Consider two glasses. Glass A has a composition of 100% SiO2. Glass B has a composition of 25% Na2O - 75% SiO2.
(a) Which sample has a shorter average bond length (r)?
(b)Which sample has a higher thermal expansion coefficient?
(c) Which sample has a lower viscosity?
a) glass A
b) glass B
c) glass B
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To determine which sample has a shorter average bond length (r), higher thermal expansion coefficient, and lower viscosity, we first need to understand the chemical composition of both glasses.
(a) Average bond length (r):
In glass, the shorter the bond length, the stronger the bonds between the atoms, leading to a more rigid structure. Glass A is composed of 100% SiO2, which forms a network of silicon-oxygen (Si-O) bonds. Glass B has a composition of 25% Na2O and 75% SiO2.
Since sodium oxide (Na2O) contains sodium (Na), which has a larger atomic radius compared to silicon (Si), we can infer that the addition of Na2O in Glass B would lead to longer bond lengths compared to Glass A. Therefore, Glass A is likely to have a shorter average bond length (r) than Glass B.
(b) Thermal expansion coefficient:
The thermal expansion coefficient measures how much a material expands or contracts with changes in temperature. In glasses, the thermal expansion coefficient is influenced by the types of elements and their atomic arrangements.
Glass B contains Na2O, which has a higher thermal expansion coefficient compared to SiO2. Sodium ions tend to have weaker bonds and higher mobility compared to silicon in the SiO2 network. Therefore, Glass B with the Na2O-SiO2 composition is likely to have a higher thermal expansion coefficient than Glass A, which contains only SiO2.
(c) Viscosity:
Viscosity is a measure of a fluid's resistance to flow. In glasses, viscosity is mainly influenced by the strength and mobility of the atomic bonds. Bond length and atomic arrangement affect the ease of flow and intermolecular interactions in glasses.
Since Glass B contains Na2O, which introduces additional sodium ions and potentially weaker interatomic bonding compared to Glass A, it is likely to have a lower viscosity. Sodium ions tend to enhance the fluidity of glasses because they disrupt the rigid SiO2 network, making it easier for the glass to flow.
In summary:
(a) Glass A (100% SiO2) is likely to have a shorter average bond length (r) than Glass B (25% Na2O - 75% SiO2).
(b) Glass B (25% Na2O - 75% SiO2) is likely to have a higher thermal expansion coefficient than Glass A (100% SiO2).
(c) Glass B (25% Na2O - 75% SiO2) is likely to have a lower viscosity than Glass A (100% SiO2).