U2.2: Covalent Structure Flashcards
Rules for drawing the lewis formula?
- Find total valence electrons of both atoms
- Make a bond between the atoms
- Put the remaining electrons in the outside
- All electrons are either dots or crosses or line
(could be double or triple bonds)
Define delocalisation of electrons
Delocalisation is when pi electron pairs are not confined to two adjacent bonding atoms but extend over three or more atoms
Formula for formal charge?
Valence electrons - the number of bonds - the number of lone electrons
V - B - L
How do we properly use the formal charge
- Ensure that the positive charge at the end is as close to zero as possible
What is true about elements beyond period 2 and their octets?
- Only elements beyond period 2 can actually have expanded octets
Name of shape (MG and EDG) and bond angle with 3 bonded pairs 0 lone pairs
MG & EDG: Trigonal
120*
Name of shape (MG and EDG) and bond angle with 2 bonded pairs and 1 lone pair
MG: Bent Shape
EDG: Trigonal Planar
118*
Name of shape (MG and EDG) and bond angle with 4 bonded pairs and 0 lone pairs
MG: Tetrahedral
EDG: Tetrahedral
109.5*
Name of shape (MG and EDG) and bond angle with 3 bonded pairs and 1 lone pair
MG: Trigonal Pyramidal
EDG: Tetrahedral
107*
Name of shape (MG and EDG) and bond angle with 2 bonded pairs and 2 lone pairs
MG: Bent Shape
EDG: Tetrahedral
105*
Name of shape (MG and EDG) and bond angle with 5 bonded pairs and 0 lone pairs
MG & EDG: Trigonal Bipyramidal
90* / 120*
Name of shape (MG and EDG) and bond angle with 4 bonded pairs 1 lone pair
MG: See Saw Shape
EDG: Trigonal Bipyramidal
89, 119
ALL 3 Names of shape (MG and EDG) and bond angle with 3 bonded pairs and 2 lone pairs
1)
MG: Trigonal Planar
EDG: Trigonal Bipyramidal
120*
2)
MG: T shape
EDG: Trigonal Bipyramidal
89*
3)
MG: Linear Shape
EDG: Trigonal Bipyramidal
180*
Name of shape (MG and EDG) and bond angle with 6 bonded pairs and 0 lone pairs
MG & EDG: Octahedral
90, 180
Name of shape (MG and EDG) and bond angle with 5 bonded pairs and 1 lone pair
MG: Square Pyramidal
EDG: Octahedral
90, 85
Name of shape (MG and EDG) and bond angle with 4 bonded pairs and 2 lone pairs
MG: Square Planar
EDG: Octahedral
90*
Why’s there a decrease in bond angle when an electron domain’s replaced by a lone pair
Each additional lone pair of e- occupies a bit more space than the e- stored in bonds therefore causing an increased repulsion with the bonding e- which will push the bonds inwards hence, decreasing the bond angle.
What’s pauliing’s rule in terms of electronegativity differences and determining whether a bond in polar, non polar or ionic
- If difference is 0-0.4, bond is non polar covalent
- If difference is 0.5-1.8, bond is polar covalent
- If difference is greater than or equal to 1.9, bond is ionic
What only IMF of attraction is between 2 nonpolar elements
LDF of attraction
What IMF of attraction are between 2 polar elements
DDF of attraction + LDF (but it’s weaker)
List the 3 intermolecular forces of attraction in terms of increasing strength
London Dispersion Forces < Dipole-Dipole forces < Hydrogen bonding
What molecules can have hydrogen bonds (when binding from Hydrogen to these molecules)
- Ammonia
- Water / Hydrogen
- Hydrogen Fluoride
- Any Alcohol
- Any Carboxylic Acid
- O / N / F
What bonds are present in O / N / F
Hydrogen Bonds
- DDF and LDF too but theyre weaker hence, theyre disregarded
Apart from between 2 non-polar elements, when do LDF occur? Provide an example
- When there is an even charge distribution
- E.g: Carbon Tetrachloride
What is a sigma bond
Formed by the head-onoverlapping of atomic orbitals along the nuclear axis
What is a pi bond
A pi bond is formed by the sideways overlapping of p orbitals above and below the nuclear axis
Explain 2 differences between sigma and pi bonds
- Sigma bonds can exist by themselves whereas pi bonds are always present with sigma bonds
- In sigma bonds, the electron’s density is along the nuclear axis whereas in pi bonds, the electron’s density is above and below the nuclear axis
- Sigma bonds are formed by the head-on overlap of atomic orbitals whereas pi bonds are formed by the side-to-side overlap of p orbitals
Define hybridisation
Mixing of atomic orbitals to form new hybrid orbitals of identical energy
What is the 3 rules when it comes to hybridisation of 4/3/2 bonds or electron domains? What type of bonds are present in each of the 4/3/2 bonds
4 bonds/domains = sp3 hybridisation (single bonds only)
3 bonds/domains = sp2 hybridisation (a double bond is present)
2 bonds/domains = sp hybridisation (2 double bonds are present)
Name at least 3 principles followed by the VSEPR Theory
- All pairs of e- are arranged as far apart from each other as possible to minimise the repulsion
- the shape of molecules and ions depends on the total number of electron pairs around the central atom
- Lone pairs, around the central atom, repel more than bonding pairs as they’re not bonded
- A single, double and triple bond count as 1 electron domain
Define resonance structures
Resonance structures can be formed by shifting lone pairs and double bonds forming alternative, stable bonding arrangements. All bond lengths of resonance structures are equal
What intermolecular forces of attraction are between non polar and non polar
London dispersion forces
What intermolecular forces of attraction are between polar and polar
Dipole Dipole forces (LDF are disregarded as they’re much weaker)
How to know which compounds have a simple molecular covalent structure?
- Diamond, Silicon Dioxide and Graphite all have giant covalent structures where graphite has intermolecular forces whereas the other have electrostatic attractions
- any other compound has a simple molecular covalent structure that have intermolecular forces
Discuss whether Kekule’s structure of benzene is accepted, giving physical and chemical evidence
- Draw Benzene with ring //
- Physical evidence: C-C bond lengths are all equal, C-C bond strengths intermediate between single and double bonds
- Chemical evidence: Unlike ethene, for example, benzene undergoes substitution reactions and not addition reactions, doesn’t decolourize bromine water
Define hydrogenation
Occurs when a C-C double bond is broken and replaced with a H2 molecule
3 main properties of buckmisterfullerene
- High Melting Point
- Can’t conduct electricity
- Is not hard
Suggest why the molecular geometry of methane requires the p orbitals of the Carbon atom to be hybridised
- MG is tetrahedral
- p orbitals are at 90* to each other
Explain how hybridisation affects the pairing and relative energies of the carbon valence electrons to enable the formation of four equivalent C-H bonds
- The paired s electrons are not paired to anything in the hybrid orbitals
- Hybridisation causes the energies of the valence orbitals to become equal
Explain the Group 15 trend when they are a compound with Hydrogen
- NH3 has a high BP due to Hydrogen Bonding
- Going down the group, electronegativity decreases
- Hence, the other molecules form either LDF or DDF which, both, are weaker than Hydrogen Bonding
- BP Increases from PH3 to SbH3 as the central atom contains more electrons and this makes the atom more polarisable.
What bonds do compounds or molecules that have resonance have?
intermediate bonds
Role of ozone in the stratosphere?
Absorbs Ultraviolet rays
State why resonance structures are needed to represent bonding in some compounds
Resonance structures are present due to the Pi bonding electrons being delocalised
Outline the relationship between structures of compounds and boiling points (linear and branched)
- Linear structure has a higher bp than a branched structure
What are the requirements for compounds to be soluble in water
- Interactions betw. water molecules must be broken when a solute gets added
What is the strength of ionic bonds compared to polar covalent bonds and why
- ESF of attraction is stronger in ionic bonds than the IMF of attraction in polar covalent bonds
Define resonance
Resonance is when there are more than 1 position for multiple bonds or for Pi bonds
Discuss the bonding in the resonance structures of Ozone
- Lone pairs on p-orbital of Oxygen atom overlap
- Delocalisation with pi electrons form double bonds
- Bond length and strength is an intermediate between single and double bonds
State and explain the acid-base character of x according to the Lewis Theory
- x has a lone pair of electrons that can be donated thus it acts as a lewis base
OR - x can accept a lone pair of electrons thus it acts as a lewis acid
How to determine polar bonds?
- H bonding
- Lack of symmetry
- EN Difference of 0-0.4
- Presence of Lone pairs
- Net Dipole moments (causing lack of symmetry)
How to determine non-polar bonds?
- Diatomic
- Noble Gas
- Contains only C and H
- Has symmetry (CF4, CCl4, CO2)
- EN Difference of 0.5-1.8 (above and equal to 1.9 is ionic bonding at that point)
- Dipole moments cancel out
Description of the bonding in SiO2?
- Each oxygen atom is covalently bonded to 2 Silicon atoms
outline the solubility of covalent structures and how to dissolve them.
- insoluble in water
- they are not charged or polar
- Thus, to dissolve non-polar covalent structures, they need to be added to non-polar solvents
If the formal charge of 3 different resonance structures is 0, how do we know which structure to use?
- The most electronegative element MUST be having a negative charge.
Describe the covalent bond between C and H in a molecule and how it is formed (talk about sigma and pi bonds too) (2)
- Strong Electrostatic attraction between shared pair of electrons and nuclei of both atoms
- Sigma bonds are formed by the head-on overlap of atomic orbitals
- Pi bonds are formed by the sideways overlap of p orbitals
Identify 3 allotropes of Carbon and describe their structures (4)
- Fullerene: 3D structure. each C atom is bonded to 3 other C atoms joined in a ball structure
- Diamond: 3D structure. each C atom is bonded to 4 other C atoms, tetrahedral structure of C atoms form a covalent network
- Graphene(one layer of graphite): 2D structure. each C atom is bonded to 3 other C atoms, hexagonal ring-layered structure of C atoms where there are weak IMF of attraction between the layers
in which homologous series are hydrogen bonds formed the most in
Carboxylic acid
