S2.2 The covalent model SL Part 2 Flashcards
Covalent network structures
A very large structure composed of hundreds or thousands of atoms covalently bonded together.
Allotropes
Alternative forms of an elemental substance. For example, diamond and graphite are allotropes of carbon.
Delocalised
An electron that is not associated with one specific atom and is free to move within the molecular structure.
London dispersion forces
The temporary instantaneous dipole formed due to the rapid and random motion of electrons.
What are the properties of diamond and its structure?
Diamond has a tetrahedral structure with strong covalent bonds, resulting in extreme hardness, high melting and boiling points, but poor electrical conductivity.
What makes graphite unique among carbon’s allotropes?
Graphite consists of hexagonal layers, allowing electrons to move freely, making it a good conductor and useful as a lubricant due to weak forces between layers.
What are the key properties of fullerenes and graphene?
Both show unique properties such as electrical conductivity and strength, attributable to their distinct geometries and electron delocalization.
Describe the structure and properties of silicon.
Silicon has a tetrahedral geometry similar to diamond, leading to significant hardness and high melting/boiling points but limited conductivity.
What are the properties of silicon dioxide and its structure?
SiO₂ forms a network of silicon atoms covalently bonded to oxygen, showcasing high thermal stability and strength, vital for its use in glass and sand.
Intramolecular forces
The covalent bonds between atoms within a molecule.
Intermolecular forces
Attractive (or repulsive) forces that exist between the molecules of a substance.
What are the types of intermolecular forces under the van der Waals forces category?
Dipole-dipole, dipole-induced dipole, and London (dispersion) forces.
How can you deduce the type(s) of intermolecular force present in molecular compounds?
By analyzing the electronegativity differences and molecular structure.
What characterizes London Dispersion Forces?
Present in all molecules, they are the weakest type, based on temporary dipoles caused by electron movement.
What are the main features of Dipole-Dipole Forces?
They occur in molecules with permanent dipoles, are stronger than London forces, and are based on electrostatic attractions between oppositely charged ends of polar molecules.
Describe Dipole-Induced Dipole Forces.
They arise when a polar molecule induces a dipole in a nonpolar molecule, with strength varying based on the molecule’s susceptibility to electric fields.
Hydrogen bonding
The intermolecular attraction between two molecules which both contain a hydrogen bonded to a highly electronegative element such as oxygen, fluorine or nitrogen.
Which elements commonly participate in hydrogen bonding when bonded to hydrogen?
Fluorine, oxygen, and nitrogen.
What role does hydrogen bonding play in the properties of water?
It is crucial for many of water’s properties, such as its high boiling point and the ability to dissolve many substances.
How does hydrogen bonding influence biological molecules?
It plays a significant role in the structure and function of biological molecules, including the stability of DNA’s double helix structure.
What is the order of intermolecular forces from strongest to weakest according to their influence on boiling points and molecular interactions?
Hydrogen bonding > Dipole-dipole forces > London (dispersion) forces.
How does molecular size and polarity affect the strength of London (dispersion) forces?
Larger molecular size increases London force strength due to more electrons and enhanced temporary dipole formations. Polarity influences these forces, but non-polar molecules also experience them through induced dipoles.
What are the typical physical properties of covalent substances in terms of volatility, electrical conductivity, and solubility?
Covalent substances exhibit low volatility, poor electrical conductivity, and solubility that depends on polarity. Volatility and conductivity are low due to the absence of free ions or delocalized electrons, and solubility follows the principle of “like dissolves like”.
Solvent front
The distance from the pencil line to where the solvent (the mobile phase) reached on the chromatography paper at the time it was removed from the solvent.
What is the difference between a dye and a pigment?
Dyes are synthetic, dissolve in water, and form a transparent solution, being small, polar molecules. Pigments are naturally derived, insoluble in water, remain suspended in solution, and are larger, non-polar molecules requiring a binder to dissolve.
What are the two phases in chromatography and how does separation occur?
Chromatography involves a stationary phase (chromatography paper) and a mobile phase (solvent). Separation is based on the intermolecular forces between the mixture’s components and the phases, with components having greater affinity for the mobile phase traveling further.
How are RF values calculated and interpreted in chromatography?
RF values are calculated by dividing the distance traveled by the substance by the distance traveled by the solvent, ranging from 0 to 1. Values near 0 indicate high affinity for the stationary phase, and values closer to 1 suggest higher affinity for the mobile phase, aiding in substance identification.
