UPCAT Chemistry — Molecular Theory — VSEPR, IMFA & KMTFlash Cards

What does VSEPR stand for and what is its main purpose?

VSEPR stands for Valence Shell Electron Pair Repulsion theory. Its main purpose is to predict the 3D molecular geometry by assuming that electron pairs around a central atom repel each other and arrange themselves to minimize repulsion.

A molecule has 2 bonding pairs and 0 lone pairs. What is its molecular geometry and bond angle?

Molecular geometry: Linear. Bond angle: 180°. Example: BeF₂ or CO₂. The two bonding pairs arrange themselves as far apart as possible in a straight line.

Determine the molecular geometry of NH₃ (3 bonding pairs, 1 lone pair).

NH₃ has trigonal pyramidal geometry. The lone pair on nitrogen occupies more space than bonding pairs, compressing the H-N-H bond angles to approximately 107° (less than the tetrahedral 109.5°).

Why do lone pairs affect molecular geometry differently than bonding pairs?

Lone pairs occupy more space than bonding pairs because they are attracted to only one nucleus, while bonding pairs are shared between two nuclei. This causes lone pairs to repel other electron pairs more strongly, compressing bond angles.

What is the molecular geometry of water (H₂O) and explain why it's bent?

H₂O has bent (angular) geometry with bond angle ~104.5°. Oxygen has 2 bonding pairs and 2 lone pairs. The lone pairs repel the bonding pairs, compressing the H-O-H angle from the tetrahedral 109.5° to 104.5°.

What are the three main types of intermolecular forces (IMFA)?

1. London Dispersion Forces (weakest) - present in all molecules 2. Dipole-Dipole Forces - between polar molecules 3. Hydrogen Bonding (strongest) - between molecules with H bonded to N, O, or F

Explain what causes London dispersion forces and give an example.

London dispersion forces are caused by temporary dipoles formed when electrons randomly cluster in one region of an atom/molecule, creating an instantaneous dipole that induces dipoles in neighboring molecules. Example: Forces between nonpolar molecules like CH₄ or noble gases like He.

Why do larger molecules have stronger London dispersion forces?

Larger molecules have more electrons, creating larger electron clouds that are more easily polarized. This leads to stronger temporary dipoles and greater attractive forces between molecules. Example: I₂ (solid) vs F₂ (gas) at room temperature.

What are the requirements for hydrogen bonding to occur?

Requirements: 1. Hydrogen must be covalently bonded to N, O, or F (highly electronegative atoms) 2. The H must interact with a lone pair on another N, O, or F atom Examples: H₂O, NH₃, HF can form hydrogen bonds.

Arrange these in order of increasing boiling point: CH₄, NH₃, H₂O. Explain your reasoning.

Order: CH₄ < NH₃ < H₂O Reasoning: CH₄ has only weak London forces. NH₃ can form hydrogen bonds (stronger). H₂O can form more hydrogen bonds (2 lone pairs on O) and has the strongest intermolecular forces, requiring the most energy to overcome.

State the five postulates of the Kinetic Molecular Theory (KMT).

1. Gas particle volume is negligible compared to container volume 2. No attractive forces between particles 3. Particles in constant random motion 4. Collisions are completely elastic 5. Average kinetic energy is directly proportional to absolute temperature

What does 'completely elastic collisions' mean in KMT?

Completely elastic collisions mean that no kinetic energy is lost during collisions between gas particles or with container walls. The total kinetic energy of the system remains constant, though individual particles may gain or lose energy.

According to KMT, what happens to molecular motion at absolute zero (0 K)?

At absolute zero (0 K), all molecular motion ceases according to KMT. This is because average kinetic energy is directly proportional to absolute temperature, so when T = 0 K, kinetic energy = 0, meaning no particle movement.

How does KMT explain gas pressure?

Gas pressure results from collisions of gas particles with container walls. More frequent and forceful collisions create higher pressure. Pressure depends on: 1. Number of particles (more particles = more collisions), 2. Temperature (higher T = faster particles = harder collisions), 3. Volume (smaller volume = more frequent collisions).

Calculate the average kinetic energy of gas molecules at 300 K using KE = (3/2)kT.

Given: T = 300 K, k = 1.38 × 10⁻²³ J/K Step 1: KE = (3/2)kT Step 2: KE = (3/2)(1.38 × 10⁻²³)(300) Step 3: KE = (1.5)(4.14 × 10⁻²¹) Answer: KE = 6.21 × 10⁻²¹ J per molecule

How do IMFA affect the validity of KMT for real gases?

Real gases deviate from KMT because: 1. Intermolecular forces exist (violates 'no attractive forces' postulate), 2. Particle volume becomes significant at high pressure (violates 'negligible volume' postulate). These effects are most noticeable at low temperature and high pressure.

Explain why polar molecules generally have higher boiling points than nonpolar molecules of similar size.

Polar molecules have dipole-dipole forces in addition to London dispersion forces, while nonpolar molecules only have London forces. The additional dipole-dipole attractions require more energy to overcome during boiling, resulting in higher boiling points.

Why does NH₃ have a higher boiling point than PH₃ despite similar molecular shapes?

NH₃ can form hydrogen bonds because N is highly electronegative, while PH₃ cannot form hydrogen bonds because P is less electronegative. Hydrogen bonding in NH₃ creates stronger intermolecular forces, requiring more energy to overcome, resulting in a higher boiling point.

Predict the molecular geometry of SF₄ (4 bonding pairs, 1 lone pair).

SF₄ has a seesaw (distorted tetrahedral) geometry. The lone pair occupies an equatorial position in the trigonal bipyramidal electron arrangement, causing distortion. Bond angles are less than 109.5° due to lone pair repulsion.

Why is ice less dense than liquid water?

In ice, water molecules form an ordered hexagonal crystal structure held together by hydrogen bonds, creating open spaces. In liquid water, molecules are closer together due to random motion. The open structure of ice makes it less dense, so ice floats on water.