Unit 8-Acids and Bases Flashcards
Kw
ion-product constant for water
Illustrates amount of water ionized
Can be used for acids and bases
Kw = [H+] [OH-]
Kw at 25 degrees Celsius
1.0 * 10^-14
Type of solution based on concentration of H+ and OH-
[H+] > [OH-] = acidic
[H+] = [OH-] = neutral
[H+] < [OH-} = basic
pH =
-log [H3O+]
pOH = -log [OH-]
Sig figs for pH is decimals
As [H3O+] increases…
The pH decreases
When concentration changes by a factor of 10, pH will change by a factor of 1
Sum of pH and pOH at 25 degrees Celsius
14.00
Autoionization of water is…
Endothermic
Weak Acids
Partially ionize in water so to solve, you have to use a Ka expression
Large Ka =
stronger acid
Percent Ionization
(x/[initial concentration of weak acid]) *100
Stronger acids have a greater percent ionization
As the concentration of a weak acid decreases, the percent ionization increases because products decrease more than reactants so reaction shifts right to form more ions
Kb
Base dissociation constant
Refers to weak base added to water and forming conjugate acid and hydroxide
Anions of Weak Acids
F- = weak base forms HF = weak acid (conjugate acid)
HCl = strong acid forms Cl- =negligible
Relationship between Ka and Kb
Stronger acids have weaker conjugate bases
Stronger acids have negligible conjugate bases
Ka * Kb = Kw = 1.0 * 10^-14
pKa + pKb = 14.00
Strong Acid + Strong Base
Use before after and stoichiometry to find limiting reactant
ER determines pH
Weak Acid + Strong Base
Use stoichiometry (moles) to find LR and ER
Strong base excess = determines pH
Weak acid and conjugate base are excess = both determine pH
Both substances are limiting so conjugate base is left = determines pH
ER dissolves in water so…Use ICE table
Ka or Kb if weak acid and weak base are present
Only weak base = use Kb ICE table
Half-Equivalence Point
pH = pKa
pOH = pKb
([weak acid] = [conjugate base])
Buffers
Solutions that contain a weak acid-conjugate base or weak base-conjugate acid pair
Withstands pH changes when strong acid/base is added to them
Two ways to make a buffer
Add a salt of the weak acid or base
Ex. Add sodium acetate to a solution of acetic acid
Add a strong acid/base to a solution of weak base or weak acid
Ex. Add sodium hydroxide to a solution of acetic acid
Strong Acid-Strong Base Titrations (Base is being added to the Acid)
Initial pH = calculate with concentration of initial acid
Between initial pH and equivalence point = added base is LR so perform stoichiometry to find concentration of acid after base was added. Use this concentration of acid to find pH
Equivalence point = 7.00
After equivalence point = base is excess so concentration of base finds pH
Weak Acid-Strong Base Titrations (Base is added to the Acid)
Initial pH = use Ka ICE table to find out how much dissociated
Between initial pH and equivalence point = added base is LR; stoichiometry to find concentration of weak acid and conjugate base after base was added. Use ICE table to calculate pH; half-way is pH = pKa
Equivalence point = HA is all converted into CB so use Kb ICE table
After equivalence point = HA is LR so excess base determines pH
Determining Ka from Titration Curve
Find volume of base needed to reach equivalence point and half it
This volume is the half-way point/half-equivalence point
pH = pKa
Weak Acid-Strong Base vs. Strong Acid-Strong Base
Solution of weak acid has higher initial pH
pH changes is smaller in weak acid titration
pH equivalence point is above 7.00 in weak acid titration
Titrations of Polyprotic Acids
Polyprotic Acids have more than one proton
It has 2+ Ka values and 2+ equivalence points
Weak Base-Strong Acid Titrations Graph (Acid added to Base)
Opposite of weak acid-strong base titrations
Initial pH = ICE table to find concentration of base dissolved
pH at equivalence point = conjugate acid present so ICE table
After equivalence point = excess strong acid determines pH
Factors that Affect Acid Strength
Bond Polarity (Bond Polarity increases, stronger acid forms)
Bond Length (Bond strength decreases, stronger acid forms
Conjugate Base Stability (greater stability of CB, stronger acid; when resonance structures can be drawn for the conjugate base, this means that the acid has great stability due to the delocalized pi electrons)
Binary Acid Trends
Within a group=Bond strength is the most important factor
Down a group = bond strength decreases and strength of acid increases
Within a period=Bond polarity is the most important facotr
Across a period = Bond polarity increases and strength of acid increases
Oxyacid
Contains an atom bonded to 1+ oxygen atoms
Inductive Effect
The attraction of electrons in adjacent bonds by more EN atoms
Rules of Oxyacid Strength
Same # of O-H groups and O atoms = acid strength increases as the EN value of Y increases
Same central Y atom - acid strength increases with increase in # of O atoms
Comparing pH and PKa to find concentration of acid and conjugate base in buffer solution
pH < pKa = acid forms a higher concentration in the original solution
pH > pKa = conjugate base forms a higher concentration in the original solution
When does an indicator change color?
Around the equivalence point; bc of rapid change in pH it changes color not exactly at the equivalence point
Henderson-Hasselbalch Equation
Used to find the pH of a buffer
pH = pKa + log [A-]/[HA]
X in the ICE table is essentially negligible
Using Henderson-Hasselbalch Equation to determine information of a reaction
Acid and conjugate base concentrations are equal:pH=pKa
Conjugate base concentration than greater than acid: pH > pKa
Conjugate base concentration is less than acid: pH<pKa
Buffer Capacity
Amount of acid/base the buffer can neutralize before it starts changing pH
Depends on concentrations of weak acid and conjugate base use to make buffer
A buffer can be made for any pH by adjusting relative concentrations of the weak acid/weak base
Higher concentrations =
greater buffer capacity
Making a buffer at a specific pH
Choose an acid with a pKa close to desired pH (H+ concentration) for the buffer
Henderson-Hasselbalch equation can be used to determine ideal ratio for a certain pH
