Discrete molecules 2.2 Flashcards
What is a discrete (simple) molecule?
An electrically neutral group of two or more atoms held together by chemical bonds
How can covalent and polar covalent bonding be shown?
Using a dot-and-cross diagram
What is a displayed (full structural) formula?
Shows each bonding pair as a line drawn between the two atoms involved
When does a dative covalent bond form?
When an empty orbital of one atom overlaps with an orbital containing a non-bonding pair (lone pair) of electrons of another atom
How is a dative covalent bond represented?
An arrow starting from the atom providing the pair of electrons and going towards the atom with the empty orbital
What does electron pair repulstion theory state?
The shape of a molecule or ion is caused by repulsion between the pairs of electrons (bond and lone pairs), that surround the central atom.
The electron pairs arrange themselves around the central atom so that the repulsion between them is at a minimum, and separation is at a maximum
lone-lone > lone-bond > bond-bond
What do you do if a molecule has multiple bonds?
Treat each multiple bond as if it contained only one pair of electrons
What shape has 2 bond pairs and 0 lone pairs? Give an example.
Linear. BeCl2
What is the bond angle in a linear molecule?
180
What shape has 3 bond pairs and 0 lone pairs? Give an example.
Trigonal planar. BCl3
What is the bond angle in a trigonal planar molecule?
120
What shape has 2 bond pairs and 1 lone pairs? Give an example.
Bent (V-shape). H2O
What is the bond angle in a bent (V-shape) molecule?
104.5
What shape has 4 bond pairs and 0 lone pairs? Give an example.
Tetrahedral. CH4
What is the bond angle in a tetrahedral molecule?
109.5
What shape has 3 bond pairs and 1 lone pairs? Give an example.
Trigonal pyramidal. NH3
What is the bond angle in a trigonal pyramidal molecule?
107
What shape has 5 bond pairs and 0 lone pairs? Give an example.
Trigonal bipyramidal. PCl5
What is the bond angle in a trigonal bipyramidal molecule?
90 and 120
What shape has 6 bond pairs and 0 lone pairs? Give an example.
Octahedral. SF6
What is the bond angle in an octahedral molecule?
90 and 180
What is a dipole?
Exists when two charges of equal magnitude but opposite signs are separated by a small distance
What is the result of two elements in a molecule having dipoles that cancel each other out?
Resulting molecule will have no overall dipole so is ‘non-polar’
What is the result of two elements in a molecule having dipoles that reinforce each other?
Molecules will possess an overall dipole so is ‘polar’
What are intermolecular interactions?
Non-bonded interactions between molecules that are considerably weaker than typical covalent and polar covalent bonds
Describe the formation of London forces
- Begin with two non-polar molecules (A and B)
- Electron density fluctuates leading to an unsymmetrical electron density in molecule A, generating a dipole
- Electron density on left of molecule has increased (delta negative), electron density on right has decreased (delta positive), so instantaneous dipole created in molecule A
- Delta positive end of molecule A is closer to molecule B, so electron density of molecule B is pulled to left, generating delta negative charge on left of molecule B, and delta positive charge on right of molecule B. Creates an induced dipole in B
- Dipole of A led to induction of dipole in B, two dipoles are arranged so they interact favourably with one another. This favourable interaction is responsible for London forces of attraction between the two molecules
What are three features of London forces?
- Attractive force increases with increasing number of electrons in the molecule (shown in boiling temps of noble gases - increases. London forces are only force of attraction between noble gas molecules). More electrons in a molecule, greater fluctuation in electron density and so larger instantaneous and induced dipoles created
- They depend on the shape and size of molecules (more points of contact between molecules, greater overall London force)
- Always present between molecules, regardless of whether they have a permanent dipole or whether or not they hydrogen bond
Describe permanent dipoles
If permanent dipoles are aligned correctly, then there will be a favourable interaction and the two molecules will attract one another. However, random movement of molecules is such that dipoles are not always aligned to produce a favourable interaction. As a result, interaction between permanent dipoles is less than the interaction between instantaneous-induced dipoles. London force is usually the most significant interaction between molecules
What is the origin of London forces?
Instantaneous dipole - induced dipole interaction
What is the origin of Permanent dipoles?
Permanent dipole - dipole interaction
When is hydrogen bonding significant?
When hydrogen is bonded to very small, highly electronegative atoms (eg oxygen, nitrogen, fluorine)
How does hydrogen bonding occur in water (hydrogen bonding through oxygen)?
All compounds containing O-H group can form intermolecular hydrogen bonds. Hydrogen bond forms between oxygen atom of one water molecule and hydrogen atom of second water molecule. Since oxygen atom has two lone pairs, it can form hydrogen bonds with two other water molecules
What is the bond angle in hydrogen bonding?
180 degrees
How does hydrogen bonding occur through nitrogen? Give example
All compounds containing N-H group can form intermolecular hydrogen bonds. Ammonia - lone pair on delta negative hydrogen of one ammonia molecule attracts delta positive H of another ammonia molecule
How does hydrogen bonding occur through fluorine?
Only fluorine compound with hydrogen bonding is hydrogen fluoride. Lone pair on delta negative fluorine of one HFl molecule attracts delta positive H of another HFl molecule
What are the three intramolecular forces?
Metallic bonding, Ionic bonding, Covalent bonding
What are the three intermolecular forces?
Hydrogen bonding, Permanent dipole-dipole forces, Instantaneous dipole-induced dipole
What is the order of strength of intra and intermolecular forces from strongest to weakest?
Metallic bonding
Ionic bonding
Covalent bonding
Hydrogen bonding
Permanent dipole-permanent dipole
Instantaneous dipole-induced dipole
What is the only significant intermolecular interaction between alkane molecules? What is the trend in boiling temperatures of unbranched alkanes? Why?
London forces. As Mr (relative molecular mass) increases, boiling temperature increases. As Mr increases, number of electrons per molecule increases so instantaneous and induced dipole increases. As length of carbon chain increases, number of contact points between adjacent molecules increases. London forces exist at each point of contact between the molecule
What is the trend in boiling temperature of branched alkanes compared to unbranched isomers?
Branched chain alkanes have lower boiling temperatures. This is because more branching leads to fewer points of contact between adjacent molecules - decrease in overall intermolecular force of attraction between molecules - decrease boiling temperature
What is the trend in boiling temperature of alcohols compared to alkanes? Why?
Alcohols have higher boiling temperatures than equivalent alkanes. This is because they contain an O-H group and can therefore form intermolecular hydrogen bonds in addition to London forces
What is the trend in boiling temperatures of the hydrogen halides? Why?
Steady increase from HCl to HI. Result of increased number of electrons per molecule, results in increase in London forces
What are the two anomalous properties of water?
- Has relatively high melting and boiling temp (for molecule with few electrons)
- Density of ice at 0C is less than water at 0C
Why does water have a high melting and boiling temp?
Hydrogen bonds between water are strong. Overall intermolecular force of attraction in water is strong - takes lots of energy to overcome
Why is the density of ice less than the density of liquid at 0C?
Molecules in ice arranged in rings of six, held together by hydrogen bonds. Creates large areas of open space inside rings. When ice melts, ring structure destroyed and average distance between molecules decreases, caused an increase in density
What conditions must be met for a substance to dissolve?
- Solute particles must be separated from each other so can be surrounded by solvent
- Forces of attraction between solute and solvent must be strong enough to overcome solvent-solvent forces and solute-solute forces
How do ionic solids dissolve in water?
- Delta negative end of water attracts sodium ions sufficiently to remove them from the lattice. Sodium ions surrounded by water molecules. Interaction between sodium ions and water molecules is ion-dipole interaction
- Delta positive end of water attracts chloride ions. Chloride ions become surrounded by water molecules. Chloride ions hydrogen bonded to water molecules
Process known as hydration. Energy released known as hydration energy
What compounds cannot form hydrogen bonds with water (cannot dissolve in water)?
Non-polar molecules eg alkanes (attraction between alkane and water not strong enough to disrupt hydrogen bonded system between water molecules)
Polar molecules (do not form hydrogen bonds with water, hydrogen bonds formed are weak)
Halogenoalkanes
