Chemical Bonding 5 Flashcards
Describe the basis of attraction for the following types of chemical bonding:
1: ionic bonding
2: covalent bonding
3: metallic bonding
4: hydrogen bonding
5: permanent dipole-permanent dipole interaction
6: dispersion forces
1: electrostatic forces of attraction between the oppositely charged ions
2: electrostatic forces of attraction between the nuclei and the shared electron pair
3: electrostatic forces of attraction between the lattice of positive ions and a sea of delocalised electrons
4: electrostatic forces of attraction between the (delta +) H bonded to F/O/N and the (delta -) on F, O, or N
5: electrostatic forces of attraction between the (delta +) of an atom in a polar molecule and the (delta -) of an atom in another polar molecule
6: electrostatic forces of attraction between the temporary (delta +) of an atom in a non-polar molecule with an instantaneous dipole and the (delta -) of an atom in another non-polar molecule with an induced dipole.
Element in Period 3 (melting point):
Na (98°C)
Mg (650°C)
Al (703°C)
Si (1410°C)
P4 (44°C)
S8 (119°C)
Cl2 (-101°C)
Ar (-189°C)
In terms of structure and bonding, explain
1: why the melting point increases from Na to Mg to Al.
2: why Si has a very high melting point.
3: why the melting point decreases from S8 to P4 to Cl2 to Ar.
1: Na, Mg and Al have giant metallic structure held by strong metallic bonding between positive cations and the sea of delocalised electrons. Their melting points are relatively high, and increases from Na to Al because the strength of metallic bonding increases as there is an increase in the number of valence electrons that can be contributed per atom and a decrease in the size of metal cations. Thus, more energy is needed to break the increasingly stronger metallic bonds from Na to Mg to Al.
2: Si has a giant molecular structure. A lot of energy is needed to break the many strong and extensive Si-Si covalent bonds throughout the giant lattice.
3: S8, P4 and Cl2 are non-polar molecules with simple molecular structures and weak dispersion forces between their molecules. Ar is monoatomic with weak dispersion forces between atoms. S8 has the greatest number of electrons and thus the most polarisable electron cloud, followed by P4, then Cl2, then Ar. The strength of dispersion forces decreases in the same order, and the energy needed to overcome the dispersion forces during melting decreases. Thus, S8 has the highest melting point, followed by P4, Cl2, and then Ar.
In terms of structure and bonding, explain
1: why the electrical conductivity increases from Na to Mg to Al
2: Si is a semiconductor.
3: the electrical conductivity of P4, S8, Cl2 and Ar is 0.
1: Na, Mg and Al have giant metallic structures held by strong metallic bonding between positive cations and the sea of delocalised valence electrons. Metals are good electrical conductors as the metallic lattice contains a sea of delocalised electrons that are free to move and carry charge. From Na to Al, the number of valence electrons that can be contributed per atom increases, thus the electrical conductivity increases.
2: Si is a metalloid (an element with properties of metals and non-metals).
3: P4, S8 and Cl2 have simple molecular structures. Their outer electrons are held strongly in covalent bonds and are not free to move about. Ar exists as individual neutral atoms. All 4 do not contain mobile charge carriers and thus do not conduct electricity.
Substance (formula) [melting point / °C]:
ethanol (C2H5OH) [-114]
silicon carbide {SiC) [2700]
krypton (Kr) [-157]
aluminium fluoride (AlF3) [1291]
argon (Ar) [-189]
Account for the melting points of these 5 substances in terms of structure and bonding.
STRATEGY: Reorganise the substances from lowest to highest melting point and group the substances into their various types of structure classification.
(1) Both Ar and Kr are monoatomic gases with weak dispersion forces between their atoms. Since there are more electrons in Kr, the strength of the dispersion forces between Kr atoms are stronger and more energy is needed to overcome the stronger attractive forces between the atoms. Hence, Kr has a higher melting point than Ar. (2) Ethanol is a polar molecule with simple molecular structure, with relatively strong intermolecular hydrogen bonds between its molecules. Since hydrogen bonds are stronger than dispersion forces, more energy is needed to overcome the stronger attractive forces between ethanol molecules and ethanol has a higher melting point than Kr. (3) AlF3 is an ionic compound with a giant ionic lattice structure, with strong electrostatic forces of attraction between the oppositely charged Al^3+ ions and F^- ions. A large amount of energy is required to overcome the very strong attractive forces, hence its much higher melting point than ethanol. (4) SiC has a giant molecular structure and for melting to occur, a large amount of energy is required to break the strong and extensive SiC covalent bonds throughout the giant covalent lattice, accounting for its much higher melting point compared to AlF3.
Carbon and silicon are elements in Group 14 of the Periodic Table. Their oxides have very different melting points. Explain how and why the melting points of carbon dioxide and silicon dioxide are different.
Carbon dioxide has a simple molecular structure with weak dispersion forces between the molecules. Silicon dioxide has a giant molecular structure with strong and extensive covalent bond between the atoms. Covalent bond between the atoms in SiO2 is stronger and requires more energy to overcome than dispersion forces between the molecules in CO2. Hence, the melting point of carbon dioxide is much lower than silicon dioxide.
Predict, with a reason, how the boiling point of phosphorus pentoxide (P2O5) will differ from that of CO2.
P2O5 and CO2 have simple molecular structures with dispersion forces between the molecules. As P2O5 have more electrons than CO2, its electron cloud is larger and forms stronger dispersion forces that require more energy to overcome. Hence, P2O5 has a higher boiling point than CO2.
