Electronegativity and intermolecular forces Flashcards
What is electronegativity?
The relative ability of an atom to attract a pair of electrons in a covalent bond
What is a permanent covalent bond?
A bond with a permanent dipole (a dipole that doesn’t change)
Usually in polar covalent bonds
Define polar covalent bond
Shared pair of electrons where the electron pair is not shared equally between the two bonded atoms
HAS A PERMANENT DIPOLE
Explain why non-polar simple molecules don’t usually dissolve in polar solvents
There is little interaction between the molecules in the lattice and the polar solvents molecules
When will a bonded pair of electrons be shared equally?
In a non-polar bond
A bond will be non-polar when:
• the bonded atoms are the same
• “ “ “ have the same electronegativity
What are pure covalent bonds?
It’s what bonded atoms which come from the same element have
(Pair of electron = shared equally)
NON-POLAR BONDS WITH SAME ELEMENT
Explain why CO2 is non polar
The two C=C bonds have a permanent dipole = acts in opposite direction + exactly opposed one another ∴ dipoles cancel out + overall dipole = 0
Explain why H2O is polar
The two O-H bonds have a permanent dipole = acts in different direction + doesn’t exactly oppose each other ∴ overall O end = partially -ve and overall H end = partially +ve
Explain why a non-polar simple molecule can dissolve in non-polar solvents
Interactions between simple + solvent -> weakens + breaks intermolecular forces in simple lattice. Intermolecular forces form between molecules and the solvent
What is the electronegativity difference in pure covalent bonds?
0
What is the electronegativity difference in ionic bonds?
> 1.8
Why does the electronegativity change when bonded atoms are different?
- diff nuclear charges
- diff sizes
- shared pair of electrons may be closer to one nucleus than the other
Explain how an induced dipole-dipole forms (London Bonds)
Movement of electrons causes an instantaneous dipole in the molecule (which constantly changes positions) -> induces a dipole in a neighbouring molecule -> same
= attraction
Define dipole
Charge separation across a bond with one atom have a partial + charge and one with a partial - charge
How does the number of electrons affect the london forces (induced dipoles)
More electrons =
• larger the instantaneous and induced dipoles
• greater induced dipole-dipole interactions
• stronger attractive forces between molecules
Explain two anomalous features of water
- ice is less dense than liquid water ∵ the 4 H-bonds in each water molecule = hold molecules apart in open lattice structure
- Higher m.p + b.p ∵ lots of energy to break H-bonds
Which atoms do you usually get H-bonds with?
O, N, F
What are intermolecular forces?
Weak interactions between dipoles of different molecules
What are the 3 types of intermolecular forces?
1) London forces
2) permanent dipole-dipole interactions
3) H bonding
What do intermolecular forces determine compare to what covalent bonds determine?
Intermolecular = physical properties e.g. M.p + b.p Covalent = identity + chemical reactions of the molecules
Where do london forces exist?
Between all molecules
Where do permanent dipole-dipole interactions exist?
Between permanent dipoles in different polar molecules
Why do permanent dipole-dipole interactions need more energy to break?
∵ need to break london forces AND the permanent dipole - dipole
