CHEM 1062, General Chemistry | Worksheet 8.6 | University of Minnesota Twin Cities
Difficulty: Intermediate | Prerequisites: Chapter 7 (acid-base fundamentals, Lewis structures), basic equilibrium concepts.
Big picture: This worksheet ties together two ideas you have already met separately: equilibrium and acid-base chemistry. The core question is straightforward: when an acid and a base react, does the equilibrium favour the products or the reactants? The tool for answering that question is pKa. If you are comfortable drawing Lewis structures and understand what "equilibrium lies to the right" means, you are ready for this material. If not, revisit Chapter 7 and the equilibrium introduction first.
pKa is a number that tells you how strong an acid is: the lower the pKa, the stronger the acid. In any acid-base reaction, equilibrium favours the side with the weaker acid (the higher pKa). Compare the pKa of the acid on the left with the pKa of the conjugate acid on the right, and the equilibrium shifts toward whichever is weaker.
pKa
The negative base-10 logarithm of the acid dissociation constant (Ka). It quantifies how readily an acid donates a proton in aqueous solution. In simple terms, pKa is a scorecard for acid strength: a small number means the acid gives up its proton easily, a large number means it holds on tight.
Acid (Brønsted-Lowry)
A species that donates a proton (H⁺) to another species. Think of it as the molecule that hands off a hydrogen ion during the reaction.
Base (Brønsted-Lowry)
A species that accepts a proton (H⁺) from another species. In simple terms, the molecule with a lone pair ready to grab a hydrogen ion.
Conjugate acid
The species formed when a base accepts a proton. For example, when NH₃ (a base) picks up H⁺, it becomes NH₄⁺, the conjugate acid.
Conjugate base
The species formed when an acid donates a proton. When H₂O loses H⁺, it becomes OH⁻, the conjugate base.
Equilibrium position
The point at which the forward and reverse reactions proceed at equal rates, so the concentrations of reactants and products stay constant. "Lies to the right" means products are favoured; "lies to the left" means reactants are favoured.
Lewis structure
A diagram showing all valence electrons (as dots or lines) in a molecule. Used here to track which atom donates or accepts the proton via curved-arrow notation.
Curved-arrow notation
Arrows drawn from an electron-rich site (a lone pair) toward an electron-poor site (typically H⁺) to show the movement of electron pairs during a reaction.
When asked "which is the strongest acid?", you are really asking which species donates a proton most readily.
Comparing H₂O, NH₃, and CH₄: H₂O is the strongest acid of the three. It has the lowest pKa (15.7), meaning it gives up a proton far more easily than NH₃ (pKa 33) or CH₄ (pKa > 55).
Why the difference? Oxygen is more electronegative than nitrogen, which is more electronegative than carbon. Higher electronegativity on the atom bonded to H stabilises the conjugate base (the anion left behind after losing H⁺), making proton loss more favourable.
The ranking: H₂O > NH₃ > CH₄ in acid strength. Equivalently, CH₄ < NH₃ < H₂O. The order tracks with electronegativity across Period 2.
pKa is the single most useful number for comparing acids in this course.
Lower pKa = stronger acid. A pKa of 15.7 (water) means water is a far stronger acid than ammonia (pKa 33). The difference is not small: each pKa unit represents a factor of 10 in Ka.
Reference pKa values from this worksheet:
H₂O: pKa 15.7
NH₄⁺: pKa 9.8
NH₃: pKa 33
CH₄: pKa > 55
Reading the scale: Strong mineral acids (HCl, H₂SO₄) have pKa values near or below 0. The species on this worksheet are all weak acids with pKa values well above 0. CH₄ with pKa > 55 is essentially non-acidic under normal conditions.
pKa and conjugate pairs go together. Every acid has a conjugate base, and every base has a conjugate acid. When you know the pKa of an acid, you also know something about the strength of its conjugate base: the weaker the acid (higher pKa), the stronger its conjugate base.
This is the practical payoff of understanding pKa. Given a reaction, you can predict which side is favoured.
The rule: Equilibrium favours the side with the weaker acid (higher pKa). The reaction proceeds from the stronger acid toward the weaker acid.
Worked example 1: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Identify the acids on each side. On the left, H₂O acts as the acid (donates H⁺). On the right, the conjugate acid is NH₄⁺.
Compare pKa values: H₂O has pKa 15.7; NH₄⁺ has pKa 9.8.
NH₄⁺ is the stronger acid (lower pKa), so equilibrium favours the side that has NH₄⁺ as the acid, which is the left (reactants).
Conclusion: equilibrium lies to the left, favouring reactants.
Note: OH⁻ is a stronger base than NH₃, which also supports this conclusion.
Worked example 2: NH₂⁻ + H₂O ⇌ NH₃ + OH⁻
On the left, H₂O is the acid (pKa 15.7). On the right, NH₃ is the conjugate acid (pKa 33).
H₂O is the stronger acid (lower pKa), so the reaction proceeds to the right.
Conclusion: equilibrium lies to the right, favouring products.
This makes chemical sense: NH₂⁻ is a very strong base and will readily abstract a proton from water.
The general method (three steps):
Identify the acid and base on each side of the equation.
Find or compare the pKa of the acid on the left with the pKa of the conjugate acid on the right.
Equilibrium favours the side with the higher-pKa acid (the weaker acid).
pK_a = -\log_{10}(K_a)K_a = 10^{-pK_a}Reference pKa table for this worksheet:
Species | pKa |
|---|---|
NH₄⁺ | 9.8 |
H₂O | 15.7 |
NH₃ | 33 |
CH₄ | > 55 |
pKa values are used constantly in pharmacology: a drug's pKa determines whether it will be ionised or neutral at physiological pH, which controls whether it can cross cell membranes. This is why aspirin (a weak acid, pKa ~3.5) is absorbed in the acidic environment of the stomach but not in the more basic intestine.
Buffer solutions in biological systems (blood, for instance) rely on conjugate acid-base pairs whose pKa sits near the target pH. Understanding which direction equilibrium lies is exactly how chemists design buffers that resist pH changes.
"Higher pKa means stronger acid." This is backwards. A higher pKa means a weaker acid. The scale runs in the opposite direction to what many students expect.
Confusing the acid with the base in a reaction. Students sometimes compare the wrong pair of species. You need to compare the pKa of the acid on the reactant side with the pKa of the conjugate acid on the product side, not the bases.
Thinking equilibrium means the reaction does not occur. Equilibrium lying to the left does not mean nothing happens. It means the forward reaction occurs but the reverse reaction is faster, so reactants predominate at equilibrium.
Forgetting that water can act as an acid or a base. Water is amphoteric. In one reaction it donates H⁺ (acid); in another it accepts H⁺ (base). The role depends on what it is reacting with.
⚠️ Expect a question asking you to predict equilibrium direction given pKa values. The three-step method (identify acids, compare pKa, call the direction) is the core skill being tested.
⚠️ You may be asked to draw Lewis structures with curved arrows showing proton transfer. Practice the arrow from the lone pair on the base to the H on the acid.
⚠️ Ranking acids by strength (given pKa or given periodic trends like electronegativity) is a common exam item.
⚠️ The relationship between acid strength and conjugate base strength is frequently tested: strong acid = weak conjugate base, and vice versa.
True or false: A species with a pKa of 4 is a stronger acid than one with a pKa of 10. (True)
Fill in the blank: Equilibrium in an acid-base reaction favours the side with the ______ acid. (weaker / higher-pKa)
True or false: OH⁻ is a stronger base than NH₃. (True)
Fill in the blank: pKa = −log₁₀(______). (Ka)
True or false: CH₄ is a stronger acid than H₂O. (False)
Q: Given that the pKa of HF is 3.2 and the pKa of CH₃COOH (acetic acid) is 4.75, which is the stronger acid?
A: HF is the stronger acid because it has the lower pKa.
Q: For the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, does the equilibrium lie to the left or the right? Explain using pKa values.
A: Equilibrium lies to the left. The conjugate acid on the product side (NH₄⁺, pKa 9.8) is a stronger acid than H₂O (pKa 15.7) on the reactant side, so the reverse reaction is favoured.
Q: In the reaction NH₂⁻ + H₂O ⇌ NH₃ + OH⁻, identify the acid and base on each side.
A: Left side: H₂O is the acid, NH₂⁻ is the base. Right side: NH₃ is the conjugate acid, OH⁻ is the conjugate base.
Q: Why does equilibrium in the reaction NH₂⁻ + H₂O ⇌ NH₃ + OH⁻ lie to the right?
A: H₂O (pKa 15.7) is a much stronger acid than NH₃ (pKa 33). Equilibrium favours the side with the weaker acid, so it shifts to the right (products).
Q: A student claims that because NH₄⁺ has a lower pKa (9.8) than H₂O (15.7), NH₄⁺ must be formed preferentially in the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. What is wrong with this reasoning?
A: The student has the logic reversed. Because NH₄⁺ is a stronger acid than H₂O, NH₄⁺ will donate its proton back more readily. This drives the equilibrium to the left (toward reactants), not to the right.
This material connects directly to buffer chemistry (typically covered next in CHEM 1062), where you will use pKa to select appropriate conjugate pairs and calculate buffer pH via the Henderson-Hasselbalch equation.
The Lewis-structure and curved-arrow skills practised here carry forward into organic chemistry, where proton-transfer mechanisms appear in nearly every reaction chapter.
The idea that equilibrium favours the weaker acid/base pair is a specific case of the broader thermodynamic principle that systems shift toward lower free energy.
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