Chapter 17: Water and Aqueous Systems

Intermolecular forces (IMF)

attractive forces between molecules

Intramolecular forces

bonds between individual elements

Dipole-Dipole forces

occurs when a polar molecule (a molecule with a permanent partial charge on one side) attracts to the partial charge of another

• charges must be opposite

• between molecules

Hydrogen bonds

• type of dipole-dipole force (strongest one)

• occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine

• between molecules, when a partial charge attracts another partial charge

London dispersion (LD) forces

• main IMFs in nonpolar molecules

• LD increases by size

  • molecules with a greater surface area exposed are more polarizable, allowing for greater LD forces

• ALL SUBSTANCES have LD forces

• cause: polarizability of molecule

• very weak force because it is not a permanent dipole, but rather a temporary dipole

Polarizability

• how easy it is to cause an uneven distribution of electrons

• larger molecules contain a larger electron cloud and are therefore more polarizable

Permanent vs. Temporary dipole

• Permanent: occurs in a polar molecule due to difference in electronegativity

• Temporary: uneven distribution of electrons (charge) by chance, which transfers to neighboring molecules and later fades away

What IMFs does water have?

hydrogen bonds, LD forces

What is the main IMF in water?

hydrogen bonds

Hydrocarbons are ________:

nonpolar

• LD is dominant

• oils, fats, greases are nonpolar

Organic compounds with C and O are ______:

polar

• Ex: C₆H₁₂O₆ is polar

• they easily dissolve in water because like dissolves like

• however they do not dissociate because they are not ions

  • van’t hoff factor is 1

Unique Properties of Water

• Normal BP of water is 100 degrees C, and normal FP is 0 degrees C

  • basically a high BP and low FP

  • it takes a lot of energy to break hydrogen bonds between water molecules

• Low vapor pressure

  • water doesn’t vaporize easily, seen with high BP, due to strong IMFs (hydrogen bonds)

  • the more stronger the IMFs, the lower the vapor pressure

• High specific heat

  • it takes 4.18 J/gc to break a hydrogen bond

• High surface tension

  • strong IMFs lead to high surface tension because the molecules in liquid state bond to each other but not the air —> skin-like layer

  • minimizes surface area if high because of inward force

• Low density of ice

  • water behaves normally (V decreases with decreasing temperature) until 4 degrees C

  • after 4 degrees C, water forms open frame honeycomb structure due to hydrogen bonds

  • because density = mass/volume, if volume increases then density decreases

Solute

stuff that is dissolved

Solvent

stuff that solute is being dissolved into

LIKE dissolves LIKE

• nonpolar dissolves nonpolar substance

• polar dissolves polar substance

Soap can act as a/an:

• surfactant

• emulsifying agent

Surfactant

breaks the hydrogen bonds and makes beads of water spread out

Emulsifying agent

makes two substances that generally don’t dissolve, dissolve

• Ex: dissolves oil and water

• acts as a “bridge”

Sodium Stearate

C₁₇H₃₅COONa

Miscible

dissolve in each other normally (follows like dissolves like)

• Ex: H₂O + CH₃OH

Immiscible

don’t dissolve in each other normally

• Ex: water and oil

Electrolyte

• can conduct electricity when dissolved in water (aqueous) or molten

• strength of electrolyte is determined by degree of dissociation

• to be an electrolyte it must be able to be broken down into its ions

• Strong: NaCl_(aq) (dissociates fully)

• Weak: HgCl₂ (only dissociates partially)

Exceptions to definition of an electrolyte

• while molecular compounds do no conduct electricity (because they don’t dissociate into ions)

• acids are electrolytes

  • Strong: HCl (dissociates fully)

  • Weak: HC₂H₃O₂ (dissociates partially)

hydrate —→

hydrate —→ anhydrate/anhydrous + water of hydration