Problem 92 - Chemistry: The Central Science

Question

Silicon has a limited capacity to form linear, \(\mathrm{Si}-\mathrm{Si}\) bonded structures similar to those formed by carbon. (a) Predict the molecular formula of a hydride of silicon that contains a chain of three silicon atoms. (b) Write a balanced equation for the reaction between oxygen and the compound you predicted in part (a).

Step-by-Step Solution

Answer

The molecular formula of a silicon hydride containing a chain of three silicon atoms is Si3H8. The balanced equation for the reaction between oxygen and this compound is 3 Si3H8 + 6 O2 -> 3 SiO2 + 4 H2O.

Step 1: (a) Predict the molecular formula of a hydride of silicon that contains a chain of three silicon atoms.

To predict the molecular formula of a hydride of silicon containing a chain of three silicon atoms, we need to know about silicon's bonding properties. Silicon belongs to Group 14 in the periodic table, and like carbon, it is a tetravalent atom. Thus, it can form four bonds with other atoms. In a silicon hydride with a linear chain of three silicon atoms, each silicon will have three bonds among themselves, and one bond with hydrogen to complete the four bonds. The number of hydrogen atoms bonded to each silicon atom depends on the position of the silicon atom in the chain. So, in a linear chain of three silicon atoms, the middle silicon atom will form two silicon-silicon bonds - one on each side - and two remaining bonds will be with hydrogen atoms. The two terminal silicon atoms will form only one silicon-silicon bond each, leaving three bonds for bonding to hydrogen atoms each. The molecular formula for this silicon hydride, based on the chain of three silicon atoms and counting their hydrogen bonds, will be Si3H8.

Step 2: (b) Write a balanced equation for the reaction between oxygen and the compound predicted in part (a).

Now, let's write a balanced equation for the reaction between oxygen (O2) and the compound we predicted in part (a), Si3H8. To write a balanced equation, we need to make sure that the number of atoms of each element on both sides of the equation are equal. When silicon hydride burns in the presence of oxygen, it forms silicon dioxide (SiO2) and water (H2O) as products. Unbalanced equation: Si3H8 + O2 -> SiO2 + H2O Now, let's balance the equation. 1. Balance the silicon atoms: 3 Si3H8 + O2 -> 3 SiO2 + H2O 2. Balance the hydrogen atoms: 3 Si3H8 + O2 -> 3 SiO2 + 4 H2O 3. Balance the oxygen atoms: 3 Si3H8 + 6 O2 -> 3 SiO2 + 4 H2O The balanced equation is: 3 Si3H8 + 6 O2 -> 3 SiO2 + 4 H2O

Key Concepts

Chemical Bonding in Silicon Hydrides

Understanding chemical bonding is crucial when dealing with compounds like silicon hydrides. As students of chemistry, it's important to recognize that silicon, although similar to carbon, has its distinct ways of forming bonds. Silicon is in the same group as carbon in the periodic table, Group 14, and shares the tetravalency feature, meaning it can form four bonds. However, the nature and geometry of these bonds can vary.

When silicon atoms bond to form a chain, as in a silicon hydride with three silicon atoms, their bonding pattern becomes interesting. The middle silicon atom, being surrounded by two other silicon atoms, forms two Si-Si bonds and completes its tetravalence with two Si-H bonds. On the other hand, the terminal silicon atoms only form one Si-Si bond each, leaving room for three Si-H bonds each. This distinct bonding arrangement is vital for predicting the silicon hydride's molecular formula.

• Central Si atom: 2 Si-Si bonds + 2 Si-H bonds
• Terminal Si atoms: 1 Si-Si bond + 3 Si-H bonds each

So, remember the basic rule that each silicon in the structure seeks to complete four bonds, whether they're with other silicon atoms or with hydrogen.

Molecular Formula Prediction of Silicon Hydrides

The ability to predict molecular formulas lies at the heart of chemical synthesis and analysis. For chemists, grasping how to predict the composition of a new compound such as a silicon hydride is a skill developed through understanding the valency and bonding nature of its constituent elements.

Given that we have a hydride (a compound containing hydrogen) with three silicon atoms, we can use the aforementioned principles of chemical bonding in silicon to arrive at the molecular formula. We've established that the central silicon forms two bonds with adjacent silicon atoms and bonds with two hydrogens while the terminal silicon atoms bond with three hydrogens each after forming a single Si-Si bond.

In this context, we can deduce that the molecular formula is Si₃H₈. Here's how that breaks down:

• Each of the two terminal silicon atoms contributes three hydrogens: 2 Si at 3 H each = 6 H
• The central silicon atom contributes two hydrogens: 1 Si at 2 H = 2 H
• Total hydrogen atoms = 6 H + 2 H = 8 H

Thus, coupled with the three silicon atoms, we have Si₃H₈ as our molecular formula. It's essential to account for all valences to ensure the formula is correct.

Chemical Reaction Balancing: Combustion of Silicon Hydrides

Balancing chemical reactions is a skill necessary for any chemistry student and ensures the fundamental law of mass conservation is upheld in chemical reactions. When writing and balancing equations, equal numbers of each type of atom must be present on both sides, signifying that matter is neither created nor destroyed.

In the case of the combustion of the silicon hydride Si₃H₈ with oxygen, we follow a methodical approach to balance the equation:

• Start with the compound with the most complex composition, so here, we balance Si from Si₃H₈.
• Next, balance hydrogen by adjusting the coefficient before the water (H₂O).
• Finally, balance oxygen, keeping in mind it is present in both products: silicon dioxide (SiO₂) and water (H₂O).

Through this systematic balancing approach, we arrive at the balanced equation: \[3 \mathrm{Si}_{3}\mathrm{H}_{8} + 22 \mathrm{O}_{2} \rightarrow 9 \mathrm{SiO}_{2} + 12 \mathrm{H}_{2}\mathrm{O}\]
This clearly illustrates that the processes such as combustion are governed by predictable stoichiometric ratios, allowing for accurate predictions of product formation.