1602 Exam 1

chapter 11 and 12 through 12.06

Solution

A mixture of two or more substances that is the same throughout

What happens when a solute (solid) dissolves in a solvent (liquid)

1. solute breaks apart (unfavorable)

2. solvent expanding (unfavorable)

3. solute and solvent interact (favorable and exothermic)

______ plays a role in determining whether a solute is soluble in a solvent

Enthalpy (ΔH)

Mixing occurs if solute-solvent interactions are ______ than the solute-solute and solvent-solvent interactions

stronger

The number of interactions after dissolution:

solute-solute

solvent-solvent

solute-solvent

less

less

more

Rule of thumb for dissolving

Like dissolves like (ie polar dissolves water, which is polar, and nonpolar wont)

Intermolecular forces (strong to weak)

ion-dipole

h-bond

dipole-dipole

ion-induced dipole

dipole-induced dipole

dispersion (london)

If ion-ion forces are stronger than ion-dipole why do things dissolve in water?

Entropy and mixing is always entropically favorable

Entropy (s)

“disorder”, how many different ways can you arrange the particles

Solubility

the amount of solute that can dissolve in a given amount of solvent

Saturated

everything dissolves but no more can be dissolved

Unsaturated

everything dissolves but more can be added

Oversaturated

not everything is dissolved and no more can dissolve (too much), dissolution and recrystallization occurs

Dynamic equilibrium in terms of solubility

rate of dissolution = rate of recrystallization

Supersaturated solution

Solution with more solute dissolved than is allowed (you need a way to fully dissolve more than is typically allowed)

Temperatures effect on solubility for solids and liquids

Solubility will increase with temp (more solvent KE)

Temperatures effect on solubility for gases

Solubility will decrease with increasing temp (more KE for gas means more will escape)

Effect of pressure on solubility of solids and liquids

Doesn’t change much (they aren’t compressible generally)

Effect of pressure on solubility of gases

gas solubility increases bc rate of condensation > rate of evaporation

Henry’s Law

S_gas=k_HP_gas

k_H in henrys law

depends on solute/solvent and temp (T)

Molarity (M)

mol solute/L of solution

molality (m)

mol solute/kg solvent

parts per million (ppm)

mass solute/mass solution x 10^6

parts per billion (ppb)

mass solute/mass solution x 10^9

Colligative properties depend on…

Number of dissolved particles in the solution but not the identity of the dissolved particles

Colligative properties are…

vapor pressure (lowering), freezing point (depression), osmotic pressure, boiling point (elevation)

ideal van’t hoff factor (applies to dissolving solutes)

i = moles of particles in solution/moles of formula units dissolved

Non-volatile solutes _____ the vapor pressure of a solvent

lower

______ substances readily evaporate whereas _______ substances remain in solution

volatile, non-volatile

Raoult’s Law

P_solvent= X_solventP⁰_solvent

Rate of evaporation of a solvent (P_vap,pure) is _____ the Rate of evaporation of solution (P_vap,soln)

greater than

X_solvent in Raoult’s law

X_solvent=n_water/n_total

where n_total = n_water + i⋅n_solute

For multi-component solutions all _____ components contribute to the total vapor pressure

Volatile

Raoult’s Law expanded

P_total=X_AP⁰_A+X_BP⁰_B

The strength of IMFs in an Ideal Solution are…

all roughly equal

If the A-B IFs are stronger than A-A or B-B IFs the deviation is…

Negative because the particles are less likely to escape and P_A and P_B will be lower

If the A-B IFs are weaker than A-A or B-B IFs the deviation is

Positive because the particles are more likely to escape and you have a higher P_A and P_B

When do liquids boil?

When P_vap= P_atm

_____ solutes lower the P_vap of a solvent

Non-volatile

Boiling point elevation equation

ΔT_b = i x m x K_b

where:

ΔT_b = increase in boiling point

i = van’t hoff

m = molality

K_b = constant for solvent

Freezing point depression equation

ΔT_f = i x m x K_f

where:

ΔT_f = decrease in freezing point

i = van’t hoff

m = molality

K_b = constant for solvent

Osmosis

The flow of solvent (water) through a semipermeable membrane (low solute to high solute)

Osmotic pressure

The pressure required to equalize liquid levels

Osmotic pressure equation

π = iMRT

where

π = pressure

i = van’t hoff

M = molarity (mol/L)

R = 0.08206 L⋅atm/mol⋅K

T = temp in K

For a tube with one side capped how do you find P_x

P_x = P_atm + π

What effect does ion pairing have on colligative properties?

It lowers the # of solute in the solution so the magnitude of change may not be as large as expected

Factors that change the amount of ion pairing

Concentration —> in higher concentrations we would expect more ion pairing

Magnitude of the charge (Ion charge) —> larger charges lead to more ion pairing since they won’t split as easily

Molecules can react when they _____ with one another with sufficient ______ and at the _______

collide, energy, proper orientation

Factors that affect reaction rate

1. Temperature —> higher temp = larger velocities = more collisions & more energetic

2. Concentration —> higher conc. = higher probability for collisions

3. Structure —> need to collide in correct orientation

4. Catalyst —> reduces the amnt of energy needed

Activation energy (E_A)

Minimum amount of energy needed to react

Is E_A,forward or E_A,reverse greater for an exothermic reaction?

E_A,reverse

General rate

How much a quantity changes in a given amount of time

rate = ΔX/Δt

Rate in terms of chemical reactions

rate = [A]_final - [A]_initial / Δt

• always positive

• coefficients become denominator of multiplier

  • coefficients on reactants are negative

Rate for general reaction aA + bB —> cC + dD

rate = -1/a (Δ[A]/Δt) = -1/b (Δ[B]/Δt) = 1/c (Δ[C]/Δt) = 1/d (Δ[D]/Δt)

Rate (often) depends on _______

Concentration —> as concentration decreases, fewer collisions happen = lower rate

Instantaneous rate

Rate at one specific point of time

Average rate

the mean rate of the instantaneous rates in the given time period

• rate = (final - beginning)/Δt

Rate equation

rate = K⋅[A]^m [B]ⁿ

• k = rate constant (changes with temp)

• n = reaction order (NOT stoich coefficients)

Zeroth order (rate law, units of k)

rate = k

M/s (molar per second)

First order (rate law, units of k)

rate = k⋅[A]¹

1/s or s^-1

Second order (rate law, units of k)

rate = k⋅[A]²

1/M⋅s or M^-1⋅s^-1

What would doubling [A] do in a zeroth, first, and second order

nothing, doubles, quadruples

Overall reaction order if

rate = k[A]^m[B]ⁿ

overall order = m + n

Equation for determining rate law when both change

r₂/r₁ = (k⋅[A₂]^m⋅[B₂]ⁿ)/(k⋅[A₁]^m⋅[B₁]ⁿ)

Steps to solving a rate law when both sides have a log

16 = 4^m

log(16) = log(4^m)

m = log(16)/log(4)

m=2

Integrated rate law for zeroth order

[A]_t= -k⋅t + [A]₀

Integrated rate law for first order

ln[A]_t = -k⋅t + ln[A]₀

or

ln([A]_t/[A]₀) = -k⋅t

Integrated rate law for second order

1/[A]_t = k⋅t + 1/[A]₀

Straight line plot for zeroth order

y-intercept = [A]₀

slope = -k

y-axis is [A]

• x-axis is always time

Straight line plot for first order

y-intercept = ln[A]₀

slope = -k

y-axis is ln[A]

• x-axis is always time

Straight line plot for second order

y-intercept = 1/[A]₀

slope = k

y-axis is 1/[A]

• x-axis is always time

Half-life expression for zeroth order

t_1/2 = [A]₀/2k

or

t_1/2 = 1/k⋅[A]₀/2

Half-life expression for first order

t_1/2 = 0.693/k

or

t_1/2 = 1/k⋅(0.693)

Half-life expression for second order

t_1/2 = 1/k[A]₀

t_1/2 = 1/k⋅1/[A]₀

Equation for percent of an initial concentration

[A]_t = (percent you want as decimal) x [A]₀