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C = Structure and Bonding = > Intermolecular Forces > Flashcards

Intermolecular Forces Flashcards

1
Q

Define electronegativity.

A

Ability of an atom to attract the SHARED PAIR of electrons (1)

in a COVALENT bond. (1)

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2
Q

What factors affect electronegativity?

A

> Nuclear charge =
as it increases, electronegativity increases.

> Atomic radius =
as it decreases, electronegativity increases.

> Electron shielding =
as it decreases, electronegativity increases.

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3
Q

What is the most electronegative element?

A

Fluorine

(4.0 on Pauling’s scale) - largest nuclear charge for its electron shielding - small atomic radius.

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4
Q

How do you get a nonpolar bond?

A

Both bonding elements have the same electronegativities.
OR
They could have different electronegativities, but if the atom shape is symmetrical then they cancel out.

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5
Q

How do you get a polar bond?

A

Bonding atoms have different electronegativities,

in an asymmetrical atom shape.

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6
Q

What is the strongest type of intermolecular force? [1 mark]

A

Hydrogen bonding.

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7
Q

What is the weakest type of intermolecular force?

A

Van der Waals forces.

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8
Q

Are Van der Waals forces stronger in smaller or larger molecules?

A

Larger - since there’s more electrons.

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9
Q

Describe permanent dipole-dipole attraction.

A

Some molecules w/ polar bonds have permanent dipoles - forces of attraction between dipoles and neighbouring molecules.

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10
Q

How are intermolecular forces represented in a ball & stick diagram?

A

Dotted lines.

Cl₂ ━ Cl₂ —– Cl₂ ━ Cl₂

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11
Q

What are the 3 types of intermolecular forces?

A

Hydrogen bonding

Dipole-dipole forces

Van der Waals

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12
Q

What is a dipole?

A

“Di” = 2, “pole” = charge

Molecules with an area of slight negative charge and area of slight positive charge.

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13
Q

What is a dipole-dipole force?

A

When there is an attraction between oppositely charged atoms between 2 dipole molecules.

E.G.
H𝛿⁺ ━ Cl𝛿− —- H𝛿⁺ ━ Cl𝛿−

The partially negative Cl charge is attracted to the partially positive H charge.

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14
Q

Does the following substance have dipole-dipole forces between molecules?

PCl₅

A

NO…

PCl₅ does not have a dipole so cannot form dipole-dipole interactions.

(way I figured this out - if they are on the same period they won’t form dipoles)

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15
Q

Does the following substance have dipole-dipole forces between molecules?

NaCl

A

NO…

NaCl is an ionic compound which does not consist of molecules, only ionic bonds.

Therefore NaCl cannot have dipole-dipole interactions.

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16
Q

Does the following substance have dipole-dipole forces between molecules?

CHCl₃

A

YES…

CHCl₃ has a dipole as the hydrogen side is partially positive and the chlorine side is partially negative.

Therefore, CHCl₃ has dipole-dipole interactions between its molecules.

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17
Q

Does the following substance have dipole-dipole forces between molecules?

HF

A

YES…

HF has a dipole as the hydrogen side is partially positive and the fluorine side is partially negative.

Therefore, HF has dipole-dipole interactions between its molecules.

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18
Q

Explain how dipole-dipole interactions arise between ICl molecules.

A

There is a difference in electronegativity between iodine and chlorine. This means that the ICl bond has a dipole and is polar.

Dipole-dipole interactions arise from the attraction between the 𝛿⁺ side on one molecule and the 𝛿− molecule on another.

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19
Q

The strength of a dipole depends on…

A

the difference between the charges.

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20
Q

What is the relationship between dipole strength and dipole-dipole forces?

A

The stronger the dipole, the stronger the dipole-dipole forces.

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21
Q

What is the relationship between difference between charges and dipole strength?

A

The larger the difference between charges / electronegativity, the stronger the dipole.

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22
Q

What is a hydrogen bond?

A

A very strong dipole-dipole force.

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23
Q

When does hydrogen bonding occur?

A

ONLY when hydrogen is bonded to either…

N - nitrogen
O - oxygen
F - fluorine

Because these elements are the most electronegative and form strong dipole-dipole bonds.

There’s a strong force of attraction between H nucleus & lone pair of electrons on N, O, F.

So look for N-H, O-H, F-H bonds, won’t count if it’s not bonded together directly.

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24
Q

What steps would you use to draw an example of hydrogen bonding?

A

1# Include partial charges on each atom.
2# Draw any lone pairs.
3# Draw the hydrogen bonding between the lone pairs and the atom (dotted line)
4# Make sure they all lie on a straight line.

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25
Q

What are temporary dipoles?

A

When the movement of electrons nonpolar molecules causes there to be a slight negative charge on one side, causing there to be a temporary dipole.

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26
Q

What’s a dipole in simple terms?

A

When there is a slight negative or positive charge on either side of a molecule due to the movement of electrons or the electronegativity.

27
Q

What are permanent dipoles?

A

When electrons are closer to the more electronegative atom, forming partial opposite charges on either side, aka a dipole.

28
Q

Explain what happens when a molecule with a temporary dipole moves close to a molecule without a temporary dipole.

A

The electrons will repair each other.
This causes the electrons to move to the opposite side of molecule B…
causing a temporary dipole to form.

Molecule A has induced a dipole in molecule B.
The partial charges on each molecule attract.

This forms temporary induced dipole-dipole forces.

29
Q

What are other names for a temporary induced dipole-dipole force?

A

Dispersion forces.

London forces.

Van der Waals forces.

30
Q

What is a van der Waals force?

A

The attractive forces between MOLECULES.

31
Q

How are Van der Waals forces formed?

A

When the movement of electrons in one molecule creates a temporary dipole.
This induces or creates a temporary dipole in a nearby molecule.

This results in the attraction between partial charges of the nearby molecules.
Aka Van der Waals forces.

32
Q

POLAR molecules can experience both…

A

Dipole-dipole forces
and
Van der Waals forces

33
Q

ALL polar/non-polar molecules or elements existing as 1 atom have…

A

Van der Waals forces.

Even gases like helium/argon.

34
Q

How do you know if a molecule is polar?

A

The molecule would have LONE PAIRS of electrons around the central atom.

35
Q

How do you know if a molecule is non-polar?

A

The molecule has NO lone pairs of electrons around the central atom.

36
Q

What is the relationship between the strength of Van der Waals forces and number of electrons?

A

The strength of Van der Waals forces increases as the number of electrons increases.

37
Q

Explain…
As you go down the group, the strength of Van der Waals forces…

A

As you go down the group, the strength of Van der Waals forces INCREASES.

This is bc the number of electrons increase as you go down.
Meaning there’s likely to be a larger charge difference between the molecules.
Creating temporary dipole forces.

38
Q

Predict which of these noble gases will experience a stronger Van der Waals force.

Helium (He)
Argon (Ar)
Krypton (Kr)
Radon (Rn)

A

Radon (Rn)

Has the most number of electrons, more likely for there to be a greater charge difference which causes a temporary dipole.

39
Q

Based on the description, name the intermolecular force.

This intermolecular force originates from temporary dipoles.

This intermolecular force originates from N, O or F atoms bonded to hydrogen.

This intermolecular force originates from the asymmetric distribution of polar bonds.

A

This intermolecular force originates from temporary dipoles =
Van der Waals force

This intermolecular force originates from N, O or F atoms bonded to hydrogen =
Hydrogen bond

This intermolecular force originates from the asymmetric distribution of polar bonds =
Dipole-dipole force

40
Q

Explain how Van der Waals forces arise between two molecules [3 marks]

A

The electrons in the first molecules move. When the electrons are unevenly distributed, a temporary dipole forms. This temporary dipole induces a dipole in the next molecule. The opposite charges of each dipole then attract each other.

41
Q

Why are there no dipole-dipole forces between SF₆ molecules?

A

F is more electronegative than S, so the S-F bond is polar and has a dipole.

SF₆ has octahedral geometry, and so, the polar bonds are distributed symmetrically.

Therefore, the dipoles cancel out and SF₆ doesn’t have a dipole.

42
Q

What happens when a molecular solid or liquid is boiled/melted?

A

The molecules can move even further apart. So, the intermolecular forces have become weaker.

43
Q

Explain why bromine molecules have a higher melting/boiling point compared to chlorine molecules. [2 marks]

A

Bromine contains more electrons, so it experiences stronger Van der Waals forces than chlorine.
More energy is required to overcome these forces.

44
Q

If 2 molecules have the same number of electrons, which molecule will have a higher melting/boiling points?

A

The molecule that has hydrogen bonding which is stronger than Van der Waals forces.

45
Q

Do molecules with intermolecular forces conduct state in solid, liquid or gas state?

A

No.
Never.

Even if their intermolecular forces are weak, the electrons are still unable to move freely as the atoms are held by covalent bonds and the electrons are being shared.

46
Q

Why is ice less dense than liquid water?

A

In liquid water…
H bonds constantly break and reform as molecules move about.

In ice…
H bonds hold the molecules in fixed positions, this makes them slightly further apart than liquid water.

47
Q

Draw the formation of a dative covalent bond in ammonia.

A

N in the centre.
3 H atoms around it, and 1 H+ ion.

1 H has a normal line.
1 H has a thick line
1 H has a dotted line.

N points an arrow to H+ ion.

48
Q

Which of these molecules are non-polar and why?

 Be ━ F (1.5)       (4.0)

   I ━ I  (2.5)     (2.5)

  C ━ H  (2.5)      (2.1)

 Be ━ FBe ━ F  (2.5)     (3.0)
A

I ━ I and C ━ H

Because their electronegativities are either the same, or just very similar.
This means they don’t have a dipole so they are NON-POLAR.

Be ━ F and Be ━ F are POLAR

Because they have a larger difference in electronegativities and so they have a dipole.

It is considered polar if the electronegativities have a difference of 0.5 or more.

49
Q

What electronegativity difference considers molecules to be polar?

A

0.5 or more difference in Pauling scale.

50
Q

What electronegativity difference considers molecules to be non-polar?

A

< 0.5

51
Q

Is the C ━ H bond polar or non-polar?

A

Non-polar

YOU ARE EXPECTED TO KNOW THIS IN THE EXAM.

52
Q

Does the presence of a single, double or triple bond influence whether a molecule has a dipole?

A

No.

All the shaired electrons will just be attracted to the more electronegative atom.

53
Q

How can you represent polar molecules in a covalent bond?

A

N δ- ━ Hδ+ (Using delta positive/negative to show partial charges)

<——+
N ━ H (Using a polar vector - + is always near the partially positive atom)

54
Q

What difference in electronegativity between to atoms form an ionic compound, and why?

A

2.5 or more.

The large difference in electronegativity makes the ‘shared’ pair of electrons to move so closely to the more electronegative element (e.g. F / Cl) that it belongs to the atom and the electrons are transferred.

55
Q

What difference in electronegativity between to atoms form a covalent compound?

A

< 2.5

56
Q

Which difference in electronegativities cause atoms to bond covalently or ionically?

A

Ionic =
2.5 or above

Covalent =
< 2.5

57
Q

What electronegativity do ELEMENTS form covalent bonds (are non-metals)?

A

2.1 or more.

58
Q

What electronegativity do ELEMENTS form metallic bonds (are metals)?

A

1.5 or less.

59
Q

Why do lower electronegativities form metallic bonds?

A

The lower the electronegativity…

the less electrons are held tightly…

easier to delocalise…

60
Q

Elements with electronegativities between 1.5 and 2.1 such as Si and B are called…

A

Metalloids

They share properties of metals and covalently bonded molecules.

(Beyond the spec)

61
Q

How do you predict how elements bond?

A

Using the BOND TRIANGLE.

(Beyond the spec)

62
Q

What are the axes on the bond triangle?

A

Y-axis = Difference in electronegativities

X-axis = Average electronegativity

63
Q

Polyatomic ionic compounds contain both…

A

Ionic and covalent bonding.

64
Q

Why do polyatomic ionic compounds contain ionic and covalent bonding?

A

Ionic bonding =
Holding the cation and polyatomic anion

Covalent bonding =
Exists between the atoms in the polyatomic anions (CO₃, OH, PO₄, NH₄)

C = Structure and Bonding = (6 decks)

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