Acid-Base Equilibrium Mastery
Deep dive into acid-base chemistry covering pH calculations, Ka/Kb, buffer design, and titration analysis. Master the Henderson-Hasselbalch equation and predict solution properties.
Learning Objectives
- ✓Calculate pH for strong and weak acid/base solutions
- ✓Apply Ka and Kb to predict acid-base behavior
- ✓Design and analyze buffer solutions
- ✓Interpret titration curves and identify equivalence points
1. Acid-Base Definitions
Arrhenius: acids produce H+, bases produce OH-. Bronsted-Lowry: acids donate protons, bases accept protons. Lewis: acids accept electron pairs, bases donate electron pairs. The Bronsted-Lowry definition is most commonly used in general chemistry.
Key Points
- •Bronsted-Lowry: acid = proton donor, base = proton acceptor
- •Conjugate acid-base pairs differ by one H+
- •Lewis acids have empty orbitals; Lewis bases have lone pairs
- •Water is amphoteric (can act as acid or base)
2. Strong vs Weak Acids and Bases
Strong acids completely dissociate: [H+] = [HA]initial. The 7 common strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4. Most acids are weak and require equilibrium calculations using Ka. Likewise, strong bases (NaOH, KOH) dissociate completely.
Key Points
- •Strong acids: 100% dissociation, no equilibrium needed
- •Weak acids: partial dissociation, use Ka expression
- •Strong bases: metal hydroxides (NaOH, KOH, Ba(OH)2)
- •Weak bases: ammonia and amines
3. pH Calculations
For strong acids: pH = -log[H+]. For weak acids: set up Ka equilibrium, solve for [H+]. If Ka << [HA]initial, use approximation [H+] = sqrt(Ka x C). Check that x < 5% of C; if not, use quadratic formula. Remember: pH + pOH = 14 at 25C.
Key Points
- •Strong acid: pH = -log[acid concentration]
- •Weak acid: pH = -log(sqrt(Ka x C)) if approximation valid
- •Quadratic needed if x > 5% of initial concentration
- •For bases: calculate pOH first, then pH = 14 - pOH
4. Buffer Solutions
Buffers resist pH change when acid or base is added. They contain a weak acid and its conjugate base (or weak base and conjugate acid). The Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). Maximum buffer capacity occurs when [A-] = [HA], i.e., pH = pKa.
Key Points
- •Buffer = weak acid + conjugate base (or weak base + conjugate acid)
- •Henderson-Hasselbalch: pH = pKa + log([base]/[acid])
- •Best buffer capacity when [A-] ≈ [HA]
- •Buffer range: pKa plus or minus 1
5. Titration Curves
Titration curves plot pH vs volume of titrant. Strong acid-strong base: steep rise at equivalence, pH = 7. Weak acid-strong base: equivalence pH > 7 (conjugate base is basic). At half-equivalence point: pH = pKa because [HA] = [A-].
Key Points
- •Strong-strong equivalence: pH = 7
- •Weak acid-strong base equivalence: pH > 7
- •Weak base-strong acid equivalence: pH < 7
- •Half-equivalence point: pH = pKa, [HA] = [A-]
6. Polyprotic Acids
Polyprotic acids have multiple ionizable protons (H2SO4, H3PO4, H2CO3). Each proton has its own Ka value, and Ka1 > Ka2 > Ka3. The first proton is easiest to remove. For titration, each equivalence point corresponds to one proton.
Key Points
- •Ka1 >> Ka2 >> Ka3 (each proton harder to remove)
- •Treat each dissociation separately
- •Multiple equivalence points in titration
- •The pH after first equivalence depends on Ka2
High-Yield Facts
- ★Kw = [H+][OH-] = 1.0 x 10^-14 at 25°C
- ★Ka x Kb = Kw for a conjugate acid-base pair
- ★Strong acid added to buffer: HA increases, A- decreases
- ★Percent ionization = ([H+]eq/[HA]initial) x 100%
- ★Indicators change color when pH equals their pKa plus or minus 1
Practice Questions
1. What is the pH of 0.10 M acetic acid? (Ka = 1.8 x 10^-5)
2. Design a buffer at pH 5.00 using acetic acid (pKa = 4.74) and sodium acetate.
FAQs
Common questions about this topic
Use it when your approximation (x = sqrt(Ka x C)) gives x > 5% of the initial concentration. This typically happens when Ka is relatively large or the acid is very dilute. Set up Ka = x^2/(C-x) and solve the quadratic.
Choose an indicator whose color change range (pKa plus or minus 1) includes the equivalence point pH. For strong acid-strong base (eq. pt. pH 7), use bromthymol blue. For weak acid-strong base (eq. pt. pH > 7), use phenolphthalein.