shapes of molecules and intermolecular forces Flashcards
what is electron pair repulsion theory
an electron has a negative charge so electron pairs repel one another so that they are arranged as far apart as possible
why is electron pair repulsion theory useful
its a model used in chemistry for explaining and predicting the shapes of molecules and polyatomic ions
why do lone pairs repel more than bonding pair charge clouds
lone pair charge clouds are slightly closer to the central atom of a molecule and occupy more space than a bonding pair. additionally a lone pair is slightly more electron dense than a bonded pair.
which has a stronger repulsion
lone pair/lone pair or lone pair/bonding pair
lone pair/lone pair
which has a stronger repulsion
lone pair/bonding pair or bonding pair/bonding pair
lone pair/bonding pair
what does a normal line in 3D structures show
____________
a bond in the plane of the paper
what does a bold wedge show in 3D structures
a bond coming out off the plane of paper towards you
what does a dotted wedge show in 3D structures
a bond going out of the plane of paper away from you
what happens to the size of the angle when a lone pair is added
the bond angle reduces by about 2.5 degrees
what would the name , shape and angle size be for 0 lone pairs and 2 pairs of electrons
linear 180 B–A–B
what would the name, shape and angle size be for 0 lone pairs and 3 pairs of electrons
trigonal planar 120
B
|
A
/ \
B B
what would the name , shape and angle size be for 1 lone pair and 3 pairs of electrons
non linear 117.5
..
A
/ \
B B
What would the name and angle be for 4 electron pairs and 0 lone pairs
Tetrahedral 109.5
What would the name and angle be for 1 lone pair and 4 electron pairs
Pyramidal 107
What would the name and angle be for 2 lone pairs and 4 electron pairs
Non linear 104.5
What would the name and angle be for 2 lone pairs and 5 electron pairs
Trigonal planar 120
What would the name and angle be for 5 electron pairs and 3 lone pairs
Linear 180
What would the name and angle be for 0 lone pairs and 6 electron pairs
Octahedral 90
What would the name and angle be for 4 lone pairs and 6 electron pairs
Linear 180
What happens when we have a double bond
Each double bond is treated as a bonded region , in the same way as a bonded pair
Why are atoms held together
Because their nuclei are both pulling on the same shared pair of electrons
What effects electronegativity
Nuclear charge , shielding and atomic size
What is the definition for shielding
Sheilding is the repulsion between electrons in different inner shells , shielding reduces the net attractive force between the positive nucleus and the outer shell electrons
What is the definition for electronegativity
Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
What is the definition for polarity
Polarity is a separation of electric charge leading to a molecule or it’s chemical groups having an electric dipole (a separation of charges)
How is electronegativity measured
The Pauling scale , numbers run from 0-4
How does the difference in electronegativity affect the dipole
The greater the difference in electronegativity between bonded atoms the greater the permanent dipole
What are the trends and reasons in electronegativity across periods
Electronegativity increases this is because nuclear charge is increasing and atomic radius decreases ( the smaller the atomic radius the closer the bonding electrons will be to the nucleus ) and no extra shielding as outer electrons are in the same shell
What’s the trend and reason for electronegativity as we go (down)the group
Electronegativity decreases because Sheilding increases , outer electrons are further away , atomic radius increases.
If there is a electronegativity difference of 0 what is the bond type
Covalent 100%
If there is an electronegativity difference of 0-1.8 what is the bond type
Polar covalent
If there is a electronegativity difference of more than 1.8 what is the bond type
Ionic
Is a molecule of hydrogen polar or non polar and why
Non polar because there is no difference in electronegativity because it is 2 identical bonding atoms which means there is an equal attraction for the bonding pair of electrons. 100% covalent
If an atom is more electronegative what happens with the force
It exerts a greater force of attraction on the bonding pair of electrons and the bonding pair will be closer to that atom
What is a permanent dipole
Is a small charge difference across a bond that results from a difference in electronegativities of the bonded atoms . A polar covalent bond has a permanent dipole
What charge does the more electronegative atom have
Delta -
What charge does the less electronegative charge have
Delta positive
What can bond polarity influence
Boiling point , solubility , structure and chemical reactivity
What does a polar molecule have
A polar molecule has a overall dipole
What is a non symmetrical molecule
Charge difference exists around the whole molecule because dipoles do not cancel out
What happens when the molecule is symmetrical
The dipoles cancel out , it becomes a non polar molecule with polar bonds
Why are no Pauling electronegativity values given to the Noble gases
Atoms don’t form covalent bonds because they have no affinity for electrons due to their electron structure
What are the three types of intermolecular forces
Induced dipole -dipole interactions (London forces) , permanent dipole-dipole interactions and hydrogen bonding
What is the bond enthalpy of covalent bonds
150-500
What is the bond enthalpy of London forces
1-10
What is the bond enthalpy of hydrogen bonds
10-40
What is the bond enthalpy of permanent dipole - dipole interactions
3-25
Describe London forces (induced dipole - dipole forces )
They are the weakest of all intermolecular forces and they exist between all molecules , polar and non polar
How does an instantaneous dipole occur and induce one in neighbouring molecules
Movement of electrons produses a changing dipole in a molecule. At any instant an instantaneous dipole exists but it’s position is constantly changing. This induces a dipole in neighbouring molecules
How does the strength of a London force depend on the number of electrons in the molecule
The more electrons
The larger the instantaneous and induced dipoles
The greater the induced dipole -dipole interactions
The stronger the London forces between the molecules
How does London force strength affect bp
The stronger the London force means more energy is needed to overcome the intermolecular forces (London)
How does being branched or straight effect London forces
Straight molecules can approach more closely than branched molecules therefore packs closer and more points of contact than branched molecules . So London forces are stronger in straight molecules than branched
What are permanent dipole - dipole interactions
Only occur in polar molecules , the permanent dipole interaction is between the positive dipole of one atom and the negative dipole of the atom from another molecule
What do simple molecular substances exist as as solids
A simple molecular lattice which the molecules are held together with weak intermolecular forces but atoms within molecules are bonded together with strong covalent bonds
What bps do simple molecular substances have and why
Low because the weak intermolecular forces holding the simple molecular lattice together need little energy to be overcome
What is hydrogen bonding
A special time of permanent dipole dipole interactions found between molecules containing : an electronegative atom with a lone pair of electrons eg O,N OR F
and a hydrogen atom attached to an electronegative atom eg H-O or H-N or H-F
How does hydrogen bonding result in a strong permanent dipole
There is a large difference in electronegativity between H and N,O,F which results in a strong permanent dipole
What’s the strongest intermolecular force
Hydrogen bonding
Why is solid water (ice) less dense than liquid
In ice the hydrogen bonds hold water molecules apart in an open lattice structure . The hydrogen bonds are slightly longer and hold the water molecules further apart than in liquid water
The holes in the lattice decrease the density of water on freezing . The lattice collapses when melting and the molecules move closer - increasing density
Why does water have relatively high melting and boiling points
Extra hydrogen bonds add strength and more energy is needed to overcome the forces of attraction . Melting causes breaking in the rigid arrangement of hydrogen bonds in ice and boiling breaks all hydrogen bonds between water molecules
Why is the boiling point of hydrides so high
Increasing number of electrons in each molecule , larger instantaneous and induced dipoles , greater dipole dipole interactions, stronger attractive forces , higher bp
Why do non polar substances dissolve in non polar solvents
Intermolecular forces form between the molecules and the solvent. The interactions weaken the intermolecular forces in the simple molecular lattice and the intermolecular forces break , and the compound dissolves
Why might polar substances dissolve in polar solvents
They can attract each other , but the solubility depends on the strength of the dipole and can be hard to predict
Why is hydrophilic polar
Contains electronegative atoms that can interact with water
Why is hydrophobic non polar
Comprised of a carbon chain
Why do simple molecular structures not conduct electricity
There are no mobile charged particles
Polar bond definition
Uneven distribution of charge caused by colvalently bonded atoms with different electronegativities
Are simple covalent structures poor or good conductors of electricity in liquid state
Poor
Are simple covalent structures poor or good conductors of electricity in liquid state
Poor
