Chem 327 Exam 2

Front 1

define equilibrium

Back 1

The rates of the foward and reverse reactions are equal

Front 2

What is the equilibrium constant of a favorable (product favored) reaction?

Back 2

K>1

Front 3

Define Standard State

Back 3

solute- 1M concentration
gas- 1 bar pressure
Pure solids and pure liquids are in their standard state so they do not appear in the equilibrium expression (they equal 1)
The concentrations of the components of the equilibrium expression are actually expressed as a ratio of their concentration to the concentration in the standard state i.e. x/1, this is why K is dimensionless, all of the concentration units cancel

Front 4

What do you do with the equilibrium constants when you add reactions

Back 4

you multiply the K values

Front 5

What is the reaction quotient? What how does it relate to K

Back 5

the reaction quotient is set up in the exact same form as K excpet the present concentrations are used instead of equilibrium concentrations
if Q>K there is too much product, the reaction needs to proceed left
If Q<K there are too many reactants, the reaction needs to proceed in the forward direction to reach equilibrium
if Q=K the reaction is at equilibrium

Front 6

How do you find the equilibrium constant when you reverse a reaction?

Back 6

you take 1/k

Front 7

define equivalence point

Back 7

the quantity of titrant is added is the exact amount necessary for stoichiometric reaction with analyte

Front 8

T/F we measure the equivalence point of a titration

Back 8

false, we measure the end point as an estimate of the equivalence point

Front 9

Define end point

Back 9

a sudden change in a physical property of the solution, indicates that the equivalence point has been surpassed, it does not equal the equivalence point

Front 10

define titration error

Back 10

the difference between the end point and the equivalence point

Front 11

HOw can titration error be estimated/ minimized

Back 11

1. use an apporpriate physical property that is easy to observe
2.Use a reagent that gives an end point very close to the equivalene point
3. Do a blank titration to estimate how much titrant is needed to create observable color

Front 12

What are the characteristics of a primary standard

Back 12

-99.9% pure or better
-stable in ordinary storage
-stable when heat dried or vacuumed
-not hygroscopic

Front 13

What is the purpose of standardization and how is it done?

Back 13

Standardization is used because it is usually not possible to make titrant that is pure enough so that you know it's exact concentration. To standardize your titrant, you use the titrant to titrate a primary standard which allows you to figure out the exact concentration of your titrant

Front 14

describe a back titration

Back 14

a known excess of standard reagent is added to the analyte. A second standard reagent is used to titrate the excess of the first reagent

Front 15

When are back titrations used

Back 15

1. when the end point of the direct titration is not clear
2. when an excess of the first titrant is needed for the direct reaction to go to completion

Front 16

define saturated

Back 16

a solution that contains all the solid capable of being dissolved

Front 17

What are the 4 qualities of an ideal gravimetric precipitate

Back 17

-insoluble
- easily filtered (large particles)
- posses a known, constant composition
-heat stable

Front 18

List and describe the two phases of particle growth

Back 18

1. nucleation- dissolved molecules or ions form small crystalline aggregates capable of growing into larger particles
2. particle growth- solute molecules or ions add to an existing aggregate to form a crystal

Front 19

List three techniques used to promote particle growth over nucleation

Back 19

1. Raise the temperature to increase solubility and decrease supersaturation
2. Add precipitant slowly with vigorous mixing to avoid local superaturation
3. Keep the volume of the solution large so that the concentrtions of the analyte and precipitant are low

Front 20

What is homogenous precipitation and what the benefits of using this technique

Back 20

a gravimetric analysis in which the precipitant is generated slowly by a chemical reaction. It is beneficial because it provides a slow precipitation reaction which favors particle growth over nucleatin to give large particles that are easier to filter.

Front 21

What is the Arrhenius definition of a base and an acid

Back 21

acid- substance that increases the concentration of H3O+ in solution
base- substance that decreases the concentration of H3O+

Front 22

What is the Bronsted Lowry definition of and acid and base

Back 22

acid- proton donor
base- proton acceptor

Front 23

What is a salt

Back 23

any ionic solid

Front 24

define conjugate base/ acid

Back 24

acids and bases related to each other by the loss or gain of a proton

Front 25

write out how hydrogen ions are formed when CO2 reacts with water

Back 25

CO2 + H2O => H2CO3 => H+ +HCO3- => H+ + CO3 2-

Front 26

write out how hydrogen ions are formed when SO3 reacts with water

Back 26

SO3 + H2O => H2SO4 => H+ + SO4- => H+ + SO42-

Front 27

List the 6 strong acids

Back 27

HCl, HBr, HI, H2SO4, HNO3, HClO4

Front 28

T/F HF is a strong acid

Back 28

false HF is a weak acid even though HCl, HBr, and HI are strong acids

Front 29

List 6 strong bases

Back 29

LiOH, NaOH, KOH,RbOH, CsOH, (group1 hydroxides)
R4NOH- quaternary ammonia hydroxides

Front 30

define strong acid/ strong base

Back 30

an acid or base that completly dissociates in aqueous solution

Front 31

What is the relationship between Ka and Kb

Back 31

Ka*Kb= KW
also note that
Ka1* Kb2= Kw
Ka2* Kb1=kw for diprotics
Ka1*kb3= kw
Ka2 * Kb2= Kw
Ka3* Kb1=Kw for triprotics

Front 32

define buffer

Back 32

a solution that resists changes in pH when small amounts of aids or bases are added or when it is diluted

Front 33

What is buffer capacity

Back 33

measures how well a solution resists changes in pH when acid or base is added. The buffer capacity is maximum when pH = pka for the buffer, the buffer range is usually pka +/- 1 pH

Front 34

at what volume does pH=pka

Back 34

V=1/2 Veq

Front 35

why must indicators be used at very low concentrations

Back 35

indicators themselves are acids or bases and consume analyte or titrant

Front 36

How should you chose an indicator for your titration

Back 36

chose an indicator whose transition range overlaps the steepest part of the titration curve (the equivalence point)

Front 37

What are the 3 assumptions made when using the aproximation that pH=1/2 (pk1+pk2) for the intermediate form of a diprotic acid

Back 37

1.The major species in solution will be HL since neither the protonation deprotonatino proceeds to a considerable extent, this allows us to replace HL with F
2. The first term in the numeration is much greater than the second, drop K1Kw
3. In the denominator, K1 is much smaller than F, drop K1

Front 38

What happens to the activity coefficient when the ionic strength increases

Back 38

when ionic strength increases, activities decreases
putting more ions in solution means that it is harder for the ions of the solute to come toegther this leads to more dissolution of the solid which increase the concentraiton of the ions. in order for ksp to stay constant, the activities must decrease

Front 39

What happens to the activity coefficient when the charge increases

Back 39

the activity decreases (it's on the top of the equation but you're taking the log)

Front 40

what happens to the activity coefficient when you decrease the hydrated radius

Back 40

The activity coefficient decreases

Front 41

Don't forget to use the activity (pH=log H+ gammaH+) when calculating the pH as a final step in an activites weak acid problem

Back 41

:-)

Front 42

Don't forget to use the magnitiude of the charge as the coefficient in a charge balance equation

Back 42

:-)

Front 43

What are the 6 steps to systematic equilibrium problems

Back 43

1. pertinent reactions
2. charge balance
3. mass balance
4. equilibrium constants
5. count equations and unknowns
6. solve

Front 44

Be careful when doing mass balance equations when you have H+ or OH- present, you can't assume they are equal to the other ion because of the autoprotolysis of water

Back 44

:-)

Front 45

Don't forget to write (2x)2 in KSP problems that have a 2 molar ratio,
don't forget to add in the common ion
don't forget to also square the activity coefficient

Back 45

:-)

Front 46

How do you calculate the molecular weight of an unknown acid or base from a titration curve

Back 46

At the first equivalence point, the moles of titrant added is equal to the moles of acid or base present. Since you know the moles and the mass of acid or base you are analyzing, you can figure out the MW as g/mol. You can also use the 2nd equivlance point, you just need to divide the number of moles added by 2
Make sure you also pay attention to the size of the aliquot you are titrating. if you are only using 50 ml of a 250 ml soultion you need to divide by the total mass by 5 to get the grams in that particular aliquot

Front 47

How is the H-H equation written when taking into account activites

Back 47

pH=pka + log (X2- GX2-)/ (HX- G HX

Front 48

What do pH meters acutally measure

Back 48

activity
pH=-logA [H+] =log [H+] GH+

Front 49

How is buffer capacity calculated

Back 49

B=(number of moles of acid or base added)/[(ph change)*(volume of buffer in L)]