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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
What are temporary dipoles?
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.
26
What's a dipole in simple terms?
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
What are permanent dipoles?
When electrons are closer to the more electronegative atom, forming partial opposite charges on either side, aka a dipole.
28
Explain what happens when a molecule with a temporary dipole moves close to a molecule without a temporary dipole.
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
What are other names for a temporary induced dipole-dipole force?
Dispersion forces. London forces. Van der Waals forces.
30
What is a van der Waals force?
The attractive forces between MOLECULES.
31
How are Van der Waals forces formed?
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
POLAR molecules can experience both...
Dipole-dipole forces and Van der Waals forces
33
ALL polar/non-polar molecules or elements existing as 1 atom have...
Van der Waals forces. Even gases like helium/argon.
34
How do you know if a molecule is polar?
The molecule would have LONE PAIRS of electrons around the central atom.
35
How do you know if a molecule is non-polar?
The molecule has NO lone pairs of electrons around the central atom.
36
What is the relationship between the strength of Van der Waals forces and number of electrons?
The strength of Van der Waals forces increases as the number of electrons increases.
37
Explain... As you go down the group, the strength of Van der Waals forces...
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
Predict which of these noble gases will experience a stronger Van der Waals force. Helium (He) Argon (Ar) Krypton (Kr) Radon (Rn)
Radon (Rn) Has the most number of electrons, more likely for there to be a greater charge difference which causes a temporary dipole.
39
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.
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
Explain how Van der Waals forces arise between two molecules [3 marks]
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
Why are there no dipole-dipole forces between SF₆ molecules?
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
What happens when a molecular solid or liquid is boiled/melted?
The molecules can move even further apart. So, the intermolecular forces have become weaker.
43
Explain why bromine molecules have a higher melting/boiling point compared to chlorine molecules. [2 marks]
Bromine contains more electrons, so it experiences stronger Van der Waals forces than chlorine. More energy is required to overcome these forces.
44
If 2 molecules have the same number of electrons, which molecule will have a higher melting/boiling points?
The molecule that has hydrogen bonding which is stronger than Van der Waals forces.
45
Do molecules with intermolecular forces conduct state in solid, liquid or gas state?
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
Why is ice less dense than liquid water?
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
Draw the formation of a dative covalent bond in ammonia.
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
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)
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
What electronegativity difference considers molecules to be polar?
0.5 or more difference in Pauling scale.
50
What electronegativity difference considers molecules to be non-polar?
< 0.5
51
Is the C ━ H bond polar or non-polar?
Non-polar YOU ARE EXPECTED TO KNOW THIS IN THE EXAM.
52
Does the presence of a single, double or triple bond influence whether a molecule has a dipole?
No. All the shaired electrons will just be attracted to the more electronegative atom.
53
How can you represent polar molecules in a covalent bond?
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
What difference in electronegativity between to atoms form an ionic compound, and why?
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
What difference in electronegativity between to atoms form a covalent compound?
< 2.5
56
Which difference in electronegativities cause atoms to bond covalently or ionically?
Ionic = 2.5 or above Covalent = < 2.5
57
What electronegativity do ELEMENTS form covalent bonds (are non-metals)?
2.1 or more.
58
What electronegativity do ELEMENTS form metallic bonds (are metals)?
1.5 or less.
59
Why do lower electronegativities form metallic bonds?
The lower the electronegativity... the less electrons are held tightly... easier to delocalise...
60
Elements with electronegativities between 1.5 and 2.1 such as Si and B are called...
Metalloids They share properties of metals and covalently bonded molecules. (Beyond the spec)
61
How do you predict how elements bond?
Using the BOND TRIANGLE. (Beyond the spec)
62
What are the axes on the bond triangle?
Y-axis = Difference in electronegativities X-axis = Average electronegativity
63
Polyatomic ionic compounds contain both...
Ionic and covalent bonding.
64
Why do polyatomic ionic compounds contain ionic and covalent bonding?
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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