3.5 - Forces acting between molocules

Question 1

Permanent dipole-dipole forces

Answer 1

Permanent dipole-dipole forces are electrostatic attractions between the partial positive end of one polar molecule and the partial negative end of another.
**
For example, in gaseous hydrogen chloride (HCl):**

1. The H-Cl bond is polar due to the greater electronegativity of chlorine compared to hydrogen.
2. The hydrogen atom bears a partial positive charge (δ+) and the chlorine a partial negative charge (δ-).
3. HCl molecules align so the δ+hydrogen of one molecule is attracted to the δ- chlorine of a neighbouring molecule.

Question 2

How do hydrogen bonds form?

Answer 2

• The H-F, H-O, and H-N bonds are highly polar due to the large electronegativity differences between hydrogen and these elements. This leads to a significant partial positive charge (δ+) on the hydrogen atom and a partial negative charge (δ-) on the F, O, or N atom.
• The small size of the hydrogen atom allows it to get close to the lone pair of electrons on an adjacent F, O, or N atom.
• The lone pairs on F, O, and N atoms are regions of high electron density and therefore high partial negative charge.
• The positively charged hydrogen is strongly attracted to the negatively charged lone pair, forming a hydrogen bond between the molecules

Question 3

Properties of water and ice (2)

Answer 3

• Ice is less denser than water -In solid ice, water molecules are arranged in a 3D lattice held together by hydrogen bonds. Upon melting, some of these hydrogen bonds break. Since hydrogen bonds are relatively long compared to covalent bonds, this causes ice to be less dense than liquid water.
• Water and ice have high melting and boiling points -Water has relatively high melting and boiling points compared to other molecules of similar size. This is due to the strong hydrogen bonds between H2O molecules, which require more energy to break.

Question 4

Hydrogen bonding (2 criterias)

Answer 4

• strongest IMF
• For hydrogen bonding to occur, two criteria must be met:

1. The molecule must contain a hydrogen atom covalently bonded to either fluorine (F), oxygen (O), or nitrogen (N).
2. There must be a lone pair of electrons on the F, O, or N atom of an adjacent molecule available to interact with the hydrogen.

Question 5

how to explain the difference in b.p due to IMFs

Answer 5

• Methanal is a polar molecule due to the carbonyl group (C=O), so it experiences both permanent dipole-dipole forces and induced dipole-dipole forces. The permanent dipole-dipole forces are stronger than the induced dipole-dipole forces alone.
• Ethane is a non-polar molecule, so it only experiences induced dipole-dipole forces.

As a result, more energy is required to overcome the stronger intermolecular forces in methanal, leading to a higher boiling point, even though the molecules are of similar size and have the same molecular mass (Mr = 30).

Question 6

Properties of simple molecular forces (3)

Answer 6

• Low melting and boiling points- Weak intermolecular forces require little energy to overcome, so covalent compounds often have low melting and boiling points and may be liquid or gaseous at room temperature. Stronger intermolecular forces lead to higher melting and boiling points.
• Solubility in water- Polar molecules, especially those capable of hydrogen bonding (e.g., ethanoic acid), can interact favorably with water molecules and are soluble. Non-polar molecules that only have induced dipole-dipole forces (e.g., hexane) are hydrophobic and insoluble in water.
• Electrical conductivity - Covalent compounds do not conduct electricity, regardless of polarity. Even though polar molecules have permanent dipoles, they are electrically neutral overall and do not carry charge.

Question 7

Factors affecting the strength of induced dipole-dipole forces (2)

Answer 7

1.** Size**- Larger atoms and molecules have more electrons and a greater volume of electron density that can become polarised, creating stronger temporary dipoles.
2. Surface area- Molecules with a larger surface area also have stronger induced dipole-dipole forces as more of the electron cloud is exposed for interactions.

Consequently, substances with stronger induced dipole-dipole forces tend to have higher boiling points.

Question 8

VDW forces - how do they arise?

Answer 8

Electrons in atoms are constantly moving. At any instant, there may be more electrons on one side of the atom than the other, creating a temporary dipole.
This temporary dipole can induce an opposite dipole in a neighbouring atom, causing a weak electrostatic attraction between the atoms.
This induced dipole can then induce further dipoles in other nearby particles.
Although these dipoles are constantly forming and disappearing as the electrons move, the overall effect is a net attraction between the atoms or molecules.

Question 9

3 types of intermolecular forces in order of strength

Answer 9

• Induced dipole-dipole forces (also known as London dispersion forces or Van der Waals’ forces).
• Permanent dipole-dipole forces.
• Hydrogen bonding.

Question 10

VDW forces in molecular lattices and noble gases

Answer 10

• In solid iodine, I2molecules are held together by strong covalent bonds.
• These I2 molecules are then attracted to each other by weak induced dipole-dipole forces, forming a molecular lattice

the weak induced dipole-dipole forces allow them to condense into the liquid and solid states at very low temperatures.

Question 11

Characteristics of the IMF

Answer 11

Question 12

3 types of intermolecular forces in order of strength

Answer 12

• Induced dipole-dipole forces (also known as London dispersion forces or Van der Waals’ forces).
• Permanent dipole-dipole forces.
• Hydrogen bonding.