Chemical bonding Flashcards
Factors affecting metallic bond strength
- Charge density of the cation
- larger the magnitude of the charge density, due to increasing cation charge and decreasing cation radius.
the stronger the attraction between cations and delocalised electrons, stronger metallic bond - No. of delocalised e-
- the larger the number of delocalised e-, the more extensive the interactions between the cations and delocalised e-, the stronger the metallic bond
Factor affecting ionic bond strength
- Lattice energy …….> charge/radius
1) Compare charge
2) Compare radius
- Magnitude of lattice energy is larger > stronger ionic bond
Sigma and Pi Bonds
Sigma bond (1 in a single, double, and triple covalent bond) —> head-on overlap
Pi bond - side-on overlap (only in multiple bonding)
Double bond - one S, one pi bond
Triple bond - one S, pi bond
> Degree of overlap of the orbitals in S bond is greater and more effective as compared to pi bond
> eg. C // C bond is stronger than C/C but not by a difference of two times because pi bond is weaker than s bond. this is because s bond has a greater degree of overlap and is more effective than a pi bond.
Factors affecting covalent bond strength
- Size of the atom
- bigger size, more diffuse the orbitals of the atoms, the less effective the overlap of the orbitals , the weaker the covalent bond. - Multiplicity of the bond
- the greater the number of shared electron pairs, the stronger the electrostatic forces of attraction between the shared pairs of electrons and the positive charged nuclei, the stronger the covalent bond - The polarity of the bond
- Polar cov. bond vs Non-polar cov. bond
- eg. H-N vs H-O vs H-F
A polar covalent bond is stronger than a non-polar covalent bond due to the increased bond strength that results from the additional electrostatic attraction between the two partial charges.
The greater the diff in electronegativity values, the more polar the covalent bond, the stronger the esfa between the two partial charges, the stronger the covalent bond. - The proximity of the lone pair electrons
- closer proximity of the lone pair of e- > excessive repulsion which weakens the covalent bond formed
What determines the extent of covalent character in ionic bonds?
- Polarising power of cation > charge density
- Polarisability of the anion > size
Cation: Smaller cationic size (radius) > higher charge density of cation > larger distortion to the electron cloud of anion > stronger polarising power of cation > greater extent of covalent character
Anion: Larger anion size > electron cloud of anion will be distorted to a larger extent > electron cloud is more polarisable > greater covalent character
Covalent bonds with ionic character
Non-polar —> two atoms with same electronegativity
Polar —> two atoms of different electronegativities > electrons is pulled more closely towards the more electronegative atom
— The greater the electronegativity difference between the two atoms, the greater the bond dipole moment, the more polar the covalent bond
(arrow points at more electronegative atom)
What is the structure and bonding of metals?
S: Giant metallic structures.
B: The metal cations are held in fixed positions in an orderly arrangement and are surrounded by a sea of delocalised e-
What are the physical properties of metals?
- High BP and MP
> strong esfa btw cations and delocalised e- > large amount of energy needed to overcome… - Good electrical conductors in both solid and molten states as the delocalised e- can act as mobile charge carriers under an applied potential difference.
- Malleable and Ductile
- Mobility of the delocalised e- allows layers of cations to slide over each other without shattering the lattice structure.
What is the structure and bonding of ionic compounds?
S: Giant ionic structure.
B: The ions are held in fixed positions in an orderly arrangement by strong electrostatic forces of attraction
What are the physical properties of ionic compounds?
- High BP and MP
- due to strong electrostatic forces of attraction between oppositely charged ions > large amount of energy required.. - Only conducts electricity in molten or aqueous state.
- In solid state, the ions are held in fixed positions. Hence, there are no mobile charge carriers and ionic compounds are unable to conduct electricity. However, in molten or aqueous state, the ions are free to move and can act as mobile charge carriers > can conduct electricity. - Hard and brittle
Hard —> Their lattice structures are held by strong electrostatic forces of attractions btw oppositely charged ions.
Brittle —> A slight displacement along a plane brings together ions of like charges, resulting in strong repulsion which shatters the lattice structure.
What are the structures of covalent compounds?
S: Giant molecular structure made up of atoms held together in an extensive network of covalent bonds.
Diamond & SiO2
- each C atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement
Graphite
- the C atoms are arranged in layers made up of hexagonal rings
- In every layer, each C atom is covalently bonded to three other C atoms to form an extensive network of planar hexagonal rings. The last valence e- on each C atom is delocalised into the layers, giving rise to a continuous electron cloud in between the layers.
The esfa between the layers is due to id-id attractions.
What are the physical properties of giant molecules?
Diamond & SiO2:
High MP > large amount of energy needed to overcome the strong extensive covalent bond THROUGHOUT its structure.
Graphite:
High MP > large amount of energy is needed to overcome the strong extensive covalent bond WITHIN EACH LAYER.
Diamond & SiO2
Poor electrical conductors > No mobile charge carriers to carry electrical charges
Graphite:
Good electrical conductors in the direction parallel to the layers > delocalised e- act as mobile charge carriers under an applied potential difference
Poor electrical conductors in the direction perpendicular to the layers > delocalised e- cannot jump across the layers.
Diamond & SiO2:
Very hard > Lattice structures are held by strong extensive covalent bonds
Graphite:
Flaky and slippery > the adjacent layers are held by weak id-id interactions that can slide over each other easily (lubricant)
What is the structure and bonding of SMS?
S: Simple molecular structure
B: made up of many discrete molecules held together by weak intermolecular forces of attraction.
What are the physical properties of SMS?
Low BP and MP > little amt of energy needed to overcome the weak intermolecular forces of attraction between molecules
Poor electrical conductors > No mobile charge carriers to carry electrical charges
— Some simple molecules can conduct electricity in aqueous state due to formation of mobile ions when the molecules interact with water, which can act as mobile charge carriers allowing electricity to be conducted under an applied potential difference.
Very soft > Lattice structures are held by weak imfa
Wjhat is the VSEPR Theory?
- to predict the molecular geometry of a covalent species
Greatest repulsion
lone pair-lone pair > lone pair-bond pair > bond pair-bond pair
How does the electronegativity of atoms affect repulsion between electron pairs?
eg. H2O (105°) vs H2S (92°)
Central atom more electronegative > bond pair electrons closer to the nucleus of the central atom > greater repulsion between the electron pairs.
(could work the other way like F2O vs H2O also, pulling away from nucleus of central atom)
How does the size of neighbouring atoms affect bond angle?
Larger size of neighbouring atoms > greater repulsion between the electron clouds of the neighbouring atoms > larger bond angle
(esp when size of neighbouring atoms»_space;> size of central atom)
Type of intermolecular force depending on polarity of molecule
Non-polar > Instantaneous dipole-induced dipole interactions
Polar > Permanent dipole-permanent dipole interactions (and ID-ID)
(got F,O,N) Highly polar > Hydrogen bonds (and ID-ID)
Factors affecting ID-ID interactions (can affect BP, MP)
- No. of electrons (see Mr)
- Greater the no. of electrons > larger the electron cloud size > greater the ease of electron cloud distortion > stronger the id-id interactions - Degree of branching
- Elongated molecules > greater SA for molecular interaction > stronger ID-ID interactions
Branched molecules > smaller SA for molecular interaction > weaker ID-ID interactions
Factors affecting PD-PD interactions (can affect BP, MP)
- Overall dipole moment
- Larger overall dipole moment > means it is more polar > its molecules are held by stronger PD-PD interactions
Factors affecting Hydrogen bonds (can affect BP, MP)
- No. of H bonds per molecule (eg. H2O vs HF)
- When extent of hydrogen bonds is greater (more H bonds per molecule) > stronger H bonds - Dipole moment of H-X bond
- Larger the dipole moment of the H-X bond (H-F > H-O > H-N) , the stronger the H bonds
Exp: Extent of H bonds is the same. However, F is more electronegative them N. The hydrogen bonds between HF molecules are stronger due to H-F bonds having a larger dipole moment than H-N bonds.
Why is the molar mass of some carboxylic acids in vapour phase found to be twice its mole mass as calculated from its molecular formula?
In polar solvent such as water, dimerisation of carboxylic acid is less likely to occur due to the higher tendency of carboxylic acid molecules to form hydrogen bonds with water molecules rather than its own molecules.
Why is the boiling point of 2-nitrophenol lower than that of 4-nitrophenol even though they have the same molecular formula?
For 2-nitrophenol, the close proximity of the phenol— and nitro— groups allows for the formation of intramolecular hydrogen bonds. This means that fewer sites are now available for the formation of intermolecular hydrogen bonding with other 2-nitrophenol molecules. Hence, the extent of intermolecular hydrogen bonding with 2-nitrophenol molecules is lesser for the case of 2-nitrophenol > lower bp.
For 4-nitrophenol, the phenol— and nitro— groups are too far apart to allow for the formation of intramolecular hydrogen bonds. This means that more sites are now available for the formation of intermolecular hydrogen bonds with other 4-nitrophenol molecules. Hence the extent of intermolecular hydrogen bonding with 4-nitrophenol molecules is greater > higher bp.
Solubility
Idea:
To dissolve solute into solvent,
energy released from the formation of new interactions between solute-solvent particles
> or = (to be sufficient to overcome)
the existing interactions between solute-solute particles and solute-solvent particles
depending on qn.. see which is solute, solvent and solute-solvent
Ionic compounds - ionic bonds (very strong)
Water - hydrogen bonds (strong)
Non-polar - id-id
SMS - imfa
etc…
