6 - Shapes of molecules and intermolecular forces Flashcards
What is electron pair repulsion theory?
model used in chemistry for explaining & predicting shapes of molecules & polyatomic ions
- electron pairs surrounding central atom determine shape of molecule/ion
- electron pairs repel -> arranged as far apart as possible
- arrangement minimises repulsion & holds atoms in definite shape
- diff numbers of electron pairs results in different shapes
Bonded-pair vs lone-pair repulsion
Lone pair of electrons slightly closer to central atom and occupies more space than bonded pair. So lone pair repels more strongly than bonding pair.
(Increasing repulsion)
bonded-pair/bonded-pair < bonded-pair/ lone pair < lone pair/lone pair
Shape of molecule with 4 bonded pairs, 0 lone pairs and bond angle
Tetrahedral
109.5 degrees
Bond angle and Shape of molecule with 3 bonded pairs, 1 lone pair
Pyramidal
107 degrees
Bond angle and Shape of molecule with 2 bonded pairs, 2 lone pairs
Non linear
104.5 degrees
Bond angle and Shape of molecule with 2 bonding regions, 0 lone pairs
Linear
180 degrees
Bond angle and Shape of molecule with 3 electron regions, 0 lone pairs
Trigonal planar,
120 degrees
Bond angle and Shape of molecule with 6 bonding regions, 0 lone pairs
Octahedral
90 degrees
Bond angle and Shape of molecule with 5 bonding regions, 0 lone pairs
Trigonal Bipyramidal
90 degrees and 120 degrees
Bond angle of CH4
109.5 degrees
Bond angle of NH3
107 degrees
Bond angle of H2O
104.5 degrees
Define electronegativity
The ability of an atom to attract the bonding electrons in a covalent bond.
How is electronegativity measured?
Using Pauling electronegativity values
Electronegativity increases towards F
- going right = more protons, Nuclear charge becomes more positive, more electronegative
– going up = smaller radius, more electronegative
As you go left to right, atom size slightly decreases because more protons, and electrons are more attracted to them
What makes an element more electronegative?
1) nuclear charge.
When there is more positive charge it is more electronegative
2)radius of atom.
Smaller radius atoms are more electronegative because electrons are closer, so there is less shielding
Electronegativity difference = 0
(Pure) covalent
Electronegativity difference = 0 to 1.8
Polar covalent
Electronegativity difference > 1.8
Ionic bond
What is a nonpolar bond
The bonded electron pairs are shared equally between the bonded atoms.
The bonded atoms are the same OR have same/similar electronegativity
Eg;
Cl2
Hydrocarbons
What is a polar bond?
The bonded electron pair is shared unequally between bonded atoms. Bonded atoms are different and have different electronegativity values resulting in a polar covalent bond.
Electronegativity difference must be greater than 0.5.
Dipoles in polar bonds
In H – Cl bond, H has a small partial positive charge and Cl has a small partial negative charge.
Separation of opposite charges is called a dipole
A dipole in a polar covalent bond does not change. PERMANENT DIPOLE.
Polarity in polar molecules
For molecules with 2+ atoms there may be 2+ polar bonds
Polar molecule requires polar bonds with dipoles that do not cancel due to their direction. CO2 does not have a permanent dipole but H2O does
Define intermolecular forces. What types?
An attractive force between molecules // weak interactions between dipoles of different molecules
Can be London forces, permanent dipole-dipole interactions or hydrogen bonding
Covalent vs intermolecular forces
Covalent: strong, holds atoms in a molecule together. Determine identity and chemical reactions of molecules
Intermolecular: weak interactions between dipoles of different molecules. largely responsible for physical properties like melting & boiling points
What are London (dispersion) forces / induced dipole-dipole interactions?
How do they arise?
1) Weak intermolecular forces that exist between ALL molecules (polar or non polar)
Act BETWEEN induced dipoles in different molecules
2) movement of electrons produces changing dipole in molecule. @ any moment, instantaneous dipole will exist (bits it’s position is always shifting). instantaneous dipole induces dipole on neighbouring molecule. induced dipole further induced dipoles on neighbouring molecules, which then attract one another.
Which London forces are stronger?
Link to BP
Bigger atoms = more electrons = bigger instantaneous dipoles = stronger London forces between particles = increased energy needed to overcome intermolecular forces = higher BP
Define permanent dipole-dipole interaction
An attractive force between permanent dipoles in neighbouring POLAR molecules (polar -> electronegativity difference greater than 0.5)
Define a simple molecular substance
Made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula, such as neon Ne, hydrogen H2, water H2O and carbon dioxide CO2
Define simple molecular lattice (structure, basically)
Structure of simple molecular lattice
Three dimensional structure of molecules, bonded together by weak intermolecular forces
In solid state, simple molecules form a regular structure called simple molecular lattice. Molecules held in place by WEAK intermolecular forces. Atoms within each molecules bonded together STRONGLY by covalent bonds
Melting and boiling points of simple molecular substances (bonding properties)
All simple molecular substances are covalent, & can be solidified into simple molecular lattices by reducing temperature
In simple molecular lattice, weak intermolecular forces can be broken even by the energy present at low temperatures. Low melting and boiling points.
When a simple molecular lattice is broken apart during melting, only WEAK intermolecular forces BREAK, covalent bonds are strong and do not break.
Electrical conductivity in simple molecular substances
There are no mobile charged particles and simple molecular structures. With no charge particles that can move there is nothing to complete an electrical circuit. Therefore, non-conductors of electricity
Can polar solute dissolve in polar solvents?
Yes
Polar solute molecules and polar solvent molecules can attract each other. Process is are similar to dissolving of an ionic compound.
Can nonpolar solute dissolve in nonpolar solvent?
Yes
When a simple molecular compound is added to a nonpolar solvent, intermolecular forces FORM BETWEEN the molecules and the solvent. The interactions WEAKEN the intermolecular forces in the simple molecular lattice. These BREAK and the compound DISSOLVES.
Can polar solute dissolve in nonpolar solvents?
No
Can nonpolar solute dissolve in polar solvent?
No
When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules. The intermolecular bonding within the polar solvent is too strong to be broken.
Solubility of alcohols
The solubility depends on the strength of the dipole and can be hard to predict. Some compounds such as alcohol is contain both polar (O—H) and nonpolar (carbon chain) parts in their structure and can dissolve in both polar and nonpolar solvents
Solid structure of simple molecular lattices
Covalently bonded molecules attracted by intermolecular forces
Define Hydrogen bonding
Spec: intermolecular bonding between molecules containing N, O or F and the H atom of -NH, -OH or -FH
Glossary: A strong dipole-dipole attraction between an electron deficient hydrogen atom of -NH, -OH or -HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule
Describe anomalous properties of water resulting from hydrogen bonding, eg density of ice compared to water.
With two lone pairs on the oxygen atom and two hydrogen atoms, each water molecule can form for hydrogen bonds.
The hydrogen bonds extend outwards holding water molecules slightly apart and forming an open tetrahedral lattice full of holes.
The bond angle about the hydrogen atom involved in the hydrogen bond is close to 180°.
The holes in the open lattice structure decrease the density of water on freezing.
When ice melts, the ice lattice collapses and molecules move closer together. So liquid water is denser than solid ice.
Describe anomalous properties of water resulting from hydrogen bonding, eg its relatively high MP & BP
Water has London forces between molecules. Hydrogen bonds are extra forces, over and above the London forces.
An appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher MP & BP than would be expected just from London forces.
When the ice lattice breaks, the rigid ARRANGEMENT of hydrogen bonds and ice is BROKEN. When water boils, the hydrogen BONDS BREAK completely.
