UPCAT CHEM - COMPUNDS 1

What shape has 2 bonding pairs, 0 lone pairs?

Linear — 180°. All bonds point in exactly opposite directions. Dipoles cancel → nonpolar. Example: CO2, BeCl2

What shape has 3 bonding pairs, 0 lone pairs?

Trigonal planar — 120°. Flat, like a Mercedes logo. All bonds in same plane. Dipoles cancel → nonpolar. Example: BF3

What shape has 4 bonding pairs, 0 lone pairs?

Tetrahedral — 109.5°. 3D shape, bonds spread equally in all directions. Dipoles cancel → nonpolar. Example: CH4, NH4+

What shape has 3 bonding pairs, 1 lone pair?

Trigonal pyramidal — ~107°. Like a 3-legged pyramid. Lone pair pushes bonds downward and closer together. Asymmetrical → polar. Example: NH3, PCl3

What shape has 2 bonding pairs, 2 lone pairs?

Bent / V-shaped — ~104.5°. Two lone pairs push bonds even closer together. Asymmetrical → polar. Example: H2O, SO2

Why do lone pairs decrease bond angles?

Lone pairs are attracted to only ONE nucleus (vs bonding pairs attracted to two). So lone pairs spread out more and push bonding pairs closer together, decreasing the angle.

What is a polar molecule?

Central atom HAS lone pairs OR surrounding atoms are different → dipoles do NOT cancel → net dipole exists. Examples: H2O, NH3, HCl

What is a nonpolar molecule?

Central atom has NO lone pairs AND all surrounding atoms are the same → dipoles cancel perfectly → no net dipole. Examples: CO2, CH4, BF3. Note: CO2 has polar bonds but is nonpolar because C has no lone pairs and both O are identical.

What is a dipole moment?

An arrow showing which direction electrons are being pulled toward the more electronegative atom. Net dipole = sum of all arrows. If arrows cancel = nonpolar. If arrows don't cancel = polar.

What is London Dispersion Force (LDF)?

Weakest IMF. Caused by random movement of electrons creating temporary/instantaneous dipoles that induce dipoles in neighboring atoms. Present in ALL molecules. Most significant for nonpolar molecules since it's their only IMF. Also called Van der Waals.

What is Dipole-Dipole Force (DDF)?

IMF between polar molecules. Permanent dipoles attract each other (+ end attracted to - end). Stronger than LDF. Also called Van der Waals.

What are Van der Waals forces?

Umbrella term covering both London Dispersion Force AND Dipole-Dipole Force. Does NOT include hydrogen bonding or ion-dipole.

What is Hydrogen Bonding?

Strong IMF where H bonded to F, O, or N is attracted to lone pairs on F, O, or N of a neighboring molecule. F/O/N pull H's electron so strongly H becomes almost a bare proton — strongly attracted to nearby lone pairs. Note: H2S also shows hydrogen bonding.

What is Ion-Dipole Force?

Strongest IMF. Attraction between a full ion (like Na+) and a polar molecule's dipole. Example: NaCl dissolving in water.

What is the IMF strength order?

London Dispersion < Dipole-Dipole < Hydrogen Bonding < Ion-Dipole

What is Dipole-Induced Dipole force?

IMF between a polar molecule and a nonpolar molecule. The permanent dipole of the polar molecule induces a temporary dipole in the nonpolar molecule → they attract.

Effect of HIGH IMF on boiling point

HIGH boiling point — more energy needed to overcome strong intermolecular attractions and change phase

Effect of HIGH IMF on melting point

HIGH melting point — same reason as boiling point

Effect of HIGH IMF on vapor pressure

LOW vapor pressure — molecules can't easily escape the liquid phase

Effect of HIGH IMF on viscosity

HIGH viscosity — strong attractions between molecules resist flow

Effect of HIGH IMF on surface tension

HIGH surface tension — molecules pull inward more strongly

Effect of HIGH IMF on heat capacity

HIGH heat capacity — more energy needed to raise temperature by 1°C

Which has highest boiling point: CH4, HCl, H2O, NH3?

H2O — has hydrogen bonding (strongest IMF among these). Order: H2O > NH3 > HCl > CH4

How do you name a binary acid (H + halogen, no oxygen)?

hydro + (halogen root) + ic acid. Examples: HCl = hydrochloric acid, HBr = hydrobromic acid, HF = hydrofluoric acid. Only use this name when dissolved in water (aqueous). In gas form = regular compound name (e.g. hydrogen chloride).

How do you name an oxyacid whose anion ends in -ate?

Change -ate to -ic acid. Examples: SO4(2-) = sulfate → H2SO4 = sulfuric acid. NO3- = nitrate → HNO3 = nitric acid.

How do you name an oxyacid whose anion ends in -ite?

Change -ite to -ous acid. Examples: SO3(2-) = sulfite → H2SO3 = sulfurous acid. NO2- = nitrite → HNO2 = nitrous acid.

How do you know if you're naming a binary acid vs a regular compound?

Binary acid: formula starts with H + halogen AND context says aqueous/acid. Regular compound: same formula in gas/pure form. H + oxygen-containing anion = always an oxyacid regardless of state.