2.5 - Shapes of molecules and intermolecular forces Flashcards
What are the two types of electron pairs molecules and ions posses ?
- Bonding pairs ( the two shared electrons in a covalent bond )
- Lone pairs ( two electrons in a pair not involved in bonding = non-bonding pairs )
What is the shape of molecules and ions determined by ?
The electron pairs surrounding the central atom of a molecule/ ion determine the shape of its molecule/ ions
What is ‘repulsion theory‘ ?
Repulsion theory is a model used in chemistry used for explaining and predicting shapes of molecules and poly atomic ions
Explain how electron pairs surrounding the central atom determine the shape of a molecule/ ion ?
- Electron pairs repel each other as they have alike charges
- This means electron pairs are arranged as far as possible
- Arrangement of electron pairs minimises repulsion and therefore holds bonded atoms in a definite shape
- Different number of electron pairs result in different shapes
Why do lone pairs repel more than bonding pairs ?
Lone pairs are more compact than bonding pairs and therefore lone pairs repel more than bonding pairs
What is the effect of lone pairs being more compact than bonding pairs on the bond angle ?
Since Lone pairs are more compact than bonding pairs this reduces the bond angle to a small extent
What is the name, shape and bond angle of molecules with two electron pairs ?
What is the name, shape and bond angle of molecules with three electron pairs ?
What is the name, shape and bond angle of molecules with four electron pairs ?
What is the name, shape and bond angle of molecules with five electron pairs ?
What is the name, shape and bond angle of molecules with six electron pairs ?
What is the name, shape and bond angle of molecules with three bonding pairs and one lone pair ?
pyramidal
What is the name, shape and bond angle of molecules with two bonding pairs and two lone pairs ?
What is the name, shape and bond angle of molecules with four bonding pairs and two lone pairs ?
What is the name, shape and bond angle of molecules with two double bonds ?
linear, 180o
What is the name, shape and bond angle of molecules with one triple and one single bond ?
linear, 180o
What is the name, shape and bond angle of molecules with one double bond and four single bonds ?
planar, 118o
What is the method to figure out the shape of molecules/ ions ?
- Work out how many outer shell electrons are on the central atom (accounting for any charge on the species)
- Work out how many atoms are bonded to the central atom (= the number of bonding pairs)
- Work out how many lone pairs there are (which is half the difference between (a) and (b))
Do you know how to explain the shape of molecules and ions such as linear, non-linear, trigonal planar, pyramidal, tetrahedral and octahedral using electron pair repulsion ?
Yes
eg. It is … shape because there are … bonding pairs and … lone pairs ( and lone pairs repel more than bonding pairs )
What are the bonding angles for the following : CHv4, NHv3 and Hv2O ?
CHv4 = 109.5o
NHv3 = 107o
Hv2O = 104.5o
What is a polar bond ?
A bond formed when there is a difference in the electronegativity of two covalently bonded atoms ( the bond will have a dipole )
What is a permanent dipole ?
Partial charges
eg. H ( δ+ ) - Cl ( δ- )
Cl is more electronegative than H, so has a greater share of bonding electrons
What is a polar molecule ?
A molecule where the dipoles of the molecule do not cancel out resulting in the molecule being polar. These dipoles are permanent.
What is a non-polar molecule ?
A molecule where the dipoles of the molecule cancel out meaning it is non-polar. Therefore these dipole are not permanent.
When are molecules non-polar ?
When the molecule is symmetrical
What are the three types of intermolecular forces ?
- Hydrogen bonding
- Permanent dipole-dipole
- London force/ Induced dipole-dipole
What is the order of strength of the different intermolecular forces ?
1 ) Hydrogen bonding
2 ) Permanent dipole-dipole
3 ) London force/ Induced dipole-dipole
Explain what is London force and how it comes about ?
- London force is the weakest type of intermolecular force
- This occurs due to constant movement of electrons which causes uneven distribution of charge which causes temporary dipoles
- This induces a temporary dipole in the adjacent molecule which produces an induced dipole-dipole attraction
What is an induced dipole-dipole interaction also known as ?
London forces
What factors affect London force ?
- Number of electrons
- Shape of molecule
Explain how the number of electrons affects London force ?
- The greater the number of electrons , the greater the size of the temporary and induced dipoles
- This means the London force is stronger between molecules
Explain how the shape of the molecule affects London force ?
- Unbranched molecules have a greater surface area and greater surface contact between molecules than branched molecules
- This means the London force is greater between molecules
Explain what is permanent dipole-dipole interaction and how it comes about ?
- Permanent dipole-dipole interaction only happens between polar molecules
- It is the attraction between the permanent dipoles of adjacent polar molecules
What factors affect permanent dipole-dipole interaction ?
Magnitude of electronegativity difference
Explain how the magnitude of the difference in electronegativity affects permanent dipole-dipole interaction ?
- The greater the difference in electronegativity between two bonded atoms, the greater the permanent dipole
- The greater the permanent dipole on a molecule, the stronger the permanent dipole-dipole interaction between molecules
What is a hydrogen bond ?
- A special type of permanent dipole-dipole interaction
- It acts between a hydrogen atom in a molecule and a lone pair of electrons on an electronegative atom
Where does a hydrogen bond occur ?
It occurs between a very electronegative atom ( ie. oxygen, nitrogen or fluorine ) with a lone pair of electrons and a hydrogen atom bonded to a very electronegative atom
What should you include the drawing hydrogen bonds ?
- Dipoles
- Lone pairs
- Hydrogen bonds
Do you know how to draw hydrogen bonds ?
Yes
What are the anomalous characteristics of water due to hydrogen bonding ?
- Ice is less dense than water
- Water has a relatively high melting and boiling point
Explain why ice is less dense than water ?
- Hydrogen bonds hold water molecules apart in an open lattice structure
- Therefore the water molecules are farther part in ice than in water and it floats
Explain why water has a relatively high melting and boiling point ?
- Water contains London forces between molecules
- Water contains hydrogen bonds between water molecules
- This means that more energy is required to break hydrogen bonds between water molecules causing water to have a higher melting and boiling point
Explain the structure of simple molecules ?
- Atoms are chemically joined together by strong covalent bonds
- Molecules are joined together by weak intermolecular forces
- This forms a simple molecular lattice structure
What are the features of the simple molecular lattice structure ?
- High melting / boiling point
- Cannot conduct electricity
- They are insoluble in polar solvents
Explain why simple molecules have a low melting/ boiling point ?
- They have strong covalent bonds between atoms but weak intermolecular forces between molecules
- Therefore, not a lot of energy is required to break weak intermolecular forces between molecules and simple molecules have a low melting / boiling point
Explain why simple molecules cannot conduct electricity ?
- They have no mobile ions and therefore cannot conduct electricity
Explain why simple molecules are insoluble in polar solvents ?
- They are typically non-polar molecules
- This means they are not soluble and are not dissolved by a polar solvent
