4 — chemical bonding, structure and properties of materials Flashcards
Structure and bonding differences betw ionic bonding, metallic bonding and covalent bonding
IB: giant ionic lattice structure — strong electrostatic forces of attraction between oppositely-charged ions
MB: giant metallic structure — strong electrostatic forces of attraction between positively-charged ions and ‘sea of delocalised mobile electrons’
CB:
X has a simple molecular structure — atoms r held tgt by strong covalent forces but weak intermolecular forces of attraction
Giant covalent structure (eg diamond graphite and silicon dioxide) — strong covalent bonds between atom
Difference between melting and boiling points for the bondings
IB: high melting and boiling point.
- x has a giant ionic structure and hence large amount of energy is needed to overcome the strong electrostatic forces of attraction between oppositely-charged ions
MB:
N has a high melting and boiling point. N has a giant metallic lattice structure and hence large amount of energy is needed to overcome the strong electrostatic forces of attraction between positive metal ions and ‘sea of delocalised mobile electrons’
CB:
Monomer m has a low melting and boiling point. M has a simple molecular structure thus small amount of energy is needed to overcome the weak intermolecular forces of attraction.
Y has the highest melting and boiling point as it has a giant covalent structure. Largest amount of energy is needed to overcome the strong covalent bonds between C atoms.
Polyester is a macromolecule that has a giant covalent structure thus small amount of energy is needed to overcome the weak intermolecular forces of attraction between ester molecules. Hence, polyester has a low melting and boiling point. However, the polyester has a higher melting point than (its monomer) as a larger amount of energy than (ester monomer) is needed to overcome the stronger intermolecular forces of attraction due to its larger molecular size.
Solubility
IB:
- usually soluble in water but insoluble in organic solvents
- water molecules r attracted to ions which weakens the electrostatic forces of attraction betw ions
- ions r pulled from giant lattice structure and compound dissolves to form aqueous solution
MB: insoluble in both organic solvents and water
CB:
- CO2 is a non-metal and acidic oxide which dissolves water to form an acidic solution
- most are soluble in organic solvents; insoluble in water
- Graphite is insoluble in both organic solvents and water as the covalent bonds between atoms are too strong to be broken
Electrical conductivity
IB:
- conducts electricity in aq and molten state due to presence of mobile ions
- in solid state, ions vibrate and rotate about fixed position and r immobile, hence, there r no mobile ions to conduct electricity
MB:
- conducts electricity in solid and molten state
- presence of ‘sea of delocalised mobile electrons’, thus presence of mobile electrons allow metals to conduct electricity
CB:
- Methane and CO2 exist as molecules and do not conduct electricity in any states due to the absence of mobile ions or electrons.
MUST MENTION WHICH STATES THEY CONDUCT IN
- EXCEPTIONS:
- simple molecular structure: some substances dissociate in water to form aq solutions with presence of mobile ions that conduct electricity
- giant covalent structure — graphite:
Each carbon atom in graphite is covalently bonded to 3 other Carbon atoms. 1 electron per Carbon atom is delocalised. Hence presence of delocalised mobile electron enables graphite to conduct electricity
properties of giant ionic structure
IB: hard but brittle
- the strong electrostatic forces of attraction betw opp-charged ions makes ionic compounds resistant to deforming and hence they tend to be hard
1. When enough force is applied, ions move away from lattice positions and ions of same charge approach each other
2. Repulsive forces betw ions of same charge becomes larger than attractive forces
3. Ionic lattice is deformed of structure and thus shatters
- High melting boiling point
- Soluble in water but insoluble in organic solvents
- Conducts electricity only in molten or aqueous state but not solid state
- Hard but brittle
Extra properties of giant metallic lattice structure
- Malleable and ductile
- malleable: can be hammered, bent or flattened into thin sheets
- ductile: pulled into wire without breaking
- pure metals have a regular structure. The layers of atoms can slide over one another easily without disrupting the metallic bonding when enough force is applied. - Good conductors of heat (both molten and solid state)
- ions are very closely packed together in an orderly manner thus allows transfer of heat energy by vibration quickly from 1 ion to another. Movement of mobile electrons further speed up rate of transfer of heat energy.
Pure metals are soft as the layers of atoms are orderly arranged and can slide over one another easily without disrupting the metallic bonding when enough force is applied.
- Conducts electricity in solid state
- Insoluble in both organic solvents and H2O
- High melting boiling point
Extra properties of giant covalent structure
Use as cutting tool:
Diamond is hard as
- giant molecular structure
- large amount of energy is needed to break the strong covalent bonds betw C atoms in the giant covalent structure
Why graphite is used as lubricant:
- Graphite is soft and slippery as small amount of energy is needed to overcome the weak intermolecular forces of attraction betw layers of carbon atoms in graphite. Hence, layers of atoms can slide over one another easily.
Simple molecular structure:
1. Low mp bp
2. Insoluble in H2O, soluble in organic solvents
3. Cannot conduct electricity in all states (other than NH3, CO2 and HCl which dissociates in water to form mobile ions)
Giant covalent structure:
1. High mp bp
2. Insoluble in all solvents
3. Cannot conduct electricity in all states other than graphite
4. Graphite soft and slippery, diamond hard
Conduction of electricity can have a different effect on metals and on solutions of ionic compounds. Describe this difference. [1]
When electricity is passed thru metal, there is no chemical change but solutions of ionic compounds will be broken down when electricity passes thru.
Covalent bond
A covalent bond is formed when atoms share electrons to achieve stable octet electronic configuration
Alloy
A mixture of a metal with one of a few other elements
Explain why alloys are harder to break than their constituent elements./ Why cracks were found on the alloy casing during extrusion.
Alloys consists of atoms of different sizes which disrupts the orderly arrangement of atoms in pure metal, making it difficult for the layers of atoms to slide over one another easily when a force is exerted during extrusion (hence causing the cracks). Thus making alloys STRONGER and HARDER than pure metal.
Compare the bonding and structures of the 3 compounds. [4]
All 3 compounds contain strong covalent bonds between atoms.
Propanol has a simple molecular structure while SiO2 and poly(propane) has a giant covalent structure.
Weak intermolecular forces of attraction exists between molecules of propanol and between chains of poly(propane) while only strong covalent bonds exist between atoms in SiO2.
Explain why the melting points of the 3 compounds differ from each other [4]
Large amount of energy is needed to break the strong covalent bonds between atoms in SiO2, resulting in high melting point.
Lesser energy is needed to overcome the weak intermolecular forces of attraction between propanol molecules and between the chains of poly(propene), resulting in low melting point.
Compared to propanol, more energy is required to overcome the stronger intermolecular forces of attraction in poly(propene) due to its larger molecular size compared to propanol.
Suggest how the superoxide ion does not follow bonding rules [1]
One of the oxygen atoms in superoxide ion did not achieve stable electronic configuration as it only has 7 valence electrons instead of the usual 8 electrons
Superoxide oxygen ion: O2^-
Compare the densities between oxides of C, Si and Al [1]
Carbon monoxide has a low density while silicon dioxide and aluminium oxide have high density. Carbon monoxide is a gas at room temperature and pressure and has a larger volume.
On the other hand, silicon dioxide and aluminium oxide are solids at room temperature and pressure and has a smaller volume.
