Chapter 6 - Shapes of Molecules and intermolecular forces (MODULE 2)

Question 1

What does the electron-pair repulsion theory explain?

Answer 1

The shapes of molecules and polyatomic ions

This theory is based on the repulsion between electron pairs around a central atom.

Question 2

How do electron pairs arrange themselves according to the electron-pair repulsion theory?

Answer 2

As far apart as possible

This arrangement minimizes repulsion and maintains a definite shape for bonded atoms.

Question 3

What shape does a methane (CH₄) molecule have?

Answer 3

Tetrahedral

Methane has four equal H—C—H bond angles of 109.5°.

Question 4

What visual aids do chemists use to represent three-dimensional molecular shapes?

Answer 4

A solid line represents a bond in the plane, a solid wedge comes out of the plane, and a dotted wedge goes into the plane.

Question 5

How do lone pairs of electrons compare to bonded pairs in terms of repulsion?

Answer 5

Lone pairs repel more strongly

This is due to lone pairs being closer to the central atom.

Question 6

What is the bond angle reduction for each lone pair present?

Answer 6

About 2.5°

Lone pairs decrease the bond angle between bonded pairs.

Question 7

What shape and bond angle does ammonia (NH₃) have?
- Why

Answer 7

Pyramidal, approximately 107.5°
- Ammonia has one lone pair and three bonded pairs.
- Lone pairs repel more than bondeed regions

Question 8

What shape does water (H₂O) have, and what is its bond angle?
- Why

Answer 8

Non-linear, approximately 104.5°
- Water has two lone pairs and two bonded pairs.
- Lone pairs repel more than bondeed regions

Question 9

What is the shape of carbon dioxide (CO₂) and its bond angle?
- Why

Answer 9

Linear, 180°
- The two double bonds count as two bonded regions.
- bonded regions repel as much as possible

Question 10

What shape and bond angle does boron trifluoride (BF₃) have?
- Why

Answer 10

Trigonal planar, 120°
- BF₃ has three bonded pairs around the central boron atom.
- bonded regions repel as much as possible

Question 11

What shape and bond angle does sulfur hexafluoride (SF₆) have?
- Why

Answer 11

Octahedral, 90°
- SF₆ has six bonded pairs of electrons around the central sulfur atom.
- bonded regions repel as much as possible

Question 12

What shape does the ammonium ion (NH₄⁺) have?
- Why

Answer 12

Tetrahedral
- NH₄⁺ has four bonded pairs around the central nitrogen atom.
- bonded regions repel as far as possible

Question 13

What shapes do the carbonate (CO₃²⁻) and nitrate (NO₃⁻) ions have?
- Why?

Answer 13

Trigonal planar
- Both ions have three regions of electron density.
- bonded regions repel as much as possible

Question 14

What shape does the sulfate (SO₄²⁻) ion have?
- Why

Answer 14

Tetrahedral
- SO₄²⁻ has four centers of electron density around the central sulfur atom.
- bonded regions repel as much as possible

Question 15

True or False: The greater the number of electron pairs around a central atom, the larger the bond angle.

Answer 15

False

The greater the number of electron pairs, the smaller the bond angle.

Question 16

Fill in the blank: The arrangement of electron pairs around a central atom is based on _______.

Answer 16

[electron-pair repulsion theory]

This theory helps to predict molecular shapes.

Question 17

What is electronegativity?

Answer 17

The attraction of a bonded atom to the pair of electrons in a covalent bond

Electronegativity determines how electrons are shared between atoms in a bond.

Question 18

How is electronegativity measured?

Answer 18

Using the Pauling scale

The Pauling scale compares the electronegativity of different elements based on their position in the periodic table.

Question 19

What does a large Pauling value indicate in terms of electronegativity?

Answer 19

Atoms of the element are very electronegative

Question 20

Which element is the most electronegative?

Answer 20

Fluorine

Fluorine has a Pauling electronegativity value of 4.0.

Question 21

What types of atoms have the least electronegative atoms?

Answer 21

Group I metals, including lithium, sodium, and potassium

These metals have low electronegativity values compared to non-metals.

Question 22

What determines whether a bond is ionic or covalent
- what is the critical value for this

Answer 22

the difference in electronegativity.
electronegativity difference >1.8 then ionic, <1.8 then covalent

Question 23

What characterizes a non-polar bond?

Answer 23

The bonded electron pair is shared equally between the bonded atoms

Non-polar bonds occur when bonded atoms are the same or have similar electronegativities.

Question 24

When is a bond considered polar?

Answer 24

When the bonded electron pair is shared unequally between different atoms with different electronegativities

This results in a polar covalent bond.

Question 25

What is an example of a polar bond?

Answer 25

Hydrogen chloride (HCl) ## Footnote In HCl, chlorine has a higher electronegativity than hydrogen, leading to a polar bond.

Question 26

What is a dipole?

Answer 26

The separation of opposite charges in a polar bond ## Footnote A dipole occurs due to unequal sharing of electrons.

Question 27

What defines a permanent dipole?

Answer 27

A dipole in a polar covalent bond that does not change ## Footnote It is different from an induced dipole, which can change based on external factors.

Question 28

How does the shape of a molecule affect dipoles?

Answer 28

Dipoles may reinforce or cancel out depending on the molecule's shape ## Footnote The overall polarity of a molecule is influenced by its geometry.

Question 29

What is the result of the dipoles in a water (H2O) molecule?

Answer 29

The overall molecule is polar with a partial negative charge on oxygen and a partial positive charge on hydrogen ## Footnote The O—H bonds each have permanent dipoles that do not completely oppose each other.

Question 30

What is the overall dipole of a carbon dioxide (CO2) molecule?

Answer 30

Zero ## Footnote Although CO2 has two polar bonds, the dipoles act in opposite directions and cancel each other out.

Question 31

How do water molecules interact with Na+ and Cl- ions when NaCl dissolves?

Answer 31

Water molecules surround and attract Na+ and Cl- ions -Na+ ions are attracted to the oxygen end of water molecules, while Cl- ions are attracted to the hydrogen end.

Question 32

What is covalent bonding?

Answer 32

the forces between a shared pair of electrons and the bonded nuclei

Question 33

What are intermolecular forces?

Answer 33

Weak interactions between dipoles of different molecules.

Question 34

What are the three main categories of intermolecular forces?

Answer 34

* Induced dipole-dipole interactions (London forces) * Permanent dipole-dipole interactions * Hydrogen bonding interactions

Question 35

What physical properties are largely influenced by intermolecular forces?

Answer 35

Melting and boiling points.

Question 36

What determines the identity and chemical reactions of molecules?

Answer 36

Covalent bonds.

Question 37

What are the rough bond enthalpies of - London forces - permanent dipoles - hydrogen bonds - covalent bonds

Answer 37

- 1-10 kJ/mol - 3 - 25 kJ/mol - 10 - 40 kJ/mol - 150 - 500 kJ/mol

Question 38

What are London forces?

Answer 38

Weak intermolecular forces that exist between all molecules, whether polar or non-polar.

Question 39

How do induced dipoles occur in molecules?

Answer 39

Movement of electrons produces a changing dipole in a molecule. This instantaneoud dipole induces a dipole on a neighbouring molecule. Induced dipole induces further dipoles which then attract each other

Question 40

What happens to the strength of London forces with an increase in the number of electrons?

Answer 40

The larger the number of electrons, the larger the instantaneous and induced dipoles, leading to stronger induced dipole-dipole interactions, and the stronger the attractive forces between molecules

Question 41

What is the boiling point trend observed in noble gases as the number of electrons increases?

Answer 41

Larger numbers of electrons mean larger induced dipoles, requiring more energy to overcome intermolecular forces, thus increasing the boiling point.

Question 42

What are permanent dipole-dipole interactions?

Answer 42

Interactions that act between the permanent dipoles in different polar molecules.

Question 43

What is a simple molecular substance?

Answer 43

Made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula.

Question 44

What structure do simple molecular substances form in the solid state?

Answer 44

A simple molecular lattice.

Question 45

What bonds and forces are present in simple molecular lattices

Answer 45

covalent bonds between aroms in a molecule, weak intermolecular forces between molecules

Question 46

What properties do simple molecular substances exhibit?

Answer 46

* Low melting point and boiling point * May exist as solids, liquids, or gases at room temperature * Can be solidified into simple molecular lattices by reducing temperature

Question 47

Why do simple molecular substances have low melting and boiling points?

Answer 47

Weak intermolecular forces can be broken even by the energy present at low temperatures.

Question 48

What happens to covalent bonds when a simple molecular lattice is broken?

Answer 48

The covalent bonds are strong and do not break.

Question 49

How do non-polar simple molecular substances behave when added to a non-polar solvent?

Answer 49

Intermolecular forces form between the molecules and the solvent, weakening the intermolecular forces in the lattice, allowing the compound to dissolve.

Question 50

What types of solvents do polar and non polar covalent molecules dissolve in

Answer 50

polar: polar solvents non polar: non polar solvents

Question 51

Why are non polar molecules insoluble in polar solvents

Answer 51

little interactions between molecules in lattice so IM bonds too strong to be broken

Question 52

What is the solubility behavior of polar simple molecular substances in polar solvents?

Answer 52

Polar covalent substances may dissolve as polar solute molecules and polar solvent molecules can attract each other.

Question 53

What does the solubility of polar molecules in polar solvents depend on

Answer 53

the strength of the dipoles

Question 54

Why are simple molecular structures non-conductors of electricity?

Answer 54

There are no mobile charged particles in simple molecular structures.

Question 55

Fill in the blank: Induced dipoles are only .........

Answer 55

[temporary]

Question 56

What are hydrophobic and hydrophilic parts?

Answer 56

hydrophobic is the polar part that interacts with water. Hydroophilic is the non polar part that does not.

Question 57

What is a hydrogen bond

Answer 57

an interaction between an electronegative atom with a lone pair (O, N, F) and a hydrogen atom attached to an electronegative atom (HO, HN, HF)

Question 58

What is the strongest IM attraction

Answer 58

hydrogen bond

Question 59

What is the shape and bond angle around a hydrogen bond

Answer 59

180 degrees, linear

Question 60

Why is solid water (ice) less dense than liquid (water)

Answer 60

- hydrogen bonds hold water molecules apart in the lattice structure, meaning the water molecules in ice are further apart and so solid ice is less dense than liquid water

Question 61

How many hydrogen bonds can a water molecule form, and how

Answer 61

2 lone pairs on Oxygen and 2 hydrogen atoms - 4 total

Question 62

Why does water have a relatively high melting and boiling point

Answer 62

- Hydrogen bonds are extra forces acting alongside the usual London forces - more energy is needed to break these hydrogen bonds - therefore water has a higher meeting and boiling points than most other simple molecular structures

Question 63

What are 4 anomalous properties of water

Answer 63

- low density as a solid - high surface tension - high viscosity - high melting and boiling points