Electronegativity

Question 1

Define electronegativity

Answer 1

The power of an atom in a molecule to attract electrons to itself

Question 2

What is this sign and what does it usually represent on a picture of a molecule?

Answer 2

It’s a delta (lowercase) and usually represents a partial charge, as in the case of a polar molecule.

Question 3

What difference in electronegativity is required for a bond to become polar?

Answer 3

0.5 or more

Question 4

What difference in electronegativity is required for a bond to become ionic?

Answer 4

More than 1.7 (approx)

Question 5

What is this sign and what does it usually represent in chemistry?

Answer 5

It’s a mu (lowercase) and represents the dipole moment

Question 6

Provide and explain the formula for calculating the dipole moment

Answer 6

mu = Qd

mu is the dipole moment

Q is the magnitude of the charge on the two poles

d is the distance between the poles

Question 7

What unit is the dipole moment in?

Answer 7

Debyes

Question 8

What are Debyes?

Answer 8

The unit in which dipole moment is given

Question 9

What does the arrow in this picture represent?

Answer 9

The direction in which electrons are more strongly attracted in a polar bond

Remember: the little plus sign on the arrow can be interpreted as the side of the bond with a partial positive charge

Question 10

What does the dipole moment tell us about a molecule?

Answer 10

How polarised it is.

Question 11

This is carbon diozide

O = C = O

Is it a polar molecule, and why?

Answer 11

It is non-polar. The double bonds make it a symmetrical linear shape. Each oxygen attracts electrons with the same strength, but in opposing directions. The individual bond dipoles cancel each other out, and overall the molecule has no dipole moment.

Question 12

The electronegativity of chlorine is 0.61 stronger than the electronegativity of carbon. Is this a polar molecule?

Answer 12

No. Its tetrahedral shape means that the four chlorine atoms are attracting electrons in symmetrically opposite directions, so the individual bond dipoles are cancelling each other out and the overall molecular dipole moment is 0

Question 13

The electronegativity of chlorine is 0.61 stronger than the electronegativity of carbon, which is 0.45 stronger than that of hydrogen. Is this a polar molecule?

Answer 13

Yes. The hydrogen does not attract the carbon’s electrons (in fact the carbon weakly attracts the hydrogen’s). The chlorines do attract the carbon’s electrons, creating an asymmetrical attraction in the downward direction in this picture. Therefore there is a dipole moment for the molecule as a whole.

Question 14

What’s the difference between dipole-dipole interaction and hydrogen bonding?

Answer 14

Hydrogen bonding is one example of dipole-dipole interaction, however it is a particularly strong attraction and hydrogen bonding is the strongest intermolecular force.

Question 15

Which atoms can participate in hydrogen bonding?

Answer 15

Flourine, oxygen, nitrogen (and hydrogen)

Remember: FON

Question 16

What is the difference between van der Waals forces and London dispersion forces?

Answer 16

London dispersion forces is the subcategory of van der Waals force in which the instantaneous formation of weak dipoles in non-polar molecules caused by fluctuations in electron distribution align between molecules, causing a weak attraction

Remember: What you originally learned as van der Waals force is actually London dispersion force.

Question 17

Why does water have a relatively high boiling point?

Answer 17

Water molecules are attracted to each other by hydrogen bonds, which are the strongest kind of intermolecular force. Therefore it takes a lot of energy to break those bonds and for water to become gas.

Question 18

Of pentane and hexane, which has the higher boiling point and why?

Answer 18

Hexane has the higher boiling point. Both molecules are non-polar, but the larger number of carbons creates a slightly greater overall attraction for the electrons, and this as well as the larger surface area means the London dispersion forces between molecules will be slightly stronger and will require more energy to break.

Remember: These are both saturated hydrocarbons (-ane).

Question 19

Which of these two molecules will have the highest boiling point and why?

Answer 19

The one on the left (pentane) has the higher boiling point. The branching structure of the molecule on the right (neopentane) means there is less surface area for London dispersion forces to form between molecules. As these are both saturated hydrocarbons and therefore nonpolar, they are only attracted to one another by London dispersion forces. The weaker forces for neopentane require less energy to break and therefore boiling point is lower.

Question 20

Which of these two molecules has the higher boiling point and why?

Answer 20

3-hexanol has the higher boiling point, because the molecules are attracted to each other with hydrogen bonds. The 3-hexanone molecules are attracted to each other with dipole-dipole interactions. The dipoles are formed between the carbon and oxygen atoms, whereas the dipoles in 3-hexanol are between oxygen and hydrogen atoms, meaning the partially negatively charged oxygen on one molecule is attracted to the partially positively charged hydrogen on the next.

Question 21

Explain the phrase “like dissolves like”

Answer 21

Polar solvents can dissolve polar (and ionic) solutes

Non-polar solvents can dissolve non-polar solutes

However each cannot dissolve the other