Acid-Base Reactions in Detail
Master the Bronsted-Lowry theory, conjugate acid-base pairs, the acid dissociation constant Ka, titration curves, and the selection of appropriate indicators.
Bronsted-Lowry Acid-Base Theory
According to the Bronsted-Lowry theory, an acid is a proton (H+) donor and a base is a proton acceptor. Every acid-base reaction involves the transfer of a proton from the acid to the base, forming a conjugate acid-base pair.
Conjugate Acid-Base Pairs
HCl
Acid
H2O
Base
Cl-
Conjugate base
H3O+
Conjugate acid
HCl donates H+ to H2O. The pair HCl/Cl- and H2O/H3O+ are conjugate pairs.
Amphoteric substances (e.g. water, H2O) can act as both an acid and a base. Water donates a proton to become OH- or accepts a proton to become H3O+.
Acid Strength and Ka
The acid dissociation constant (Ka) quantifies the strength of a weak acid. For HA(aq) ⇌ H+(aq) + A-(aq), Ka = [H+][A-]/[HA]. A larger Ka means a stronger acid (greater ionisation).
Strong Acids
Fully ionise in water. Ka is very large.
Examples: HCl, HNO3, H2SO4
HA → H+ + A- (complete, one-way arrow)
Weak Acids
Partially ionise. Ka is small (< 1).
Examples: CH3COOH (Ka = 1.8 × 10-5)
HA ⇌ H+ + A- (equilibrium, double arrow)
pH and pKa
pH = -log10[H+] (lower pH = more acidic)
pKa = -log10Ka (lower pKa = stronger acid)
At the half-equivalence point of a weak acid titration: pH = pKa
Titration Curves and Indicators
A titration curve plots pH against volume of titrant added. The shape depends on the acid and base strengths. The equivalence point is where moles of acid equal moles of base. An indicator must be chosen so its colour change range (transition range) overlaps with the steep section of the curve around the equivalence point.
Titration Curve Shapes
Strong Acid + Strong Base
Equivalence point at pH 7. Use methyl orange or phenolphthalein.
Weak Acid + Strong Base
Equivalence point at pH > 7. Use phenolphthalein (pH 8.2-10.0).
Choosing an Indicator
Methyl orange: Transition range pH 3.1-4.4 (red to yellow). Suitable for strong acid-strong base or strong acid-weak base titrations.
Phenolphthalein: Transition range pH 8.2-10.0 (colourless to pink). Suitable for strong acid-strong base or weak acid-strong base titrations.
Rule: The indicator's transition range must fall within the steep portion of the titration curve around the equivalence point.
Key Vocabulary
Conjugate Acid-Base Pair
Two species that differ by one proton (H+). When an acid donates a proton, it forms its conjugate base, and vice versa.
Ka (Acid Dissociation Constant)
The equilibrium constant for the ionisation of a weak acid in water. Ka = [H+][A-]/[HA]. Larger Ka means stronger acid.
Equivalence Point
The point in a titration where the moles of acid exactly equal the moles of base. The pH at this point depends on the strengths of the acid and base.
Buffer Solution
A solution that resists changes in pH when small amounts of acid or base are added. Made from a weak acid and its conjugate base (or weak base and conjugate acid).
Worked Examples
Identify the conjugate acid-base pairs in: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Step 1: NH3 accepts H+ from H2O, so NH3 is the base and H2O is the acid.
Step 2: Pair 1: NH3 (base) / NH4+ (conjugate acid)
Step 3: Pair 2: H2O (acid) / OH- (conjugate base)
Calculate the pH of 0.10 M acetic acid. (Ka = 1.8 × 10-5)
Step 1: CH3COOH ⇌ H+ + CH3COO-. Ka = [H+][CH3COO-]/[CH3COOH]
Step 2: Let [H+] = x. Then Ka = x2/(0.10 - x) ≈ x2/0.10 (assuming x << 0.10)
Step 3: x2 = 1.8 × 10-6, so x = 1.34 × 10-3 M
Answer: pH = -log(1.34 × 10-3) ≈ 2.87
Explain why phenolphthalein is suitable for a weak acid/strong base titration but methyl orange is not.
Step 1: The equivalence point of a weak acid/strong base titration is above pH 7 (typically pH 8-10) because the conjugate base of the weak acid is formed.
Step 2: Phenolphthalein changes colour between pH 8.2-10.0, which falls within the steep part of the curve near the equivalence point.
Step 3: Methyl orange changes at pH 3.1-4.4, which is well below the equivalence point and falls in the buffer region where pH changes slowly. It would change colour too early.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
According to the Bronsted-Lowry theory, a base is a substance that:
Question 2
The conjugate base of H2SO4 is:
Question 3
A weak acid with Ka = 1.0 × 10-3 compared to one with Ka = 1.0 × 10-7 is:
Question 4
The equivalence point of a weak acid-strong base titration is at pH:
Question 5
At the half-equivalence point of a weak acid titration:
Key Concepts Summary
- ●Bronsted-Lowry: Acids donate protons, bases accept protons. Every reaction creates conjugate acid-base pairs.
- ●Ka quantifies acid strength. Larger Ka = stronger acid = more ionisation.
- ●Strong acids fully ionise (very large Ka); weak acids partially ionise (Ka < 1). Both contribute H+ but to different extents.
- ●Titration curves vary in shape depending on acid/base strength. The equivalence point pH is 7 only for strong/strong titrations.
- ●Choose an indicator whose transition range falls within the steep section of the titration curve near the equivalence point.