E. Solutions and Solubility · Solutions and SolubilityStrong and Weak Acids and Bases
E3.6 E3.5 E2.1 A1.8About 70 minutes
Learning goals- I can explain the difference between strong and weak acids and bases in terms of their degree of ionization or dissociation.
- I can name common strong and weak acids and bases.
- I can use evidence (pH, conductivity, reaction rate) to classify an acid as strong or weak.
- I can calculate percentage ionization from pH, and distinguish strength from concentration.
Words to know
strong acid
An acid that ionizes almost completely (about 100%) in water: HA + H₂O → H₃O⁺ + A⁻.
weak acid
An acid that ionizes only partly (often less than 5%) in water; most of it stays as molecules: HA + H₂O ⇌ H₃O⁺ + A⁻.
strong base
A base that dissociates completely into OH⁻ ions, such as NaOH or KOH.
weak base
A base that reacts only partly with water to form OH⁻, such as ammonia.
percentage ionization
(concentration of acid that ionized ÷ initial acid concentration) × 100%; for a weak acid HA, ([H₃O⁺] ÷ c) × 100%.
⇌ (double arrow)
Shows that a reaction goes in both directions and does not go to completion.
Why can you put vinegar on fries but not hydrochloric acid?
Both are acids. Vinegar (about 0.8 mol/L acetic acid) is used on food. Hydrochloric acid at the same concentration would be dangerous to touch. The difference is not only how much acid is present: it is how much of each acid ionizes. Chemists call this the acid's strength.
Remember from the last lesson
An acid reacts with water to produce H₃O⁺(aq). pH = −log[H₃O⁺]. Electrolytes conduct electricity because of their ions.
Strong acids ionize completely
In a strong acid, essentially every molecule reacts with water:
HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq) (about 100%)
So for a strong monoprotic acid, [H₃O⁺] = the acid concentration. There are only six common strong acids, worth memorizing:
HCl, HBr, HI, HNO₃, H₂SO₄ (first hydrogen), HClO₄
Weak acids ionize only partly
In a weak acid, only a small fraction of molecules ionize at any moment. The rest stay as whole molecules. The ions keep re-forming molecules, so we use a double arrow:
CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO⁻(aq) (about 1% at 0.10 mol/L)
Most acids are weak, including acetic acid (vinegar), citric acid (citrus fruit), carbonic acid (H₂CO₃, in carbonated drinks), phosphoric acid (H₃PO₄, in some colas), hydrofluoric acid (HF) and formic acid (HCOOH, in ant stings).
Strong and weak bases
Strong bases are ionic hydroxides that dissociate completely: the Group 1 hydroxides (LiOH, NaOH, KOH) and Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂ (the part that dissolves dissociates completely).
NaOH(s) → Na⁺(aq) + OH⁻(aq)
Weak bases react only partly with water to form OH⁻. Ammonia is the most common example:
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) (about 1% at 0.10 mol/L)
The carbonate and hydrogen carbonate ions (washing soda, baking soda) are also weak bases.
Evidence for strength (A1.8)
Compare equal concentrations of two acids:
- pH: the strong acid has a lower pH (more H₃O⁺).
- Conductivity: the strong acid conducts much better (many more ions).
- Reaction rate: with magnesium ribbon or a carbonate, the strong acid bubbles much faster.
But with enough base, both acids are completely neutralized by the same amount of base. As OH⁻ removes H₃O⁺, more weak-acid molecules ionize, until all of them have reacted. This is why titration (next lesson) measures the total amount of acid, weak or strong.
Worked exampleExample 1: Percentage ionization
A 0.10 mol/L solution of acetic acid has a measured pH of 2.87. (a) Find [H₃O⁺]. (b) Find the percentage ionization. (c) What would the pH be if acetic acid were a strong acid?
- (a) [H₃O⁺] = 10⁻²·⁸⁷ = 1.35 × 10⁻³ mol/L.
- (b) Each ionized molecule makes one H₃O⁺, so the concentration that ionized = 1.35 × 10⁻³ mol/L. % ionization = (1.35 × 10⁻³ ÷ 0.10) × 100% = 1.3%.
- (c) A strong acid would give [H₃O⁺] = 0.10 mol/L, so pH = 1.00.
[H₃O⁺] = 1.3 × 10⁻³ mol/L; 1.3% ionized; a strong acid would have pH 1.00.
Strength is not the same as concentration
- Strength describes the percentage of molecules that ionize. It is a property of the acid itself.
- Concentration describes how much acid is dissolved per litre.
So you can have a dilute strong acid (0.001 mol/L HCl, pH 3.0) and a concentrated weak acid (glacial acetic acid, about 17 mol/L, which is corrosive). Use the words carefully: "weak" does not mean "safe", and "dilute" does not mean "weak".
Worked exampleExample 2: Classifying from evidence
Two acid solutions, A and B, are both 0.10 mol/L. A has pH 1.0 and lights a conductivity bulb brightly. B has pH 3.4 and lights it dimly. (a) Which is strong? (b) Find B's percentage ionization. (c) Which needs more 0.10 mol/L NaOH to neutralize 25.0 mL? (Both are monoprotic.)
- (a) A: [H₃O⁺] = 0.10 mol/L = the acid concentration, so it is completely ionized: A is strong. B is weak.
- (b) [H₃O⁺] for B = 10⁻³·⁴ = 4 × 10⁻⁴ mol/L; % = (4 × 10⁻⁴ ÷ 0.10) × 100% = 0.4%.
- (c) Neither: they need the same volume (25.0 mL). Both contain 0.00250 mol of acid, and the base removes the weak acid's H₃O⁺ until every molecule has reacted.
A is strong; B is weak (0.4% ionized); both need the same volume of base.
Just for funFormic acid gets its name from formica, the Latin word for ant, because it was first collected from ants. It is a weak acid, but if you've ever been stung by certain ants, you'll agree it is not a weak argument.
Hands-on activityStrong or weak? A fizz-rate comparison (described lab + home version)
In the school lab (teacher-supervised, goggles on): equal 2 cm pieces of magnesium ribbon are dropped into 20 mL of 0.10 mol/L HCl and 20 mL of 0.10 mol/L CH₃COOH. Students record the pH (meter), the conductivity, and the time for the magnesium to dissolve. Typical results: HCl pH about 1, bright conductivity, fast bubbling (dissolves in a minute or two); acetic acid pH about 3, dim conductivity, slow bubbling (takes much longer). With excess acid, the total volume of hydrogen eventually collected is the same, because the same amount of magnesium reacts completely in each (and both flasks hold the same amount of acid).
At home (safe version): you need two clear glasses, white vinegar (acetic acid, weak), lemon juice (mainly citric acid, also weak), water and baking soda. Put 2 tablespoons of each acid in a glass with 2 tablespoons of water. Add ½ teaspoon of baking soda to each at the same time and compare the fizz. Then keep adding baking soda in ¼-teaspoon portions until the fizzing stops, counting portions.
What to look for: both fizz, because both are acids. The number of portions needed depends on how much acid is present (concentration), not on how strong it is. Explain the difference between the lab and home versions using the words strength, concentration and ionization. Never taste, and keep acids away from eyes.
Your turn
Any four: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄.
2Write the equation for the ionization of hydrofluoric acid (weak). Which arrow do you use?
HF(aq) + H₂O(l) ⇌ H₃O⁺(aq) + F⁻(aq), a double arrow because it ionizes only partly.
3What is [H₃O⁺] in 0.025 mol/L HNO₃? Find the pH.
Strong acid: [H₃O⁺] = 0.025 mol/L; pH = −log(0.025) = 1.60.
4A 0.20 mol/L weak acid has [H₃O⁺] = 1.8 × 10⁻³ mol/L. Find the percentage ionization.
(1.8 × 10⁻³ ÷ 0.20) × 100% = 0.90%.
5Is ammonia a strong or weak base? Write its equation.
Weak. NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
6Explain the difference between a 'dilute' acid and a 'weak' acid.
Dilute: a small amount of acid per litre (low concentration). Weak: only a small percentage of its molecules ionize. A dilute acid can be strong (0.001 mol/L HCl), and a weak acid can be concentrated (glacial acetic acid).
7Why does 0.10 mol/L HCl conduct electricity better than 0.10 mol/L acetic acid?
HCl is completely ionized, so it has far more ions to carry charge; acetic acid is only about 1% ionized.
More practice: warm-up
1What does the double arrow ⇌ show?
The reaction is reversible/incomplete: it goes both ways and only partly forms products.
2Is KOH a strong or weak base?
3Name the weak acid in vinegar.
4For a strong monoprotic acid, how does [H₃O⁺] compare with the acid concentration?
5Which has the lower pH: 0.1 mol/L HCl or 0.1 mol/L CH₃COOH?
More practice: on your own
1A 0.050 mol/L solution of a monoprotic acid has pH 1.30. Is it strong or weak? Show your reasoning.
[H₃O⁺] = 10⁻¹·³⁰ = 0.050 mol/L = the acid concentration, so it is 100% ionized: strong.
2A 0.050 mol/L monoprotic acid has pH 3.00. Find its percentage ionization.
[H₃O⁺] = 1.0 × 10⁻³; % = (1.0 × 10⁻³ ÷ 0.050) × 100% = 2.0%.
3Predict which reacts faster with a piece of zinc: 1.0 mol/L HCl or 1.0 mol/L acetic acid. Explain.
HCl. It has a much higher [H₃O⁺] because it is fully ionized, so H₃O⁺ ions collide with the zinc more often.
4Explain why 25.0 mL of 0.10 mol/L acetic acid and 25.0 mL of 0.10 mol/L HCl need the same volume of NaOH for neutralization.
Both contain 0.0025 mol of acid. As OH⁻ uses up H₃O⁺, more acetic acid molecules ionize (and OH⁻ also reacts with the molecules directly), until all of the acid has reacted.
5Rank in order of increasing pH (all 0.10 mol/L): NaOH, HCl, NH₃, CH₃COOH, NaCl.
HCl < CH₃COOH < NaCl < NH₃ < NaOH (about 1.0, 2.9, 7.0, 11.1, 13.0).
Challenge
1A student says, 'Since acetic acid is only 1% ionized, you need 100 times less NaOH to neutralize it than the same amount of HCl.' Explain the error.
Neutralization removes H₃O⁺, which causes more acetic acid to ionize (the reversible reaction shifts) until all the acid has reacted. The amount of base needed depends on the moles of acid, not the percentage ionized at the start.
2Find the pH of 0.10 mol/L HCl after it is diluted 100 times, and compare with diluting 0.10 mol/L acetic acid 100 times (the percentage ionization of 0.0010 mol/L acetic acid is about 12%).
HCl: 0.0010 mol/L → pH = 3.00. Acetic acid: [H₃O⁺] ≈ 0.12 × 0.0010 = 1.2 × 10⁻⁴ → pH ≈ 3.92. Note that diluting a weak acid increases its percentage ionization.
3Sulfuric acid is called a strong acid, but its second ionization (HSO₄⁻ ⇌ H⁺ + SO₄²⁻) is incomplete. Predict whether the pH of 0.10 mol/L H₂SO₄ is greater than, less than, or equal to 0.70 (the pH if both H ionized fully) and explain.
Greater than 0.70 (but less than 1.00). The first H ionizes completely (0.10 mol/L H₃O⁺), and only part of the second ionizes, so [H₃O⁺] is between 0.10 and 0.20 mol/L.
Quiz
1. A strong acid is one that
3. 0.10 mol/L of a monoprotic acid has [H₃O⁺] = 0.0020 mol/L. Its percentage ionization is
5. Equal volumes of 0.10 mol/L HCl and 0.10 mol/L acetic acid need
Big idea (E3.6): strength is the percentage that ionizes; concentration is how much is dissolved. Strong acids and bases ionize or dissociate completely; weak ones only partly.
How to help: Ask your teen to explain why vinegar is safe on fries but hydrochloric acid of similar concentration is not, using 'ionization'. Then ask the reverse: is a strong acid always dangerous? (Very dilute strong acid is not.)
Common difficulties:
- Using 'weak' to mean 'dilute'.
- Assuming [H₃O⁺] equals the concentration for a weak acid.
- Believing weak acids need less base to neutralize.
- Forgetting the double arrow for weak acids and bases.
Safety: the magnesium/acid comparison is a teacher-supervised lab. The home version uses only vinegar, lemon juice and baking soda.