S2.2 The covalent model SL Part 2

Question 1

Covalent network structures

Answer 1

A very large structure composed of hundreds or thousands of atoms covalently bonded together.

Question 5

Allotropes

Answer 5

Alternative forms of an elemental substance. For example, diamond and graphite are allotropes of carbon.

Question 6

Delocalised

Answer 6

An electron that is not associated with one specific atom and is free to move within the molecular structure.

Question 7

London dispersion forces

Answer 7

The temporary instantaneous dipole formed due to the rapid and random motion of electrons.

Question 8

What are the properties of diamond and its structure?

Answer 8

Diamond has a tetrahedral structure with strong covalent bonds, resulting in extreme hardness, high melting and boiling points, but poor electrical conductivity.

Question 9

What makes graphite unique among carbon’s allotropes?

Answer 9

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.

Question 10

What are the key properties of fullerenes and graphene?

Answer 10

Both show unique properties such as electrical conductivity and strength, attributable to their distinct geometries and electron delocalization.

Question 11

Describe the structure and properties of silicon.

Answer 11

Silicon has a tetrahedral geometry similar to diamond, leading to significant hardness and high melting/boiling points but limited conductivity.

Question 12

What are the properties of silicon dioxide and its structure?

Answer 12

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.

Question 13

Intramolecular forces

Answer 13

The covalent bonds between atoms within a molecule.

Question 14

Intermolecular forces

Answer 14

Attractive (or repulsive) forces that exist between the molecules of a substance.

Question 15

What are the types of intermolecular forces under the van der Waals forces category?

Answer 15

Dipole-dipole, dipole-induced dipole, and London (dispersion) forces.

Question 16

How can you deduce the type(s) of intermolecular force present in molecular compounds?

Answer 16

By analyzing the electronegativity differences and molecular structure.

Question 17

What characterizes London Dispersion Forces?

Answer 17

Present in all molecules, they are the weakest type, based on temporary dipoles caused by electron movement.

Question 18

What are the main features of Dipole-Dipole Forces?

Answer 18

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.

Question 19

Describe Dipole-Induced Dipole Forces.

Answer 19

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.

Question 20

Hydrogen bonding

Answer 20

The intermolecular attraction between two molecules which both contain a hydrogen bonded to a highly electronegative element such as oxygen, fluorine or nitrogen.

Question 21

Which elements commonly participate in hydrogen bonding when bonded to hydrogen?

Answer 21

Fluorine, oxygen, and nitrogen.

Question 22

What role does hydrogen bonding play in the properties of water?

Answer 22

It is crucial for many of water’s properties, such as its high boiling point and the ability to dissolve many substances.

Question 23

How does hydrogen bonding influence biological molecules?

Answer 23

It plays a significant role in the structure and function of biological molecules, including the stability of DNA’s double helix structure.

Question 24

What is the order of intermolecular forces from strongest to weakest according to their influence on boiling points and molecular interactions?

Answer 24

Hydrogen bonding > Dipole-dipole forces > London (dispersion) forces.

Question 25

How does molecular size and polarity affect the strength of London (dispersion) forces?

Answer 25

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.

Question 26

What are the typical physical properties of covalent substances in terms of volatility, electrical conductivity, and solubility?

Answer 26

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”.

Question 27

Solvent front

Answer 27

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.

Question 28

What is the difference between a dye and a pigment?

Answer 28

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.

Question 29

What are the two phases in chromatography and how does separation occur?

Answer 29

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.

Question 30

How are RF values calculated and interpreted in chromatography?

Answer 30

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.