IMF Flashcards
What are intramolecular interactions?
They are things that happen/act within molecules:
{atoms within molecules are held together by chemical bonds}
[ionic,covalent,netallic]
What are intermolecular interactions?
They are the forces of attraction between molecules
{they are non bonded interactions}
-they don’t involve the transfer of any electrons-
[they are a result of the constant and random movement of electrons within the shells of atoms]
What are the three types of intermolecular forces?
{main}
Induced dipoles
Permanent dipoles
Hydrogen bonds
What are London forces?(on an atomic scale)
Theory : rather than electrons existing statically within orbitals, they are constantly moving within the clouds of charge.
On average, electron density is symmetrical … at any moment it is likely there are more electrons on one side of the atom than the other, temporarily the charge is unequally distributed resulting in a temporary dipole.
Like dominos, the temporary dipole can induce a temporary dipole in a neighbouring atom. The two dipoles will be attracted to each other, these dipoles are constantly being formed and destroyed so overall we observe that the atoms are attracted to each other.
What is the scale of London forces
They occur between almost everything in addition to other intermolecular forces of attraction
Pause at 3:25
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What is electronegativity
The ability to attract the bonding electrons in a covalent bond
What are the cov bonds in diatomic gases not? (Also why)
Polar because the electronegativites in the atoms are the same
Finish the sentence:
In a covalent bond between two atoms of different electronegativities …?
The bonding electrons are pulled towards the more electronegative atom .
The greater difference in electronegativity , the more polar the bond.
What is a polar molecule?
If a charge is unevenly distributed along a molecule then the molecule will have a permanent dipole. Molecules that have this are polar
What is a permanent dipole?
It describes the partial charge separation that an occur within a molecule along the bond that forms between 2 diff atoms
What factors affect the polarity of the molecule?
Shape affects whether or not the molecule is polar
Number of polar bonds if any
What do simple molecules that are polar have ?
Usually one polar bond that makes the charge distributed unevenly e.g. HCl
What do more complex molecules that are polar have?
May or may not have several polar bonds, the overall shape will determine whether the molecule has. A permanent dipole, if the molecule is arranged symmetrically then the polar bonds cancel out and it has a (temporary dipole) and thus is non polar i.e. ( - + - )
What are temporary dipoles?
Uneven distribution of charge when electrons which are in constant motion around the nucleus come into close proximity with one another (non polar or polar molecules can both do this)
Thus creating a temporary dipole
What is the most important fact about polar molecules that needs to be remembered?
If the polar bonds are arranged so that they all point in the roughly direction and they don’t cancel each other out then charge will be arranged unevenly across the whole molecule resulting in a permanent dipole thus a polar molecule e.g.
+. - +
- - - , +. + , - -
What are intermolecular forces?
Forces between molecules (much weaker than ionic or covalent)
What are Van der Waals forces?
- cause atoms to be attracted to each other
{} electrons in charge clouds are always moving super quickly so at any given moment, the electrons in an atom are more likely to be at one side of the atom rather than evenly spread out or on the other side. At this moment the atom would have a temporary dipole (moment) . This can cause another temporary dipole in the opposite direction of a neighbouring atom ( this can cause a domino effect). The two dipoles are then attracted to one another, these dipoles are constantly created and destroyed . Overall effect is for atoms to be attracted to one another
What do the intensity of van der waals forces affect?
Boiling points, melting points, viscosity
What do all van der waals forces not really share in common?
Same strength
What’s the general trend with van der waals forces?
Larger molecules have larger electron clouds thus stronger van der waals forces
Longer alkane chains have more vdw
As you go down groups (noble) no. Of e increases so the vdw increases as well as the bp
Describe what happens with the van der waals forces and a substance boiling ?
To boil a liquid you need to overcome the IMF so that particles can escape from the liquid surface , liquids with stronger vdw forces require more energy to overcome thus will have higher bps
What’s a practical you can use to demonstrate dipole (permanent) at action?
Ruler (charged) and water
More polar, more electrostatic attraction /deflection
What is hydrogen bonding?
When hydrogen is covalently bonded to fluorine, nitrogen , oxygen.
It’s the strongest IMF
They draw bonding electrons away from the hydrogen atom
What is hydrogen bonding (in depth)
The bond is so polarised and hydrogen has such a high charge density because its so small that the H atom forms weak bonds with lone pairs of fluorine/nitrogen/oxygen of other molecules . These molecules are usually organic and (-OH) / (-NH) groups
What effect does H bonding have on properties of substances ?
- strong H bonding= higher bp and mp because extra energy is required to break the bonds e.g. when water has a higher bp than other group 6 hydrides
What effect does hydrogen bonding have on water
As liquid water cools to form ice the molecules makes more h bonds and arrange themselves into a regular lattice structure, since h bonds are relatively long, the avg dist between H2O molecules is greater in ice than liquid water, thus ice is less dense than water unusually as most substances are more dense in their solid form in comparison to their liquid ones.
What’s the general behaviors of simple covalent compounds
Strong cov bonds within molecules but weak forces between molecules.
Don’t conduct electricity( no free ions or electrons to carry the charge)
Low melting points because the forces between molecules is weak
Some simple cov compounds are soluble in water but that depends on their polarity
