Lecture 3 Flashcards
Are bonds and material properties related ?
Yes ! closely related -> stronger bonds = harder / more stable material.
We can calculate some properties thanks to the bonds
Which are the most stable conformations ?
With a high degree of symmetry
ionic crystals : energy gain ?
the energy gain is cohesive energy + electron affinity - ionization energy.
The energy of the molecule is lower than the energy of the separated neutral atoms.
How is the covalent bond formed ? How is strength comparable ?
Formed by an electron pair with anti-parallel spin = electron is shared. Strength is comparable to ionic bond.
Metal bonding : how strong ? why ? are there preferred directions ?
Less trong than covalent and ionic because kinetic energy of conduction electrons is lower at larger distances (larger distances = weaker interactions). No preferred directions.
rest valence bonds : strength, examples, essential for what ?
weak, Van de Waals forces. ex : solid states formed out of gas molecules. Essential in formation of macromolecular arrangements of polymers.
polymeric molecule structures : types of bonds and where they are
1) Covalent bonds : within the organic molecule (non conducting configurations). Generated by polymerization = formation of macromolecules out of monomer building blocks.
2) Weak bonds : between the macromolecules. VERY IMPORTANT because they guarantee the formation of crystals and dominate the material properties.
Impact of number of covalent bonds on bond length, energy and valence angle.
More bonds (double, triple) = shorter bond but more stable /stronger (more energy).
C-C-C, H-C-H : 109°
C-O-C : 107°
H-C-C : 120°
How does dipole formation occur ? What does it lead to ?
It occurs because the valence electrons are often shifted towards one partner (polarization). The dipoles lead to secondary (weak) bonds between neighboring molecules.
4 types pf Van der Waal forces (in order of strength)
1) Hydrogen bonds
2) dipole-dipole (polar molecules)
3) induced dipole - dipole
4) dispersion / London forces : temporary electron fluctuation (random movement)
What is responsible for the formation of polymeric solid states (crytsalline lattice in polymers) ? Characteristics of polymers ?
Electrostatic interactions (dipole-dipole) are responsible.
Small melting point, specific solubility, uptake of liquids, dielectric behaviour, …
What does the degree of polymerization characterize ? Relation to stability ?
It means the chain length : number of identical units within a molecule.
High degree = more stability, higher transition T, better mechanical properties and chemical resistance. BUT lower degree of crystallinity and lower density.
What does the formation of crystalline solid states out of polymers depend on ? 3 aspects ?
On the infrastructure = the three dimensional arrangement within the molecule -> constitution, configuration, conformation
What is the constitution of macromolecules ?
It describes the chemical connections (arrangement of atoms along the chains, etc…) : linear or ramified, interconnected or not, symmetric or asymmetric.
Same composition but different structure.
What is the configuration ?
It describes the arrangement of atoms /groups along the chains : cis (neighboring) or trans (opposite), for example.
