Structure 2.2 Flashcards
Retardation factor
Distance travelled by component/distance travelled by the solvent from original spot
When do resonance structures occur?
When there is more than one possible place for a double bond in a molecule
Covalent bond
Sharing of electrons between atoms to form a noble gas electron configuration (known as octet rule) due to electrostatic attraction
Single covalent bond
When one pair of electrons is shared between two atoms
Lewis formula
Formula that shows all of the outer valence electrons for each atom in the molecule
What happens to the bond length and bond strength when pairs of shared electrons increases?
Generally, bond length decreases and bond strength increases as number of shared pairs of electrons increases
Coordinate covalent bond
When both electrons are contributed from the same atom
Octet rule
Atoms bond together and share electrons to gain a full outer shell
Why don’t noble gases form compounds?
They are already stable with a full outer shell.
Most electronegative atom
Flourine
Atoms lower in the periodic table… (octet rule)
Can defy the octet rule
Electronegativity
The relative attraction that an atom has for a shared pair of elections when covalently bonded.
What is electronegativity measured in?
Pauling scale
Electronegativity trends…
Increase across a period and up a group so N, O, F are most electronegative
A covalent bond between two atoms of the same element will be
Non-polar as bonding is shared equally
A covalent bond between two atoms of different elements will be
Polar as bonds will be attracted to each other
What happens if the difference between electronegativity is greater than about 1.8?
The bonding pair of electrons moves completely towards the more electronegative atom and bonding becomes ionic.
Electron affinity
The amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
Bond polarity
The distribution of electric charge across a chemical bond between two atoms.
Molecular polarity
A molecule (or polyatomic ion) is polar when one side of the molecule is more positive (or more negative) than the other
Valence Shell Electron Pair Repulsion
States that pairs of electrons are arranged around the central atom i a simple molecule or ion so that they are mutually repulsive
What does the VSEPR theory apply to?
Strictly speaking only electron domains but for most purposes, electron pairs
All electrons in a multiple bond must…
Point in the same direction
What is the order of repulsion?
Non bonding : non bonding > non bonding : bonding > bonding : bonding
To determine whether a molecule is polar or non polar, consider …
Both the electronegativities of the bonded atoms and the shape of the molecule
Molecules are polar when
Their bond dipoles do not cancel out, bent shape
Molecules are non polar when
Their bond dipoles cancel out, when dipoles are equal, linear shape
Covalent network structure
Lattice of atoms that are covalently bonded together
Covalent molecules b/m points
Are very high
Covalent molecules conductivity
Poor conductors as all electrons are localised and so cannot carry charge
Allotropes chemical and physical properties
Are different as they have different bonding and structural properties
Diamond
Bonded tetrahedrally to four other carbon atoms, is hardest natural substance
Graphite
Allotrope of carbon, trigonal planar bonding to three other carbons and so layers held together by weak bonds. Delocalised electrons so good conductor of electricity.
Buckminsterfullerence
60 carbon atoms, arranged in rings to form a sphere
Silicon/silicon dioxide
Tetrahedral bonding so no free electrons, hard, high melting point and can’t conduct.
Van der Waal’s forces of attraction
General term that covers all weak forces of attraction between molecules
London dispersion forces
Exist between all particles, thought to occur due to the formation of temporary instantaneous dipoles caused by uneven spread of electrons
What are london dispersion forces responsible for?
Attractive forces between non-polar moleculars and although they are weak, they increase with increasing mass
What is the only force of attraction between non-polar molecules?
London dispersion force
Why do molecules with only london dispersion forces have low boiling/melting points?
Only a small amount of energy is required to overcome the london dispersion forces and seperate the molecules.
Dipole-dipole forces of attraction
Polar molecules attracted to other polar molecules by electrostatic attractions
Dipole
When electrons are shared unequally between atoms in the same molecule due to a high difference in the electronegativity which causes a separation of charges
Hydrogen bonding
When a molecule containing hydrogen bonds directly to one of the three most electronegative atoms (F, N, O)
What is the order of strength of van der waal’s forces?
London dispersion < dipole-diole < hydrogen bonding < covalent < ionic
The stronger the attractive forces,
The less volatile the molecule and the higher the boiling point
What happens to solubility as the polarity of a molecule increases?
It increases as attraction to the polar water molecules also increases.
Why are covalent molecules poor conductors of electricity?
Their electrons are localised.
How can a coordinate covalent bond be shown?
An arrow instead of a line
Electron deficient
Atoms that can form stable covalent compounds with an incomplete octet
How do you know if a covalent bond is polar/nonpolar?
Electronegativity difference of 0-0.4=non polar, 0.4-1.7=polar, 1.8+=ionic
Chemical evidence to support benzene resonance structure
Undergoes substitution instead of addition reactions, only exists in one form
Physical evidence to support benzene resonance structure
- Bond lengths are all equal
- Enthalpy of hydrogenation is low
Delocalisation energy in benzene
Accounts for the extra stability due to delocalised electrons and has to be overcome before addition reactions can occur so that’s why benzene undergoes substitution reactions
What does formal charge assume?
That the octet rule always work and that all atoms in the molecule have the same electronegativity
Formal charge formula
Number of valence electrons - (non bonding electrons - 1/2 number of bonding electrons)
Sigma bond
When two atomic orbitals combine along a line to form a molecular orbital
Pi bond
When two p orbitals combine sideways to form one molecular orbital
How many sigma/pi bonds in single/double/triple bonds?
Single - 1 sigma
Double - 1 sigma, 1 pi
Triple - 1 sigma, 2 pi
sp3 hybridisation
Three p orbitals, 1 s orbital combine to form four sp3 orbitals
sp2 hybridisation
Two p orbitals, 1 s orbital, combine to form three sp2 orbitals and one unhybridized p orbital - double bond
Mixing of one s and two p atomic orbitals, which involves the promotion of one electron in the s orbital to one of the 2p atomic orbitals. Three new sp2 orbitals equal in energy-level + one unhybridized p orbital
sp hybridisation
One p orbital, 1 s orbital, form 2 hybrid orbitals and 2 unhybridized p orbitals
What shape forms sp hybridisation?
Linear
What shape forms sp2 hybridisation?
Trigonal planar
What shape forms sp3 hybridisation?
Tetrahedral
What forms the pi bonds in benzene?
The delocalised p orbitals
Paper chromatography
Used to seperate components in a mixture - spot of mixture rises up paper depending on it’s affinity for the phase
Thin layer chromatography
Alumina on a glass plate is use and components partition between solvent and paper
Liquid column chromatography
Sample is placed on a packed column, solvent is poured down the column and so seperated.
Gas-liquid chromatography
Used to analyse voliate substances in the gas phase. Mobile phase is the inert gas that carries molecules through the analyte.
What is the preferred formula?
The formula where all atoms have formal charge closest to zero
Single bond trends (hybridisation, arrangement, pi v sigma)
1 sigma bond, sp3 hybridisation, tetrahedral shape
Double bond trends (hybridisation, arrangement, pi v sigma)
1 sigma, 1 pi bond, sp2 hybridisation, trigonal planar shape `
Triple bond trends (hybridisation, arrangement, pi v sigma)
1 sigma, 2 pi bonds, sp hybridisation, linear arrangement
Benzene hybridisation
All 6 carbons have sp2 hybridisation and the 6 p atomic orbitals with one electron each form a delocalised pi bond above and below the plane of the molecule that contains 6 electrons
