Unit 3 - Properties of Substances & Mixtures

intermolecular forces

forces holding molecules together

• usually affect physical properties (boiling, melting point..)

• weaker than intramolecular

Intramolecular forces

forces holding molecule/ionic compound together

• covalent/ionic bonds

• usually affect reactivity

• stronger than intermolecular

Stronger the london dispersion forces the more/less polarizable

more

stronger a molecules intermolecular forces, higher/lower its boiling point

higher

factors that increase LDF’s

• high # of e-

• high molecular/molar mass

• size

• more polarizable

• higher bp (nonpolar molecules)

low bp = less/more polarizable = less/more e-

less, less

which has higher bp? polar or nonpolar

polar

because dipole dipole attractions in polar are stronger than just LDFs in nonpolar

Hydrogen bonding is a strong type of dipole dipole force found in molecules containing..

O-H, N-H, or F-H

because the molecules bonding with H are extremely electronegative

Why do polar molecules with F, O, N usually dissolve in water?

they can hydrogen bond with water itself

NH3 (g) dissolves in water but PH3 (g) does not… why?

because central atom N is both small and very electronegative causing hydrogen bonds strong attraction to water, but P central atom is larger and less electronegative

what is the strongest force?

ionic forces

so they have the highest mp and bp

as temp increases vapor pressure of a liq ..

increases

temp increases evaporation anyway but when you have a closed lid theres more pressure since container is full of gas

(think of closing the lid on a cooking pot to boil water)

liquids with weak ____ will have higher vapor pressures

weak intermolecular forces

Covalent network solids

extremely strong structure (stronger than ionic compounds) made from REPEATING network of covalent bonds

• extremely high mp and bp

• strong cov bonds in multiple directions

Ex: diamond, graphite (pure C), silicon dioxide SiO2 (quartz, sand), silicon carbide SiC

Boyles Law, when to use?

pressure of gas inversely proportional to its volume

P1V1=P2V2

use when temp and moles constant

Charles Law, when to use?

The volume of gas is directly proportional to its temp

V1/T1 = V2/T2

temp in kelvin!!

use when pressure and moles constant

Gay Lussac’s Law, when to use?

The pressure of gas directly proportional to its temp

P1/T1 = P2/T2

t is kelvin!!

use when volume and moles constant

Combined Gas Law

P1V1/T1 = P2V2/T2

use when moles are constant

Ideal Gas Law, when to use?

PV=nRT

use when working with

• finding/using moles

• mass→ moles conversions

• V,P,T all important or given

• nothing is changing

Dalton’s Law of Partial Pressures

Partial pressure_Gas = X_Gas * P_Total

_{X is the mole fraction!}

(When you have grams in question convert to moles first, then do the formula)

Grahams Law of Effusion, when to use?

Rate of movement of gas particles inversely proportional to their molecular mass

lighter gases move faster, heavier gases slower

use when comparing speed of 2 gases based on molar masses thru effusion/diffusion

Effusion

the escaping of molecules thru a very tiny hole in a material

Properties of an ideal gas

• has molecules that take up no space (no volume)

• has molecules with no intermolecular attractions for each other

this shit don’t exist in real world!

What kind of gases can act like ideal gases?

very small molecules such as He, H₂ and Ne that can take up very little space

• high temp

• low pressure

solvent

medium into which solute is dissolved

water is almost always the solvent

solvent

substance that gets dissolved into a medium

What is a dilute solution?

solution that has lo amnt of solute

opposite of concentrated

What is a saturated solution?

concentrated solutions that dissolved the max amnt of solute

Ionic compounds are strong electrolytes so when dissolved in water they..

dissociate completely into their component ions (can be element ions and/or polyatomic ions)

Like dissolves like

polar molecules like NH3 and C2H5OH dissolve easily in a polar solvent like H2O

nonpolar like CH4 or C6H6 won’t dissolve in H2O!

nonpolar molecules like CH4 and C6H6 dissolve easily in nonpolar solvent like CCl4

solubility of gas vs temp

as temp increases solubility decreases (inverse)

How to determine concentration of a solution using spectrophotometry? (answer in law or formula)

Beer-Lambert Law A = εbc

A= absorbance (between 0-1)

ε= molar absorptivity (constant measuring how well substance absorbs light)

b= path length in cm (width of cuvette, almost always 1cm)

c= concentration (mol/L or molarity)

relationship between absorbance and concentration

directly proportional

since molar obsorptivity and path length are constant

if theres fingerprints on the holder holding a concentration of red dye solution.. how will that affect the absorbance results?

it’s covering some spots so it’s gonna look darker (more concentrated) thus we will think more absorbance (than it actually is..)