Chapter 17 Aqueous Ionic Equilibrium Flashcards
Define Buffer
A solution that resists pH change by neutralizing the added acid or base
Buffer’s contain significant amount of either…
I) A weak acid and its conjugate base
II) A weak base and its conjugate acid
What is a real life example of a Buffer
Human blood is a biological buffer in our system
How does a Buffer work if you add a base?
Adding a base will be neutralized by the acid
How does a Buffer work if you add an acid?
Adding an acid will be neutralized by the base
A Buffer ____________ small to moderate amounts of added ________ or _________ without a ________ __________ in its _____
Neutralizes; acid/base; large change; pH
What must a solution contain when making an Acidic/Basic solution?
I) Must contain significant amounts of both a weak acid and its conjugate base
II) Most contain significant amounts of both a weak base and its conjugate acid
What are the 2 methods of calculating the pH of Buffer solutions?
Method 1: Equilibrium Ice Table
Method 2: Henderson-Hasselbach Equation
Define Henderson-Hasselbach Equation
An equation relating the pH of a buffer solution to the initial concentration of the buffer
What is the Henderson-Hasselbach Equation?
pH = pKa + log [base]/[acid]
What does pH equal in the Henderson-Hasselbach Equation?
pH = pKa when the initial concentrations of the acid and base are equal
T/F For log(x), if x is between more than 0 but less than 1, log(x) will be POSITIVE
False; for log(x), if x is between more than 0 but less than 1, log(x) will be NEGATIVE
If x is _________ than 1, log(x) will be ____________
Larger; positive
When do you know if Henderson-Hasselbach Equation is good enough to use?
Henderson-Hasselbach Equation is good enough to use when “x is small” approximation is applicable
In what 3 circumstances will the “X is small” approximation work?
I) Initial acid and conjugate base concentrations are not to DILUTED
II) Equilibrium constant (value of Ka) is fairly small
III) For Buffers: initial concentrations of acids and conjugated bases should be at least 100~1000x greater than Ka
What pH change happens in Buffers when adding OH-?
The stoichiometric amount of the weak acid is neutralized and converted to the conjugate base
What pH change happens in Buffers when adding H3O+?
The stoichiometric amount of the conjugate base is neutralized and converted to the weak acid
Adding an acid to a buffer = creating more _______
Creating more acid
Adding a base to a buffer = creating more _______
Creating more base
What equation do you use to find pKa when given pKb?
pKa + pKb = 14
Define Buffer Capacity
The amount of acid or base a buffer can neutralize
Define Buffer Range
The pH range in which the buffer can be effective
Buffer Effectiveness is made up of what?
Buffer Capacity and Buffer Range
Buffer effectiveness depends on what 2 factors?
I) The relative amounts of buffer acid and base
II) The absolute concentrations of buffer acid and base
Buffer Capacity _________ with __________ absolute concentration of the buffer components
Increases; increasing
An __________ buffer neutralizes ________ to ___________ amounts of added acid or base without a _______ _________ in its pH
Effective; small; moderate; large change
What are the 2 factors showing when a Buffer is most effective?
Factor 1: A buffer is most effective when the concentrations of acid and conjugate base are equal, with a ratio of [base]/[acid] = 1
Factor 2: A buffer is most effective when the concentrations of acid and conjugate bases are high
A buffer will be effective when [base]/[acid] is between what 2 values?
0.1 < [base]/[acid] < 10
How do you determine what acid to use when making a Buffer?
When choosing an acid to make a buffer, choose the one whose pKa is closest to the pH of the buffer
What is the range used for Buffer Range?
0.10 < [base]/[acid] < 10
Effective Buffer Range = ???
Effective Buffer Range = pKa + or - 1
What is the Effective Buffer Range equation for lowest pH?
pH = pKa + log[base]/[acid] pH = pKa + log0.10 pH = pKa -1
What is the Effective Buffer Range equation for highest pH?
pH = pKa + log[base]/[acid] pH = pKa + log10 pH = pKa + 1
A chemist needs a solution buffered at pH 3.30 and has to choose from the following list of acids and their soluble salt, which solution would make the most effective buffer at this pH? I) Ka = 2.0x10^-11 II) Ka = 1.8x10^-5 III) Ka = 4.5x10^-4 IV) Ka = 1.7x10^-1 V) Ka = very large
pKa = -logKa I) -log(2.0x10^-11) = 10.69 II) -log(1.8x10^-5) = 4.74 III) -log(4.5x10^-4) = 3.35 IV) -log(1.7x10^-1) = 0.77 V) -log(100) = -2
Correct Answer: III)
Which acid would you choose to combine with its sodium salt to make a buffer solution with a pH of 4.25? I) HClO2; pKa = 1.95 II) HNO2; pKa = 3.34 III) HCHO2; pKa = 3.74 IV) HClO; pKa = 7.54
III)
To create a buffer that maintains a pH around 7.54, which solution would you choose?
I) CH3COOH & NaCH3OO; pKa(CH3COOH) = 4.74
II) HClO & KClO; pKa(HClO) = 7.54
III) NaOH & HCN; pKa(HCN) = 9.31
IV) HNO3 & KNO3; this is a strong acid and salt
II)
Define Acid-Base Titration
Procedure determining an unknown concentration of a basic (or acidic) solution by reacting with a known concentration of acidic (or basic) solution
In an Acid-Base Titration how to you determine the unknown concentration?
Unknown concentration determined by precise volumes of acid/base that will completely react
Define Indicator
A dye whose color depends on pH of solution
What does the Acid-Base indicator indicate?
Indicates the endpoint of the titration by changing color
Define Titration Curve (pH Curve)
A plot of the pH of the solution in Erlenmeyer flask (y-axis) and the volume of the solution added (x-axis)
Define Equivalence Point
The point in the titration when the moles of base is stoichiometrically equal to the moles of the acid
BEFORE Equivalence Point what solution is in excess?
Solution in Erlenmeyer flask (y-axis) is in excess
AFTER Equivalence Point what solution is in excess?
Solution from burette is in excess
What are the 5 different types of Titration Curves?
I) Adding strong base to a strong acid II) Adding strong base to a weak acid III) Adding strong acid to a strong base IV) Adding strong acid to a weak base V) Titration of a Polyprotic Acid
What is the Equivalence Point in the Titration Curve: Adding strong base to a strong acid?
Equivalence point: pH = 7
Describe the Titration Curve: Adding strong base to a strong acid
Before equivalence point starts low from bottom, then pH = 7, then after equivalence point is high at the top
What is the Equivalence Point in the Titration Curve: Adding strong base to a weak acid?
Equivalence point: pH > 7
Describe the Titration Curve: Adding strong base to a weak acid
Before equivalence point starts somewhat low, then pH greater than 7, then after equivalence point is high at the top and peaks down
What is the Equivalence Point in the Titration Curve: Adding strong acid to a strong base?
Equivalence point: pH = 7
Describe the Titration Curve: Adding strong acid to a strong base
Before equivalence point starts high at the top, then pH = 7, then after equivalence point is somewhat low towards the bottom
What is the Equivalence Point in the Titration Curve: Adding strong acid to a weak base?
Equivalence point: pH < 7
Describe the Titration Curve: Adding strong acid to a weak base?
Before equivalence point starts somewhat high peaking down, then pH < 7, then after equivalence point is somewhat low towards the bottom
Define Polyprotic Acid
Specific acids that pH are capable of losing more than a single proton per molecule in acid-base reactions
Polyprotic Acid Titration curves are different from the rest of the Titration Curves how?
Polyprotic acid titration curves have 2 equivalence points
T/F At Equivalence Point there may not always be equal moles of acids and bases
False; at equivalence point there is always equal moles of acids and bases
Define Solubility
The quantity of the compound that will dissolve in a certain amount of liquid
Define Molar Solubility (S)
The solubility in units of moles per liter (mol/L)
Solubility Product Constant (Ksp)
The equilibrium expression for the dissolution of an IONIC COMPOUND (aka salt) into its aqueous ions
T/F There are compounds that are completely insoluble
False; nothing is COMPLETELY insoluble, just very little of it dissolves
Ksp has only ____ value at a given temperature, wheres, solubility can have ____________ values at different kinds of solutions
One; different
In order to compare the solubility of different compounds by comparing their Ksp values, what must be the same?
The compounds must have the SAME DISSOCIATION STOICHIOMETRY
The __________ the Ksp value, the _______ solubility
Larger; more
Which of the following compounds will have the highest molar solubility in pure water? I) PbSO4, Ksp = 1.82x10^-8 II) MgCO3, Ksp = 6.82x10^-6 III) AgCN, Ksp = 5.97x10^-17 IV) PbS, Ksp = 9.04x10^-29 V) NiS, Ksp = 3.00x10^-20
II)
Define Common Ion Effect
The tendency for a common ion to decrease the ionization of a weak acid or a weak base or to decrease the solubility of an ionic compound
The solubility of an ionic compound is ________ in a solution containing a _________ ion than that in _______ _________
Lower; common; pure water
What is a good example showing the Ksp equation?
Mx Ny (s) --> xM^y+ (aq) + yN^x- (aq) Ksp = [M^y+]^x [N^x-]^y
The more the common ions between compounds = ???
A reduction in solubility
Low pH leads to ______ solubility
More
High pH leads to ______ solubility
Less
Define Precipitation Reactions
Reactions that occur upon mixing 2 solutions when one of the cross products is insoluble
List compounds that are generally soluble with no exceptions
Li+, Na+, K+, NH4+, NO3- and C2H3O2-
List compounds that are generally soluble except when they are paired with a certain compound. Also identify the compound pair that makes it an exception
Cl-, Br- and I- Exception: paired w/ Ag+, Hg2 2+ and Pb2+
SO4 2- Exception: paired w/ Sr2+, Ba2+, Pb2+, Ag+ or Ca2+
List compounds that are generally insoluble except when they are paired with a certain compound. Also identify the compound pair that makes it an exception
OH- and S2- Exception for both: Li+, Na+, K+ or NH4+ Exception for S2-: Ca2+, Sr2+ or Ba2+ Exception for OH-: Ca2+, Sr2+ or Ba2+ CO3 2- and PO4 3- Exception for both: Li+, Na+, K+ or NH4+
Ksp is at ___________ only, whereas, Q is under _____ ___________
Equilibrium; any condition
Explain Q < Ksp
Unsaturated, more solid dissolves, NO precipitate forms
Explain Q = Ksp
Saturated, additional solid does not dissolve, precipitate
Explain Q > Ksp
Supersaturated, the excess solid precipitates out
Define Selective Precipitation
The addition of a reagent to a solution containing several cations, forms a precipitate with ONLY ONE of the ions
In regards to Selective Precipitation, the ion requires the ___________ amount of added ion precipitates ______
Smallest; first
Define Complex Ion
An ion that contains a central metal ion bound to one or more ligands
Define Ligand
A neutral molecule or ion that acts as a Lewis Base with the central metal ion
Define Complex Ion Formation Reaction
The reaction between an ion and ligands to form a complex ion
Define Formation Constant (Kf)
The equilibrium constant for the formation reaction
Large Kf = ???
Formation of complex ions is highly favored
Which solution is a buffer? I) HNO3 and HCl II) HNO3 and NaNO3 III) NaOH and KCl IV) HF and NaF V) NH3 and NaOH
IV)
Which of the following would be a buffer solution? I) 50 mL HCl and 50 mL NaOH II) 25 mL NaOH and 50 mL NH3 III) 25 mL NaOH and 50 mL CH3COOH IV) 50 mL HCl and 50 mL NH3
III)
Consider the titration of HClO with NaOH, determine the pH of the solution at Equivalence Point and the species that determines that pH
pH > 7; ClO-
What is the dissolution of solid Ca3(PO4)2 in water with a solubility product constant of Ksp?
Ca3(PO4)2 –> 3Ca^2+ + 2PO4^3-
What is the Ksp expression for Mg3(PO4)2?
Ksp = [Mg^2+]^3 [PO4^3-]^2
In which solution would BaSO4 be most soluble?
I) 0.10 M BaNO3
II) 0.10 M Na2SO4
III) 0.10 M NaNO3
III) B/c it has no common ions
