Silica and silica glass is held together by single covalent Si–O bonds. — Physical Chemistry Chemistry Question
Silica and silica glass
Silica and silica glass is held together by single covalent Si–O bonds.
What is the coordination number of the Si and O atoms in the structure?
Model Answer
4 and 2
The density of silica glass is 2.203 g cm–3.
What is the average volume of a SiO2 unit? How many bonds are there on the average in such a volume?
Model Answer
In 1 cm3 we have ρ NA / M(SiO2) = 2.208×10^22 SiO2 units. For one unit we obtain a volume of 4.53×10^–23 cm3.
Every Si atom forms four Si–O bonds. Hence a volume element having one SiO2 unit has four Si–O bonds.
A frequent crystal defect in silica glass is oxygen vacancy: oxygen atoms in the lattice are missing and the neighboring Si atoms of the missing oxygen stabilize by forming a Si-Si bond. An amorphous silica sample is characterized by the formula SiO1.9.
What percentage of the total number of the bonds are Si–Si bonds?
Model Answer
Let us denote the number of Si atoms in the sample by n. We have 1.9n O atoms which form 3.8n Si–O bonds. The 4n and 3.8n valence electrons contributed by the Si and O atoms form (2 + 1.9)n bonds (Si–O and Si–Si together). The remaining 3.9n – 3.8n bonds are the Si–Si bonds. Their ratio is 0.1n / 3.9n = 0.0256
So 2.56 % of the bonds are Si–Si bonds.
Give an expression for the nSi-Si/nSi-O ratio of a SiOx sample, as a function of x, where nSi-Si is the number of Si–Si bonds and nSi-O is the number of Si–O bonds.
Give the value of x where, on the average, all Si atoms form one Si–Si bond.
Model Answer
In general the required formula can be written as:
nSi-Si / nSi-O = (2 - x) / 2x = 1/x - 0.5
The question implies that the nSi-Si / nSi-O ratio is 1/6. It follows that x is 1.5 in this case.