CHM2046 Exam 2

acid-base buffer

solution that lessens the impact of pH from the addition of acid or base, consists of conjugate acid-base pair

what is an acid-base buffer a solution of

a weak acid and its conjugate base, or a weak base and its conjugate acid

what is the effect that a buffer works through

the common-ion effect

common-ion effect function

addition of a salt provides source of its constituent ion and the equilibrium will shift to the side without that ion, would reduce percent dissociation of acid

what are the common kinds of buffer solutions

weak acid + soluble ionic salt of weak acid

weak base + soluble ionic salt of weak base

eg. CH3COOH and NaCH3COOH

buffer capacity

measure of the “strength” of the buffer, its ability to maintain the pH following addition of strong acid or base

what makes a buffer more effective

the closer the component concentrations are to each other.

The closer [HA]/[A-] is to 1

when there are greater concentrations of the buffer components, what happens to a buffer

its capacity to resist pH change is greater

buffer range

the pH range over which the buffer is effective

if the concentration of one component is more than 10x the concentration of the other, how is the buffering action

POOR BOOOOO

buffers have a usable range within what

±1 pH unit of the pKa of the acid component

buffers will lose their ability to keep pH constant if what

too much acid or base is added (above its capacity)

how to choose conjugate acid-base pair for preparing a buffer

the pKa of the weak acid component should be close to the desired pH, or the pKb of weak base component

how to prepare a buffer

choose conjugate acid-base pair

calculate ratio of buffer component concentrations

determine buffer conc and calc the required volume of stock solutions and/or masses of components

mix solution and correct the pH by adding strong acid/base

how is the concentration of an acid or base determined in an acid-base titration

by neutralizing the acid or base w a solution of base or acid of known concentrations, uses a plot of pH vs volume of titrant added

equivalence point

occurs when the # of moles of OH- added equals the # of moles of H3O+ originally present, or vice versa

end point

occurs when the indicator changes color

how should an indicator be selected

so that its color change occurs at a pH close to that of the equivalence point

what is the pH at the equivalence point for a strong acid strong base titration

7.00

when pH = pKa, what

acid and conjugate base concentration are equal

what is the half equivalence point

when exactly half the acid has been consumed by the acid

what are the mol at equivalence point of strong base with weak acid

mol of strong base added equals the moles of of initial weak acid

what does the solution contain at equivalence point of a weak acid and strong base

water and the salt of the weak acids conjugate base, cannot have a pH of 7.00

what is the general formula of amino acids that act as polyprotic acids

NH2—CH(R)—COOH, where R can be 1 of 20 amino acid groups

what do amino acids as polyprotic acids consist of

weak base (—NH2) and weak acid (COOH) on the same molecule

how do the amino group and carboxylic acid group act on an amino acid and low pH

protonated, thus in this form behaves like a polyprotic acid

protonated amino acid

at low pH, amino and acid group both have attached protons

zwitter ion

at neutral pH, only amino group has proton attached

unprotonated

at high pH, there are no protons on the amino group nor the acid group

what does the graph of a titration of weak base with weak acid look like

very short vertical sections, solution is buffered before and after equivalence point

what is an acid-base indicator

a weak organic acid (HIn) whose color differs from that of its conjugate base (In-)

how do indicators work

the ratio of [HIn]/[In-] is governed by the [H3O+] of the solution, thus indicators can be used to monitor pH change during acid base reaction

most indicators cover what pH

a specific, narrow pH range, usually about 2 pH values

what does the graph of the titration of a weak polyprotic acid look like

looks like titration of weak acid w strong base but multiple put together (multiple plateaus)

“insoluble” ionic compounds are ACTUALLY what, and what do we assume

slightly soluble in aqueous solutions! we assume that the very small amt that dissolves will dissociate completely

for a slightly soluble ionic compound in water, equilibrium exists between what

solid solute and aqueous ions

Q_sp

the ion product expression for a slightly soluble ionic compound

Q_sp = K_sp when?

when the solution is saturate and the system is at equilibrium

solubility product constant

Ksp

what does the Ksp value of a salt indicate

how far the dissolution proceeds at equilibrium (saturation)

what are requirements to compare Ksp values

they have the same total number of ions in their formulas, ratio of ions doesnt matter

the higher the Ksp, what happens to solubility

greater the solubility

if ion ratios are different how do you compare solubilities

calculate molar solubility

effect of pH on solubility

the addition of H3O+ will increase the solubility of a salt that contains the anion of a weak acid

what is used to calculate the concentration of ions in a solution and whether or not a precipitate will form

the reaction quotient (Q) and the Ksp of a compound

saturated solution also mean

solution at equilibrium

if Qsp = Ksp

solution is saturated and no change will occur

if Qsp < Ksp

no precipitate will form because the solution is unsaturated

if Qsp > Ksp

a precipitate will form until the remaining solution is saturated

selective precipitation is what and used for what

the process of removing one ion but not the other, used to separate a solution containing a mixture of ions

precipitating ion

what is added to solution in order to cause Qsp of more soluble compound almost equal to its Ksp

what happens to the less soluble compound when selectively precipitating

it will precipitate in as large a quantity as possible, leaving behind the ion of the more soluble compound

what should you factor in when comparing Ksp in selective precipitation

the molar ratio

complex ion

consists of a central metal ion covalently bonded to two or more anions of molecules called ligands

metal ions act as what

good lewis acids

ligands

molecules or ions that bind to metal ions, lewis bases that form coordinate covalent bonds w the metal

all complex ions are what

lewis adducts

metal ions and ligands exist in what

equilibrium at Kf, formation constant

how do complex ions form

salt dissolves in water and water surrounds metal ion, then water can be replace by another ligand

how do ligands affect solubility

a ligand increases the solubility of a slightly soluble ionic compound if it forms a complex ion w the cation

how to get overall K of reaction of complex ions

multiply Ksp and Kf

what happens when solid Al(OH3) is treated with H3O+ or OH-

it dissolved as a result of the formation of soluble complex ions

key to separation step in the production of aluminum metal

the difference in solubility in base between aluminum hydroxide and iron III hydroxide

spontaneous change

a change that occurs without a continuous input of energy from outside the system and only occur in the direction that leads them to equilibrium

what do all chemical processes require to take place, but what if diff abt spontaneous reactions

energy, thus activiation energy; once a spontaneous process has begun, no further input of energy is needed

nonspontaneous reaction

occurs only if the surroundings continuously supply energy to the system

if a change is spontaneous in one direction, it will be

nonspontaneous in the reverse direction

1st law of thermodynamics

energy is conserved— neither created nor destroyed, but is transferred in the form of heat and/or work

total energy of universe is what

constant

what does 1st law apply to and what does it not do

applies to ALL changes, and does not allow us to predict the direction of a spontaneous change

delta H does not predict what

if a reaction is spontaneous or nonspontaneous

two factors that affect spontaneity of any physical or chemical change

1. spontaneity is favored when heat is released during the change (delta H is exothermic (negative)

2. spontaneity is favored when the change causes an increase in the dispersal of energy and matter (S= entropy increased (positive))

what do spontaneous endothermic processes result in

an increase in freedom of motion of the particles in the system, disorder

increase of entropy is shown by

solid→ liquid → gas

crystalline solid + liquid → ions in solution

localized energy of motion → dispersed energy of motion

A system with fewer microstates has what entropy

lower

a system with more microstates has what entropy

higher

entropy is a state function therefore

it is independent of the path taken between the final and initial states

microstate

each quantized energy state for a system of particles, the allowed energy state

at a given set of conditions, each microstate has the same

total energy as any other, and is therefore equally likely

the larger the number of possible microstates…

the larger the number of ways in which a system can disperse its energy

gas spontaneously expands in a container, what does increasing the volume do

increases the number of translational energy levels the particles can occupy → aka in english, increases the number of microstates and entropy

reversible process

a process that occurs in such tiny increments that the system remains at equilibrium and the direction of the change can be reverse by an infintesimal reversal of conditions

Trouton’s Rule

most liquids have a delta S vap value close to 88 J/mol k

what does the sign of delta s not predict by itself

the direction of a spontaneous reaction

all real process occur spontaneously in the direction that does what

increases entropy of the universe

ΔS_univ​ > 0

the process is spontaneous

ΔS_univ < 0

process is non-spontaneous

ΔS_univ​ = 0

process is at equilibrium

for enthalpy there is no zero point, so instead we measure

changes in enthalpy

for entropy there is a zero point so we can determine

absolute entropy values

enthalpy

describes the total heat content of a particular system

entropy

describes the disorderliness of a system

a perfect crystal has what at absolute zero

zero entropy

what is a perfect crystal

has flawless alignment of all of its particles, at absolute 0 the particles have minimum energy, so there is only one microstate

how to find the entropy of a substance at a given temperature

ΔS = nCv ln(T1/T2)

S°

standard molar entropy of a substance, measures in its standard state of J/mol k

conventions for defining a standard state

1 atm for gases, 1 M for solutions, pure substance in its most stable form for solids and liquids

what factors affect entropy

temperature, physical state of a substance, the formation of a solution, atomic size and molecular complexity

for any substance how does S° change with temperature

increases as temperature increases

with increasing temperature, what happens to energy

increases the total energy (area under the curve) and the range of occupied energy levels and microstates increase