{H}_{6}\) with chlorine: \[\text{Si}_{2}\text{H}_{6} + 6\text{Cl}_{2} \rightarrow 2\text{SiCl}_{3} + 6\text{HCl}\]

Thermal Decomposition of \(\text{H}_{2}\text{S}_{5}\) (e)

The thermal decomposition of \(\text{H}_{2}\text{S}_{5}\) results in hydrogen sulfide and elemental sulfur: \[\text{H}_{2}\text{S}_{5} \rightarrow 5\text{S} + 2\text{H}_{2}\text{S}\]

Hydrolysis of Phosphorus Pentachloride (f)

The hydrolysis of phosphorus pentachloride \(\text{PCl}_{5}\) forms phosphoric acid and hydrochloric acid: \[\text{PCl}_{5} + 4\text{H}_{2}\text{O} \rightarrow \text{H}_{3}\text{PO}_{4} + 5\text{HCl}\]

Key concepts

inorganic compounds

Inorganic compounds are generally made up of elements other than carbon and hydrogen, although there are exceptions. These compounds are typically formed through the combination of metals with nonmetals or among nonmetal elements themselves.
In the exercise, diborane, pentaborane, trisilane, disilane, and phosphorus pentachloride are all inorganic compounds. They contain elements such as boron, silicon, and phosphorus, combined with hydrogen or chlorine.
Inorganic compounds often exhibit different physical and chemical properties compared to organic compounds, which are mainly based on carbon chains and rings.

reactivity of nonmetals

Nonmetals have a tendency to gain electrons, making them highly reactive in many scenarios. They typically form covalent or ionic bonds with other elements.
In the exercise, reactivity is observed in various reactions:

• Chlorination of diborane leads to boron trichloride.
• Combustion reactions with pentaborane produce boron trioxide and water.
• Hydrolysis reactions involving trisilane and disilane produce silica and hydrochloric acid, respectively.

Nonmetals like chlorine and oxygen have high reactivity, due to their electronegativity and ability to form strong bonds with other elements.

balanced chemical equations

A balanced chemical equation has equal numbers of each type of atom on both sides of the equation. This is essential to satisfy the law of conservation of mass. In the exercise:

• The decomposition and chlorination of diborane are represented as: \[\text{B}_{2}\text{H}_{6} \rightarrow 2\text{B} + 3\text{H}_{2}\right\rightarrow 2\text{BCl}_{3}\right\]
• The combustion of pentaborane is given by: \[\text{B}_{5}\text{H}_{9} + 12\text{O}_{2} \rightarrow 5\text{B}_{2}\text{O}_{3} + 9 \text{H}_{2}\text{O}\]
• The hydrolysis of trisilane is shown as: \[\text{Si}_{3}\text{H}_{8} + 6\text{H}_{2}\text{O} \rightarrow 3\text{SiO}_{2} + 10\text{H}_{2}\]

Balancing chemical equations ensures that the same number of atoms are present on both sides, hence respecting the conservation of mass.

combustion reactions

Combustion is a chemical reaction where a substance reacts with oxygen to release energy in the form of heat and light. These reactions typically produce oxides and water. In the context of the exercise, pentaborane \(\text{B}_{5}\text{H}_{9}\) undergoes combustion:
\[\text{B}_{5}\text{H}_{9} + 12\text{O}_{2}\rightarrow 5\text{B}_{2}\text{O}_{3} + 9 \text{H}_{2}\text{O}\]
Here, pentaborane reacts with oxygen to yield boron trioxide and water. The products are often oxides of the elements in the original compound. Combustion reactions are usually exothermic, meaning they release heat.

hydrolysis reactions

Hydrolysis is a chemical process in which a molecule is split into two parts by the addition of a water molecule. It often involves the replacement of certain groups in one molecule with the OH group from the water molecule.
In the exercise:

• Trisilane hydrolysis: \[\text{Si}_{3}\text{H}_{8} + 6\text{H}_{2}\text{O} \rightarrow 3\text{SiO}_{2} + 10\text{H}_{2}\right\]
• Phosphorus pentachloride hydrolysis: \[\text{PCl}_{5} + 4\text{H}_{2}\text{O} \rightarrow \text{H}_{3}\text{PO}_{4} + 5 \text{HCl}\]

The addition of water breaks the bonds in the original compounds, leading to the formation of new products such as silica and phosphoric acid.

halogenation reactions

Halogenation involves the addition of one or more halogens (fluorine, chlorine, bromine, iodine) to a compound. It often results in the substitution of hydrogen atoms with halogen atoms.
In the step-by-step solution:

• Complete halogenation of disilane: \[\text{Si}_{2}\text{H}_{6} + 6\text{Cl}_{2} \rightarrow 2\text{SiCl}_{3} + 6\text{HCl}\]
• Diborane's reaction with chlorine: \[2\text{B} + 3\text{Cl}_{2} \rightarrow 2 \text{BCl}_{3}\]

Through these reactions, chlorine atoms replace the hydrogen atoms in disilane, and elemental boron reacts with chlorine to form boron trichloride. Halogenation is a key method for modifying the chemical properties of a molecule.

thermal decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into simpler compounds or elements when heated. Heat provides the energy needed to break the chemical bonds in the compound.
In the exercise, thermal decomposition can be observed in the breakdown of \(\text{H}_{2}\text{S}_{5}\):
\[\text{H}_{2}\text{S}_{5}\rightarrow 5 \text{S} + 2 \text{H}_{2}\text{S}\]
This reaction breaks \(\text{H}_{2}\text{S}_{5}\) into hydrogen sulfide and elemental sulfur upon heating. Thermal decomposition is often used in various industries to extract pure elements from compounds.