Chemistry IMFs and Chromatography

What is VSEPR theory?

Valence Shell Electron Pair Repulsion theory states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion.

Why does BF₃ have a trigonal planar shape?

BF₃ has 3 areas of electron density around the central boron atom. These repel as far apart as possible, giving a trigonal planar shape with 120° bond angles.

Why does NF₃ have a pyramidal shape instead of trigonal planar?

NF₃ has 3 bonding pairs and 1 lone pair (4 areas of electron density). The lone pair repels more strongly than bonding pairs, pushing the bonding pairs closer together and resulting in a pyramidal shape.

What shapes do the following molecules have: CH₄, NH₃, H₂O?

CH₄ = tetrahedral (4 bonding pairs), NH₃ = pyramidal (3 bonding + 1 lone pair), H₂O = bent/V-shaped (2 bonding + 2 lone pairs)

What makes a molecule polar?

A molecule is polar if it has: 1) Polar bonds (due to electronegativity differences) AND 2) An asymmetrical shape so that bond dipoles don't cancel out.

Why is SO₂ polar but SO₃ non-polar?

Both have polar S-O bonds, but SO₃ is symmetrical with no lone pairs on sulfur, so dipoles cancel out (non-polar). SO₂ has a lone pair and is non-symmetrical, so it remains polar.

Why is CO₂ non-polar despite having polar C=O bonds?

CO₂ has a linear shape with no lone pairs on carbon, making it symmetrical. The two polar C=O bonds point in opposite directions and cancel each other out.

Is HCl polar or non-polar? Explain.

HCl is polar because chlorine is much more electronegative than hydrogen, creating a dipole. The molecule is also linear and asymmetrical, so the dipole doesn't cancel.

What are the three types of intermolecular forces in order of increasing strength?

Dispersion forces < Dipole-dipole forces < Hydrogen bonding

What are dispersion forces and when do they occur?

The weakest intermolecular forces caused by temporary dipoles from random electron movement. They occur in all molecules but are the only force in non-polar molecules.

What is hydrogen bonding and when does it occur?

A strong type of dipole-dipole interaction that occurs between hydrogen and highly electronegative atoms (F, O, N). It's stronger than regular dipole-dipole forces.

What intermolecular forces are present in water?

Dispersion forces, dipole-dipole forces, and hydrogen bonding (the strongest possible combination).

What intermolecular forces are present in octane (C₈H₁₈)?

Only dispersion forces, because octane is a non-polar molecule.

What intermolecular forces are present in ethanol?

Dispersion forces, dipole-dipole forces, and hydrogen bonding (due to the O-H group).

Why does ammonia have a higher boiling point than other Group 15 hydrides?

Ammonia can form hydrogen bonds (H-N) in addition to dipole-dipole and dispersion forces, making the total intermolecular forces stronger and requiring more energy to break.

Why do Group 14 hydride boiling points increase steadily down the group?

All Group 14 hydrides are non-polar (only dispersion forces). Going down the group, molecules get larger with more electrons, so dispersion forces get stronger, requiring more energy to overcome.

Why does HF have a higher boiling point than HBr?

HF can form hydrogen bonds, dipole-dipole forces, and dispersion forces, while HBr can only form dipole-dipole and dispersion forces. The hydrogen bonding makes HF's intermolecular forces stronger.

Why does I₂ have a higher boiling point than F₂?

Both are non-polar (dispersion forces only), but I₂ is much larger with more electrons, creating stronger dispersion forces that require more energy to overcome.

How can I₂ have a higher boiling point than water?

Despite water having hydrogen bonding, I₂ is so large that its dispersion forces become stronger than water's combined intermolecular forces.

Why does water have unusually high melting and boiling points compared to similar-sized molecules?

Water's bent molecular shape and hydrogen bonding create a network of strong intermolecular forces. The combination of hydrogen bonds, dipole-dipole interactions, and dispersion forces requires significant energy to break.

Why is ice less dense than liquid water?

In ice, hydrogen bonds form a rigid hexagonal lattice structure that holds molecules further apart than in liquid water, where bonds constantly break and reform allowing closer packing.

Why does water have high surface tension?

Water molecules at the surface are pulled inward by strong cohesive hydrogen bonds from molecules below and beside them, creating a "skin" effect. This cohesive force is much stronger than in other liquids due to hydrogen bonding.

What gives water its high specific heat capacity?

Hydrogen bonds between water molecules must be broken and reformed as temperature changes. This requires extra energy, meaning water can absorb or release large amounts of heat with relatively small temperature changes.

Why is water an excellent solvent for polar substances?

Water's polarity and ability to form hydrogen bonds allows it to surround and separate ionic compounds and dissolve polar molecules by forming similar-strength intermolecular forces with the solute.

What is cohesion in water and why does it occur?

Cohesion is the attraction between water molecules due to hydrogen bonding. It's responsible for water's high surface tension and allows water to form droplets.

What is adhesion in water?

Adhesion is water's ability to stick to other polar surfaces due to hydrogen bonding and dipole interactions. This enables capillary action and helps water climb up plant stems.

Why does water reach maximum density at 4°C?

Between 0-4°C, breaking hydrogen bonds allows molecules to pack more closely (increasing density). Above 4°C, thermal expansion dominates (decreasing density). At 4°C, these effects balance for maximum density.

What is desalination and why is it important?

The process of removing salt and minerals from seawater to produce fresh water, using techniques like reverse osmosis or distillation. It's crucial for water supply in arid regions.

What is chlorination and why is it used in water treatment?

Adding chlorine compounds to water to kill bacteria, viruses, and pathogens, preventing waterborne diseases. Uses chlorine gas, sodium hypochlorite, or calcium hypochlorite.

What is fluoridation and what are the benefits?

Adding fluoride compounds (like sodium fluoride) to drinking water to help prevent tooth decay. It significantly reduces dental caries in the population by strengthening tooth enamel.

What is reverse osmosis and how does it work in desalination?

A process that uses pressure to force water through a semi-permeable membrane that blocks salt and other contaminants. The membrane allows water molecules through but rejects larger ions and molecules.

Why is monitoring heavy metal contamination in groundwater important?

Heavy metals like lead, mercury, and cadmium are toxic even at low concentrations and can cause serious health problems including neurological damage, kidney disease, and cancer.

What factors influence the choice of water treatment methods?

Economic factors (cost of equipment and operation), social factors (community acceptance and infrastructure), effectiveness for specific contaminants, and environmental impact of the treatment process.

According to kinetic theory, what happens to gas particles when temperature increases?

Particles gain kinetic energy, move faster, and collide with container walls more frequently and with greater force, increasing pressure.

Why are aerosol cans dangerous when heated?

Higher temperature increases particle kinetic energy, causing more frequent and forceful collisions with the can walls, potentially causing the can to explode.

What happens to a helium balloon as it rises in the atmosphere?

External atmospheric pressure decreases, so the balloon expands (internal pressure > external pressure). Eventually the balloon bursts when it expands beyond its elastic limit.

What happens to tire pressure when a tire heats up during use?

The pressure increases because higher temperature gives air particles more kinetic energy, causing more frequent and forceful collisions with the tire walls.

What volume does 1 mole of any gas occupy at STP?

22.71 L (this is the molar volume of gases at Standard Temperature and Pressure)

How do you calculate the volume of a gas at STP?

V = n × 22.71 L, where n is the number of moles and 22.71 L is the molar volume at STP.

How do you calculate moles from mass?

n = m/Mr, where n = moles, m = mass in grams, and Mr = molar mass in g/mol

How do you calculate molar mass if you know mass and volume at STP?

First find moles: n = V/22.71, then Mr = m/n

In stoichiometry problems, how do mole ratios relate to volume ratios at STP?

At STP, mole ratios from balanced equations can be used directly as volume ratios because all gases have the same molar volume.

What volume does 1.34g of methane (CH₄) occupy at STP?

Mr(CH₄) = 16.042 g/mol; n = 1.34/16.042 = 0.0835 mol; V = 0.0835 × 22.71 = 1.90 L

If 4.18g of a gas occupies 1.00L at STP, what is its molar mass?

n = 1.00/22.71 = 0.0440 mol; Mr = 4.18/0.0440 = 95.0 g/mol

If 4.60g of sodium reacts with water, what volume of H₂ is produced at STP?

2Na + 2H₂O → 2NaOH + H₂; n(Na) = 4.6/22.99 = 0.200 mol; n(H₂) = 0.200/2 = 0.100 mol; V(H₂) = 0.100 × 22.71 = 2.27 L

Why is glucose highly soluble in water but octane is not?

Glucose can form hydrogen bonds with water (similar strength forces), while octane can only form weak dispersion forces with water, which aren't strong enough to overcome water's hydrogen bonding network.

A substance is highly soluble in water but not in hexane. What can you infer about its polarity?

The substance is likely polar and can form hydrogen bonds or strong dipole-dipole interactions with water, but only weak dispersion forces with non-polar hexane.

What is the general rule for predicting solubility?

"Like dissolves like" - polar substances dissolve in polar solvents, non-polar substances dissolve in non-polar solvents. This is because similar intermolecular forces allow effective mixing.

Why are ionic compounds generally soluble in water?

Water molecules can surround and separate ions through ion-dipole interactions. The energy released from these new interactions compensates for breaking the ionic lattice and hydrogen bonds in water.

What is electronegativity?

The ability of an atom to attract shared electrons in a chemical bond. Fluorine is most electronegative, electronegativity generally decreases down groups and increases across periods.

What is a covalent bond?

A chemical bond formed by the sharing of electron pairs between atoms, allowing each atom to achieve a stable electron configuration similar to noble gases.

What is a polar covalent bond?

A covalent bond where electrons are unequally shared due to electronegativity differences, creating partial positive and negative charges on the atoms.

What does "bent" or "V-shaped" mean in molecular geometry?

A molecular shape where the central atom has two bonding pairs and at least one lone pair, creating a bent appearance with bond angles less than 109.5° (like in water: ~104.5°).

What is thermal expansion?

The tendency of matter to increase in volume when temperature increases, due to particles moving more vigorously and requiring more space.

Why is fluoride added to drinking water controversial in some regions?

While proven to prevent tooth decay, some communities debate optimal levels due to concerns about dental fluorosis (too much fluoride) or potential health effects, though scientific consensus supports water fluoridation as safe and effective.

What are the main sources of water contamination?

Industrial waste, agricultural runoff (pesticides, fertilizers), sewage, heavy metals from mining, naturally occurring minerals, and microbial pathogens from animal waste.

Why is lead particularly dangerous in drinking water?

Lead accumulates in the body over time, causing neurological damage especially in children, developmental delays, learning difficulties, and various health problems even at low concentrations.

What is hard water and why does it matter?

Water with high concentrations of dissolved minerals (mainly Ca²⁺ and Mg²⁺). It reduces soap effectiveness, causes scale buildup in pipes, but may have some health benefits from mineral content.

Why do different halogens (F₂, Cl₂, Br₂, I₂) exist in different states at room temperature?

All are non-polar (dispersion forces only). Size increases down the group: F₂ < Cl₂ < Br₂ < I₂. Larger molecules have stronger dispersion forces, requiring more energy to overcome, leading to higher melting/boiling points.

What determines whether a substance is a gas, liquid, or solid at room temperature?

The strength of intermolecular forces relative to thermal energy. Weak IMF = gas, moderate IMF = liquid, strong IMF = solid. Temperature provides energy to overcome these forces.

Why does vapor pressure increase with temperature?

Higher temperature gives more molecules enough kinetic energy to escape from the liquid surface into the gas phase, increasing the number of molecules in the vapor phase.

What is paper chromatography and how does it work?

A separation technique using paper as the stationary phase and a liquid solvent as the mobile phase. Components separate based on their different solubilities in the solvent and attraction to the paper.

What is thin layer chromatography (TLC)?

Uses a thin layer of adsorbent material (like silica gel) coated on a glass, metal, or plastic plate as the stationary phase. More sensitive and faster than paper chromatography.

What is gas chromatography (GC)?

Uses an inert gas (like helium) as the mobile phase and a liquid or solid coating inside a column as the stationary phase. Separates volatile compounds that can be vaporized without decomposition.

What is high performance liquid chromatography (HPLC)?

Uses high pressure to force a liquid mobile phase through a column packed with fine particles as the stationary phase. Provides excellent separation and is widely used in analytical chemistry.

What principle do all chromatography methods rely on?

Different components in a mixture have different affinities for the stationary phase versus the mobile phase, causing them to move at different rates and separate.

How do you calculate Rf values in paper/TLC chromatography?

Rf = distance traveled by compound / distance traveled by solvent front. Values range from 0-1, with higher values indicating greater solubility in the mobile phase.

What factors affect separation in chromatography?

Polarity of compounds, choice of mobile phase (solvent), nature of stationary phase, temperature, and flow rate of mobile phase.

Why might you choose GC over HPLC?

GC is better for volatile, thermally stable compounds and provides excellent separation. HPLC is better for non-volatile, thermally labile, or ionic compounds.

How do you predict relative boiling points of similar molecules?

Compare intermolecular forces: hydrogen bonding > dipole-dipole > dispersion. For same IMF types, larger/more electrons = stronger forces = higher boiling point.

List HF, HI, HCl, HBr in order of increasing boiling point.

HI < HBr < HCl < HF. First three increase due to increasing molecular size/dispersion forces. HF is highest due to hydrogen bonding.

How does molecular shape affect polarity?

Even with polar bonds, if the molecule is symmetrical, the dipoles cancel out making it non-polar overall (like CO₂, BF₃). Asymmetrical molecules with polar bonds remain polar (like H₂O, NH₃).

What's the relationship between intermolecular forces and solubility?

For dissolution to occur, the new intermolecular forces formed between solute and solvent must be similar in strength to the original forces being broken in both pure substances.