Exam 2: Chapter 11 and 12

If solvent-solute interactions > solvent-solvent and solute-solute interactions

ΔH_soln< 0 (exothermic)

Solution forms

If solvent-solute interactions = solvent-solvent and solute-solute interactions

ΔH_soln = 0

Solution forms

If solvent-solute interactions < solvent-solvent and solute-solute interactions

ΔH_soln > 0 (endothermic)

Solution may form

ΔH_soln=

break solvent-solvent interactions (endo) + break solute-solute interactions (endo) + form solute-solvent interactions (exo)

DRIVING FORCE IS ENTROPY

Entropy

measurement of matter or energy dispersal in a system

• more arrangements, more entropy

solubility

maximum amount of solute able to be dissolved in a solvent under given conditions

solubility = max amt of solute / amt solvent

unsaturated solution

concentration < solubility

rate dissolution > rate of recrystallization

saturated solution

concentration = solubility

rate dissolution = rate of recrystallization

oversaturated solution

concentration > solubility

rate dissolution < rate recrystallization

miscible

mix in any ratio (no solubility limit)

solubility: gases with gases

always miscible

solubility: liquids with liquids

like dissolves like (in terms of polarity)

concentration

amt solute / amt solution

• molarity: mol/L

• mass %: mass/mass x 100%

• mole fraction: moles/total moles

Energy diagrams for solution formation

Dissolution of Ionic and Covalent Compounds

Ionic: dissociate into individual ions which are surrounded by solvent

Covalent: dissolve as whole molecules (not breaking covalent bond)

electrolyte

ions in solution conduct electricity (produce ions when put in water)

ionic species and dissolves —> strong electrolyte

strong electrolyte

100% of products form

weak electrolyte

< 100% of products form

non-electrolyte

no products form (aka pathetic)

some molecular solutes form ions by reacting

Solution equilibrium

solubility curves for solids

solubility of a substance v. temperature

With Increased Temperature:

• increased rate of dissolution (by a lot)

• increased rate of recrystallization (by a smaller factor)

• so, solubility typically increases for solids at higher temperatures

temperature dependence of solubility of gases in water

increase of temperature, decrease of solubility

With Increased Temperature:

• increased rate of dissolution (by a smaller factor)

• increased rate of gas bubbling out (by a lot!)

• so, solubility typically decreases for gases at higher temperatures

impact of temperature on solubility of solids v. gas

pressure dependence of solubility of gases in water

(refers to the partial pressure of that gas above water: not dependent on pressures of other gases)

as pressure of that gas increases, solubility increases (more will end up dissolves in the water)

Henry’s Law

Concentration: molarity of gas in aqueous solution

P_gas: pressure of gas above liquid

molarity (M)

moles solute / L solution

molality (m)

moles solute / kg solvent

mole fraction (x)

moles solute / moles solution

(then multiply this by 100% for mole percent)

colligative properties

Nothing to do with the identity of the solute, only dependent on the amount

• Boiling Point T_b

• Freezing Point T_f

• Vapor Pressure (P_vap)

• Osmotic Pressure

Freezing Point Depression: ΔT_f=

i x m x k_f

i: Van’t Hoff Coefficient (for electrolytes), moles of particles formed in solution / moles solute added

m: molality of particles

k_f : freezing point constant (unique to the solvent)

Boiling Point Elevation: ΔT_b=

i x m x k_b

i: Van’t Hoff Coefficient (for electrolytes), moles of particles formed in solution / moles solute added

m: molality of particles

k_b : boiling point constant (unique to the solvent)

Equation for vapor pressure after dissolution of a compound (when solutes are nonvolatile)

P_{vap, soln} = x_solvP°_vap,solv

x_solv: mole fraction (aka how much of the surface is still solvent particles)

P°_vap,solv : vapor pressure of pure solvent

Equation for vapor pressure after dissolution of a compound (when solutes are volatile)

This means that evaporation of solute must be accounted for!

P_{vap, soln} = x_AP°_vap,A + x_BP°_vap,B

same as P_tot = P_A + P_B

Osmotic Pressure

Important Equations to Consider with Ch. 11 Questions

Equation for average rate of consumption or production of a substance, A

Δ[A] / Δt

NOT ON EQUATION SHEET: For aA + bB —> cC + dD, the Rate of the Reaction =

Rate= -1/a Rate A = -1/b Rate B = 1/c Rate C = 1/d Rate D

Rate of Reaction is NEVER regative

Higher concentration = ______ Rate

Higher Rate: since things have to collide for a reaction to occur and things are more likely to collide at higher concentrations

Differential rate laws tell us about

rate versus concentration

instantaneous rate (rate at some specific time)

Equation for differential rate law

Rate_rxn= k [A]^x[B]^y

k: rate constant

x: order of reaction with respect to [A]

y: order of reaction with respect to [B]

x and y are NOT determined by stoichiometric coefficients

Found using experimental data

units for k for zero order reaction

M/s

units for k for first order reaction

1/s

units for k for second order reaction

1/Ms

What do Avg Rate and Instantaneous Rate tell you vs. Integrated Rate Law?

Integrated Rate Law for Zeroth Order Reaction

Integrated Rate Law for First Order Reaction

NOT ON EQUATION SHEET: Alternate (Exponential) Form of Integrated Rate Law for First Order Reaction

Integrated Rate Law for Second Order Reaction

Calculate Half life (for first order reactions)

t_1/2= ln2 / k

or k = ln2 / t_1/2

Fundamental Relation for Half-Life

n: number of half lives

• Can calulate n by doing t / t_1/2

• aka time divided by how much time is considered one half life

[A]₀: initial concentration, you can choose if told substance has decayed by _%

A graph shows the correct order for a reaction if the data is _____

linear

• then slope can be used to determine k