Electronegativity and intermolecular forces

Question 1

What is electronegativity?

Answer 1

The relative ability of an atom to attract a pair of electrons in a covalent bond

Question 2

What is a permanent covalent bond?

Answer 2

A bond with a permanent dipole (a dipole that doesn’t change)
Usually in polar covalent bonds

Question 3

Define polar covalent bond

Answer 3

Shared pair of electrons where the electron pair is not shared equally between the two bonded atoms
HAS A PERMANENT DIPOLE

Question 4

Explain why non-polar simple molecules don’t usually dissolve in polar solvents

Answer 4

There is little interaction between the molecules in the lattice and the polar solvents molecules

Question 5

When will a bonded pair of electrons be shared equally?

Answer 5

In a non-polar bond
A bond will be non-polar when:
• the bonded atoms are the same
• “ “ “ have the same electronegativity

Question 6

What are pure covalent bonds?

Answer 6

It’s what bonded atoms which come from the same element have
(Pair of electron = shared equally)
NON-POLAR BONDS WITH SAME ELEMENT

Question 7

Explain why CO2 is non polar

Answer 7

The two C=C bonds have a permanent dipole = acts in opposite direction + exactly opposed one another ∴ dipoles cancel out + overall dipole = 0

Question 8

Explain why H2O is polar

Answer 8

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

Question 9

Explain why a non-polar simple molecule can dissolve in non-polar solvents

Answer 9

Interactions between simple + solvent -> weakens + breaks intermolecular forces in simple lattice. Intermolecular forces form between molecules and the solvent

Question 10

What is the electronegativity difference in pure covalent bonds?

Answer 10

0

Question 11

What is the electronegativity difference in ionic bonds?

Answer 11

> 1.8

Question 12

Why does the electronegativity change when bonded atoms are different?

Answer 12

• diff nuclear charges
• diff sizes
• shared pair of electrons may be closer to one nucleus than the other

Question 14

Explain how an induced dipole-dipole forms (London Bonds)

Answer 14

Movement of electrons causes an instantaneous dipole in the molecule (which constantly changes positions) -> induces a dipole in a neighbouring molecule -> same
= attraction

Question 14

Define dipole

Answer 14

Charge separation across a bond with one atom have a partial + charge and one with a partial - charge

Question 15

How does the number of electrons affect the london forces (induced dipoles)

Answer 15

More electrons =
• larger the instantaneous and induced dipoles
• greater induced dipole-dipole interactions
• stronger attractive forces between molecules

Question 16

Explain two anomalous features of water

Answer 16

• 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

Question 16

Which atoms do you usually get H-bonds with?

Answer 16

O, N, F

Question 17

What are intermolecular forces?

Answer 17

Weak interactions between dipoles of different molecules

Question 18

What are the 3 types of intermolecular forces?

Answer 18

1) London forces
2) permanent dipole-dipole interactions
3) H bonding

Question 19

What do intermolecular forces determine compare to what covalent bonds determine?

Answer 19

Intermolecular = physical properties e.g. M.p + b.p
Covalent = identity + chemical reactions of the molecules

Question 20

Where do london forces exist?

Answer 20

Between all molecules

Question 21

Where do permanent dipole-dipole interactions exist?

Answer 21

Between permanent dipoles in different polar molecules

Question 22

Why do permanent dipole-dipole interactions need more energy to break?

Answer 22

∵ need to break london forces AND the permanent dipole - dipole