MCAT General Chemistry-Acids and Bases
acids: create H+ in solution
bases: creates OH- in solution
Arrhenius Definition of acids and bases
Lewis acids: electron pair acceptor
Lewis base: electron pair donor
Lewis Definition of acids and bases
Acids: proton donors
Bases: proton acceptors
Bronsted-Lowry Definition of acids and bases
structure after BL-acid donates a proton
structure after BL-base accepts a proton
acid that completely dissociates in solution
acid that does not completely dissociate in H₂O
acid-ionization constant (Ka)
equilibrium for acid dissociation that indicates strength of the acid
NOTE: the larger the Ka, the stronger the acid
Products are favored, and the acid is a strong acid
Acid-ionization constant>1 indicates what about the rxn?
Reactants are favored and the acid is weak
Acid-ionization constant <1 indicates what about the rxn?
HI, HBr, HCl, HClO₄, H₂SO₄, HNO₃, and HSO₄⁻
List of strong acid (Ka>1)
Base ionization constant (Kb)
equilibrium for base dissociation that indicates strength of the base
NOTE: the larger the Kb, the stronger the base
OH⁻, OR⁻, NH₂⁻, NR₂⁻, H⁻, R⁻, O²⁻
List of strong bases
a conjugate base of a strong acid has NO basic properties in H₂O
Conjugate base of strong acid in H₂O
will be more basic than water, and be able to accept protons and donate electrons
Conjugate base of weak acid in H₂O
the conjugate acid of a strong base has NO acidic properties (will not donate any protons)
Conjugate acid of a strong base in H₂O
the conjugate acid of a weak base has weak acidic properties
NOTE: the weaker the base, the stronger the conjugate acid
Conjugate acid of a weak base in H₂O
wherever a substance can act either as a base or an acid
Conjugate bases of polyprotic acids
Yes, it has two H+
Is H₂O amphoteric?
the self ionization of H₂O
Equilibrium Expression for autoionization of H₂O
Kw=1*10⁻¹⁴ at 25C
Only varied by temperature (like other equilibrium constants, increasing as temperature increases)
measure of the concentration of H+ in solution
NOTE: log of 1=0
Formula for pH
pH<7 is acidic
pH=7 is neutral
pH>7 is basic
pH values determining acidity or basicity at STP
concentration of OH in solution
Formula for pOH
pOH+pH=14 at 25C
strength of an acid or strength of a base. a compound cannot compare pka and pkb
Definitions of pKa and pKb
Ka*Kb=10⁻¹⁴ for conjugates only
Relationship between Ka and Kb
pKa+pkb=14 for conjugates only
strong acids dissociate completely, therefore the concentration of H+ is equal to the concentration of the acid
pH calculations w/ strong acids gives what value for the concentration of products?
weak acids come to equilibrium w/ dissociated ions
Ka=[H][A-]/[HA]=x²/([original con HA]-x)
Ka calculations for weak acids gives what value for the concentration of products?
When can X in the denominator for Ka equation be considered negligible?
when an acid and a base combine to produce salt and H₂O
NOTE: all (blank) rxns go to completion regardless of the strength of base or acid
ionic compound, consisting of a cation and an anion; in H₂O, (blank) dissociate into ions
NOTE: determines whether solution will be neutral, acidic, or basic
Group I cations and larger Group II cations
Which cations of a salt do not react w/ H₂O?
NH₄⁺, Be²⁺, Cu²⁺, Zn²⁺, Al³⁺, Cr³⁺, Fe³⁺
Cations that are stronger acids than H₂O
Anions that do not react w/ H₂O
conjugate bases of a strong acid
will be more basic than water and be able to donate electrons and accept protons
conjugate bases of weak acids
rxn of a substance (salt or ion) w/ H₂O
Weak acid with conjugate base (or weak base with conjugate acid)
solution that resists changing pH when a small amount of acid or base is added
should match buffer pKa with buffered pH
Buffer choice should what?
presence of a weak acid and conjugate base in equal concentrations, or weak base and its conjugate acid
Henderson-Hasselbalch Equation (for acid)
Henderson-Hasselbalch Equation (for base)
weak acid that undergoes a color change when its converted to its conjugate base
Equation for determining color of indicator
protonated form will dominate
If [H₃O]>>Ka, then....
the deprotonated form is dominant
If [H₃O]<<Ka, then....
both protonated and deprotonated forms will be present
If [H₃O]=Ka, then...
pKa +/- 1
Range of indicator end points
experimental technique to identify an unknown weak acid or base by determining its pKa or pKb
strong acid or base added to solution containing the unknown
NOTE: (blank) of an acid is always a base and vice versa
section of titration curve where pH changes gradually
Acid-base equivalent point
point where moles added equals moles of original solution, leaving solely neutral salt
The equivalence point of a weak acid
equivalence point will be at pH>7
The Equivalence point of a weak base
equivalence point will be at pH<7
Equivalence point of a strong acid w/ a strong base
equivalence point will occur at pH=7
Half Equivalence Point
point where half the volume of titrant is added as the equivalence point; where weak acid has equal concentration with conjugate base
have more than one equivalence point due to increased number of hydrogens. With each equivalence point reached the Ka decreases
base that can accept two subsequent protons
Protonated indicator dominates
when pH<pKa, the solution is too acidic or has too many protons
Deprotonated form will dominate
when pH>pKa, what form will dominate?
pKa w/in 1 pH point of the pH to be indicated
When choosing an indicator, the pKa should w/in what value of the pH?
compound that differ by only one proton
What is a conjugate?
formula for chlorate?
formula for carbonate?
formula for phosphate?
formula for sulfate?
formula for nitrate?
formula for chloride?
formula for hypochlorate?
formula for chlorite?
formula for perichlorate?
formula for acetate?
formula for benzoate?
Which strong base is diprotic?
sulfuric acid, H₂SO₄
Which strong acid is diprotic?
HF, NH₄⁺, H₂CO₃, phenol, RCOOH, RNH₃⁺, Al³⁺, BF₃
List of useful weak acids?
Al³⁺ and BF₃
Which weak acids are lewis acids?
F⁻, NH₃, HCO₃⁻, phenol-O⁻, RCOO⁻, RNH₂, CN⁻
List of useful weak bases?
increase acidity through induction
Do electron withdrawing groups decrease or increase acidity?
decrease acidity by making it less likely to receive electrons
Do electron donating groups decrease or increase acidity?
lone pair of electrons
What must all bases have to donate electrons, and thus act like a base?
Nnnnnooooo. Only exception is conjugates
Can pKa and pKb be compared?
No this can be varied with temperature
Does the neutral pH always have to be 7?
What is the equation for calculating the pH of sulfuric acid?
What is the equation for calculating the pH using Ka and concentration of conjugate base?
What is the equation for calculating the pOH of oxide?
Yes, the buffer will resist change, but change will still occur
Will the pH change when an acid or base is added to a buffer?
the pH will remain unchanged, but the buffer capacity will decrease since the concentration of weak acid and conjugate base have decreased
What will happen to the pH when water is added to dilute a buffer solution?
for every point difference btw the pH and pKa there is one 9 in the greater percentage of protonated versus deprotonated
Explain the rule of 9s?
No, their conjugates cannot act as buffers
Is there a half equivalence point for the titration of a strong acid or strong base?
take the average of the two pKa
How do you calculate the pI for a diprotic acid?
when the indicator changes color
What is the end point of a titration?
less products than reactants, which is why the Ka is less than 1
Will weak acid have more are less products than reactants?
compound contains both polar and nonpolar structures
compound that contains both fat and water loving properties
No, they will either 100% or 0%
Will strong acids follow the rule of 9s?
exothermic process since bonds are being formed
Is neutralization an exo or endothermic process?
a strong acid with weak base will generate a protonated salt, thus lowest pH. A strong base with weak acid will generate a salt at the highest pH
In a neutralization rxn, which two reactants will generate a salt of the lowest pH? Highest pH?