Acids and Bases
Key Ideas

Acid and Bases is typically in AP Chemistry Course Content in Unit 8. This topics covers foundational ideas for different types of reactions, and mechanisms involved. Below will be a short summary covering topics 8.1 Introduction Acid & Bases to 8.10 Buffer Capacity.

This page will serve as a basis to this difficult unit, or quick review for others.

8.1 Introduction to Acid and Bases

Acids/bases can be reversed (essentially equilibrium) being related to the strength.

Be able to calculate pH or pOH also based on Kw, Ka, or Kb values. pH represents the acidity ranging from 0-14, and pOH is the reverse representing bases. This is implied through H+ protons being an acidic, and OH- hydroxide being basic.

Calculate the values of pH and pOH based on Kw and the concentration of all species present in a neutral solution of water

Key Terms:

• Bronsted Lowry Acid = gives the hydrogen (H+)

• Bronsted Lowry Base = accepts the hydrogen (H+)

8.2: pH and pOH of Strong Acids and Bases

Strong acids: HCl, HBr, HI, H2SO4, HNO3, HClO4

Strong bases: all group 1 & 2 bases paired with OH-

Tip: A mnemonic to memorize strong acids would be "So I Brought No Clean Clothes", using the first letters of each word paired with a prefix of "H" would give you the strong acid.

DISCLAIMER: Any acid not listed above is not considered one of the six strong acids.

For strong bases it is straightforward.

Strong acid + Strong bases always dissociate 100%

Percent Ionization = [H+] at eq / [HA] original x 100%

pH/pOH for strong acids/bases are very low/high

8.4: Acid-Base Reactions and Buffers

Acid-base neutralization Reactions typically follow this mechanism:

Acid + Base → Salt + Water

When doing a dilution question take into consideration:

Dilution: did you add volume to volume? M1V1=M2V2

Stoichiometry: Any strength acid + base will proceed to make the product until the limiting runs out

Equilibrium: If weak acids or bases are involved they may need to do equilibrium calculations

Strong Acid + Strong Base - Stoichiometry, for strong acid + strong base pH is determined from the excess remaining

Weak Acid + Strong Base - Stoichiometry, for weak acid + strong base pH is determined after an equilibrium calculation

Weak acid → Conjugate Base

Conjugate base + water

Weak Acid + Strong Base - Equilibrium 

This represents the Equivalence Point

This occurs when [acid] = [base] (think of them as co-limiting)

The pH is dictated by the reaction of the conjugate base (or conjugate acid if weak base + strong acid)

You will have to calculate the K value for the conjugate (often part of the prompt)

8.5: Acid-Base Titrations

Titrant → Molarity/Initial Volume/Final Volume

Analyte → Volume/Molarity/Mass

(MaVa) = (MbVb)

At volumes less than the half-equivalence point, undissociated acid is the dominant species, the moles/molarity of acid = moles/molarity of the conjugate base

At volumes greater than the half-equivalence point, the conjugate base is the dominant species

At the equivalence point, the conjugate base is the only species, and it directs the pH

Strong acids have non-reacting conjugate bases so the auto-dissociation of water directs the pH

For a weak acid, the region around the half-equivalence point is called the buffer zone since the weak acid AND the conjugate base are both present in the solution

A titration involves the slow addition of a TITRANT to an ANALYTE

The volume at which equimolar amounts of titrant and analyte have been added is called the EQUIVALENCE POINT

The equivalence point is typically used to determine either the molarity of the acid or the volume of base required to reach the equivalence point

For weak acids, the pH at half the volume of the equivalence point (aka the half-equivalence point) is equal to the pKa for the acid

For weak bases, at half the volume of the equivalence point (aka the half-equivalence point) pH = 14 - pKb for the weak base

At volumes less than the half-equivalence point, the original base is the dominant species 

At the half-equivalence point the moles/molarity of base = moles/molarity of conjugate acid

At volumes greater than the half-equivalence point, the conjugate acid is the dominant species

At the equivalence point, the conjugate acid is the only species, the pH is determined from the rxn between the conjugate acid and water

For a weak base, the region around the half-equivalence point is called the buffer zone since the weak base AND the conjugate acid are both present in the solution

8.6: Molecular Structure of Acids and Bases

Stronger acids have weak H-X bonds (Such as HCl), and weaker acids have strong H-X bonds (such as HF)

BASICALLY, STRONG ACIDS HAVE WEAK BONDS MAKING IT EASIER TO DISSOCIATE

Oxoacids or oxyacids contain an atom bonded to one or more oxygen atoms, sometimes with hydrogen atoms attached

Inductive effect the attraction of electrons in adjacent bonds by more electronegative atoms

Strong acids such as nitric acid can experience an inductive effect due to the highly electronegative oxygen atoms

Polarity in the molecule draws electrons away making the hydrogen atom making it easily ionizable

Nitric acid’s conjugate base nitrate ion is more stable (weaker) due to the negative charge being spread evenly in each resonance molecule

Weak Acids such as carboxylic acids do not experience a strong induced dipole force

Less polarity in a molecule results in the hydrogen ion being more attracted to the oxygen and thereby less ionizable

The conjugate bases are less stable which results in a stronger base

Which protons a molecule will participate in acid-base reactions, as well as the relative strength of these protons can be inferred from the molecular structure

Strong acids have very weak conjugate bases that are stabilized by electronegativity-inductive effects, resonance, or some combination of both

Carboxylic acids are one common class of weak acid

Electronegative elements tend to stabilize a conjugate base relative to its conjugate acid and so increase acid strength

8.7: pH and pKa

You can compare pH and pKa 

pH < pKa the acid formed has a higher concentration 

pH > pKa the conjugate base formed has a higher concentration

Indicators can be used to find the pH of the original solution/indicate the equivalence point

8.8: Properties of Buffers

Buffers contain both the conjugate base and acid pair

Conjugate acid reacts with base and conjugate base reacts with acid

Buffers stabilize the pH levels

8.9: Henderson-Hasselbalch Equation

Basically, pH = pKa is true at the midpoint because [A-]/[HA] = 1 so log(1) = 0

When the conjugate base concentration is greater than the acid formed, the log is positive so pH > pKa

When the conjugate base concentration is less than the acid formed the log is negative so pH < pKa

BASICALLY IF pH>pKA THAT MEANS IT IS BECOMING MORE BASIC SO MORE CONJUGATE BASE IS BEING FORMED SO pH GOES UP

IF pH<pKa THAT MEAN IT BECOMING MORE ACIDIC SO A LESS CONJUGATE BASE IS BEING FORMED

8.10: Buffer Capacity

A buffer resists changes the one closer to the ORIGINAL M/mol has a greater capacity meaning it resisted more change

Whatever is added more of has a greater capacity