E. Chemical Systems and Equilibrium · Acid-Base and Solubility EquilibriaBrønsted-Lowry Acids and Bases: Proton Transfer
E3.6 E2.1 A2.2About 70 minutes
Learning goals- I can explain the limits of the Arrhenius theory of acids and bases.
- I can define Brønsted-Lowry acids as proton donors and bases as proton acceptors.
- I can identify the Brønsted-Lowry acid and base in a proton-transfer reaction, including reactions with no water.
- I can recognize amphiprotic substances and write equations showing them act as an acid and as a base.
Words to know
Brønsted-Lowry acid
A molecule or ion that donates a proton (H⁺) to another particle in a reaction.
Brønsted-Lowry base
A molecule or ion that accepts a proton (H⁺). It must have a lone pair of electrons to bond to the proton.
proton transfer
A reaction in which an H⁺ ion moves from one particle (the acid) to another (the base).
hydronium ion
H₃O⁺(aq), a water molecule that has accepted a proton. It is how H⁺ really exists in water.
amphiprotic
Able to act as either a proton donor or a proton acceptor, depending on what it reacts with (water, HCO₃⁻, H₂PO₄⁻, HSO₄⁻). Also called amphoteric when describing acid-base behaviour in general.
monoprotic / polyprotic
A monoprotic acid can donate one proton per molecule (HCl); a polyprotic acid can donate two or more, one at a time (H₂SO₄, H₃PO₄).
A white ring with no water in sight
Put a cotton swab soaked in concentrated hydrochloric acid at one end of a long glass tube and a swab soaked in concentrated ammonia at the other end. Gases drift down the tube from both ends. Where they meet, a white ring of solid ammonium chloride appears:
HCl(g) + NH₃(g) → NH₄Cl(s)
This is clearly an acid reacting with a base. But there is no water in the reaction and no hydroxide ion anywhere. The theory you used in Grade 11 cannot explain it. In this lesson you will meet a better theory, the one chemists use for the rest of this unit and in first-year university chemistry.
(This is a teacher demonstration done in a fume hood: concentrated HCl and ammonia are corrosive and their fumes are harmful. You will only read about it.)
Remember from Grade 11
In Solutions and Solubility (SCH3U) you learned the Arrhenius theory: an acid produces H⁺(aq) in water and a base produces OH⁻(aq). You then used the modified Arrhenius theory: an acid reacts with water to form hydronium, H₃O⁺(aq), and a base such as ammonia reacts with water to form OH⁻(aq). You also calculated pH = −log[H₃O⁺] and met strong and weak acids. All of that still holds. This lesson widens the definitions.
What the Arrhenius theory cannot do
The Arrhenius theory works well for many reactions in water, but it has three limits:
- It only works in water. The HCl(g) + NH₃(g) reaction above has no solvent at all.
- It needs OH⁻ in the base. Ammonia, the carbonate ion (CO₃²⁻) and the acetate ion (CH₃COO⁻) make solutions basic without containing OH⁻. The modified theory patched this by saying they react with water, but it does not say what makes them able to do that.
- It does not explain why some salts are acidic or basic. A solution of ammonium chloride is acidic and a solution of sodium acetate is basic, even though neither contains H⁺ or OH⁻ in its formula. (You will explain these in the lesson on salt solutions.)
The Brønsted-Lowry theory (1923)
In 1923, Johannes Brønsted in Denmark and Thomas Lowry in England proposed the same new idea independently. They focused on the particle that actually moves: the proton. (A hydrogen atom that loses its electron is just a proton, so H⁺ and "proton" mean the same thing.)
- A Brønsted-Lowry acid is a proton donor.
- A Brønsted-Lowry base is a proton acceptor.
- An acid-base reaction is a proton transfer from the acid to the base.
Acids and bases are now defined by what they do in a reaction, not by what they contain. To accept a proton, a base needs a lone pair of electrons that can form a new bond to the H⁺. That is why NH₃ (a lone pair on N), H₂O (two lone pairs on O) and anions such as OH⁻, F⁻ and CO₃²⁻ can all be bases.
The ring in the tube, explained
In HCl(g) + NH₃(g) → NH₄Cl(s), the HCl molecule donates a proton and the NH₃ molecule accepts it with the lone pair on its nitrogen atom. The NH₄⁺ ion that forms and the Cl⁻ ion that is left behind attract each other and form the ionic solid. No water and no OH⁻ are needed: it is simply a proton transfer, so Brønsted-Lowry theory calls it an acid-base reaction.
How to spot the acid and the base
Compare each reactant with the product it becomes:
- The particle that loses one H and gains one negative charge (its charge goes down by 1) is the acid. HCl → Cl⁻, NH₄⁺ → NH₃, H₂O → OH⁻.
- The particle that gains one H and one positive charge (its charge goes up by 1) is the base. H₂O → H₃O⁺, NH₃ → NH₄⁺, CO₃²⁻ → HCO₃⁻.
Always check that the formula changes by exactly one H and the charge by exactly one unit.
Water can do both: amphiprotic substances
Look at what water does in two different reactions:
- With HCl, water accepts a proton and becomes H₃O⁺. Water is acting as a base.
- With NH₃, water donates a proton and becomes OH⁻. Water is acting as an acid.
A substance that can act as either a proton donor or a proton acceptor is amphiprotic. Water is the most important example. Many ions that still have a removable hydrogen and also carry a negative charge are amphiprotic too: the hydrogen carbonate ion HCO₃⁻ (in baking soda), the dihydrogen phosphate ion H₂PO₄⁻, the hydrogen phosphate ion HPO₄²⁻ and the hydrogen sulfate ion HSO₄⁻. Which way they act depends on their partner: with a stronger acid they act as a base, and with a stronger base they act as an acid.
| Arrhenius (modified) | Brønsted-Lowry |
|---|
| Acid | forms H₃O⁺(aq) in water | proton donor |
| Base | forms OH⁻(aq) in water | proton acceptor (has a lone pair) |
| Solvent needed? | yes, water | no: works in any solvent or with none |
| Acid-base reaction | H₃O⁺ + OH⁻ → 2 H₂O | any proton transfer |
| Explains NH₃, CO₃²⁻ as bases? | only by adding a reaction with water | yes: they accept protons |
Polyprotic acids give up protons one at a time
Sulfuric acid (H₂SO₄) is diprotic and phosphoric acid (H₃PO₄) is triprotic. They donate protons one per step, and each step is a separate proton transfer:
H₃PO₄(aq) + H₂O(l) ⇌ H₃O⁺(aq) + H₂PO₄⁻(aq)
H₂PO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO₄²⁻(aq)
HPO₄²⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + PO₄³⁻(aq)
Notice that H₂PO₄⁻ is a product in the first step and the acid in the second step. Each step happens to a smaller extent than the one before, because it is harder to pull a positive proton away from an ion that is already negative.
Beyond this course: first-year university chemistry adds the even broader Lewis theory, in which a base is an electron-pair donor and an acid is an electron-pair acceptor. You will not need it in SCH4U, but notice the link: a Brønsted-Lowry base uses its lone pair to grab a proton.
Worked exampleExample 1: Identify the acid and the base
Identify the Brønsted-Lowry acid and base in each reaction.
(a) HNO₃(aq) + H₂O(l) → H₃O⁺(aq) + NO₃⁻(aq)
(b) CO₃²⁻(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + OH⁻(aq)
(c) CH₃COOH(aq) + NH₃(aq) ⇌ CH₃COO⁻(aq) + NH₄⁺(aq)
- (a) HNO₃ loses an H and becomes NO₃⁻ (charge 0 → −1): HNO₃ is the acid. H₂O gains an H and becomes H₃O⁺ (charge 0 → +1): H₂O is the base.
- (b) CO₃²⁻ gains an H and becomes HCO₃⁻ (charge −2 → −1): CO₃²⁻ is the base. H₂O loses an H and becomes OH⁻: H₂O is the acid.
- (c) CH₃COOH loses the H on its –COOH group and becomes CH₃COO⁻: it is the acid. NH₃ gains an H and becomes NH₄⁺: it is the base. There is no water involved in the proton transfer, so only Brønsted-Lowry theory calls this an acid-base reaction.
(a) acid HNO₃, base H₂O; (b) acid H₂O, base CO₃²⁻; (c) acid CH₃COOH, base NH₃.
Worked exampleExample 2: An amphiprotic ion both ways
The hydrogen carbonate ion, HCO₃⁻, is amphiprotic. Write an equation for HCO₃⁻ acting as an acid with OH⁻, and another for HCO₃⁻ acting as a base with H₃O⁺.
- As an acid, HCO₃⁻ donates a proton. Removing one H⁺ from HCO₃⁻ leaves CO₃²⁻. The OH⁻ accepts the proton and becomes H₂O.
- HCO₃⁻(aq) + OH⁻(aq) → CO₃²⁻(aq) + H₂O(l)
- As a base, HCO₃⁻ accepts a proton. Adding one H⁺ to HCO₃⁻ gives H₂CO₃ (carbonic acid). H₃O⁺ donates the proton and becomes H₂O.
- HCO₃⁻(aq) + H₃O⁺(aq) → H₂CO₃(aq) + H₂O(l)
- Check: in each equation, atoms and charges balance (left −2 = right −2 in the first; left 0 = right 0 in the second).
Acid: HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O. Base: HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O. (The H₂CO₃ then partly breaks down to CO₂ and water, which is why baking soda fizzes with vinegar.)
Worked exampleExample 3: Writing a proton transfer from a description
Hydrofluoric acid, HF(aq), reacts with aqueous ammonia. Write the equation and explain why Arrhenius theory (in its original form) could not classify this as an acid-base reaction.
- HF has a hydrogen it can donate; NH₃ has a lone pair on nitrogen to accept it. So HF is the acid and NH₃ is the base.
- Move one H⁺ from HF to NH₃: HF loses H to become F⁻, and NH₃ gains H to become NH₄⁺.
- HF(aq) + NH₃(aq) ⇌ F⁻(aq) + NH₄⁺(aq)
- Original Arrhenius theory needs a base to release OH⁻, but NH₃ contains no OH⁻ and no OH⁻ appears in this equation.
HF(aq) + NH₃(aq) ⇌ F⁻(aq) + NH₄⁺(aq). NH₃ is a base because it accepts a proton, which is a Brønsted-Lowry idea, not an Arrhenius one.
Just for funBrønsted and Lowry published the same idea in the same year without working together, one in Copenhagen and one in Cambridge. Chemists sometimes joke that the idea of a proton transfer was so good it transferred itself.
Hands-on activityMake a red cabbage indicator and classify kitchen acids and bases
You need: about a quarter of a red cabbage, a knife and cutting board, a heat-safe bowl, hot water from a kettle, a strainer, 6 clear glasses or jars, a teaspoon, and test liquids: white vinegar, lemon juice, club soda, tap water, and baking soda dissolved in water (1 teaspoon in half a cup).
Grown-up note: an adult should do the chopping and pour the hot water. Never taste lab-style tests, and never use or mix cleaning products (bleach, drain cleaner, ammonia cleaners) for this activity.
- Chop the cabbage into small pieces, put it in the bowl and cover it with hot water. Let it stand for 15 to 20 minutes, then strain. The purple liquid is your indicator. (It keeps a few days in the fridge.)
- Pour about 2 cm of indicator into each glass. Add a teaspoon of a different test liquid to each, and keep one glass as plain indicator for comparison.
- Record each colour. Cabbage indicator is roughly red to pink in acids, purple near neutral, and blue to green in bases.
- For each acidic liquid, write a Brønsted-Lowry equation with water. Vinegar: CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO⁻(aq). For the baking soda solution: HCO₃⁻(aq) + H₂O(l) ⇌ H₂CO₃(aq) + OH⁻(aq). Label the acid and the base in each.
- Add a little vinegar to the baking soda glass and watch. Explain the fizzing with Example 2.
What to look for: vinegar and lemon juice turn the indicator red or pink, club soda turns it pink to reddish-purple, tap water stays close to purple, and baking soda turns it blue or blue-green. In the baking soda equation, water is the acid and HCO₃⁻ is the base; in the vinegar equation, water is the base. That is water being amphiprotic in your own kitchen.
Your turn
1Identify the Brønsted-Lowry acid and base: HBr(aq) + H₂O(l) → H₃O⁺(aq) + Br⁻(aq).
HBr loses a proton (becomes Br⁻): acid. H₂O gains a proton (becomes H₃O⁺): base.
2Identify the acid and base: NH₄⁺(aq) + OH⁻(aq) ⇌ NH₃(aq) + H₂O(l).
NH₄⁺ loses a proton and becomes NH₃: acid. OH⁻ gains a proton and becomes H₂O: base.
3Identify the acid and base: HSO₄⁻(aq) + CO₃²⁻(aq) ⇌ SO₄²⁻(aq) + HCO₃⁻(aq).
HSO₄⁻ → SO₄²⁻ (loses H, charge −1 → −2): acid. CO₃²⁻ → HCO₃⁻ (gains H, charge −2 → −1): base.
4Why must a Brønsted-Lowry base have a lone pair of electrons?
A proton has no electrons. To bond to it, the base must supply both electrons of the new bond, so it needs an unshared (lone) pair.
5Write an equation for the dihydrogen phosphate ion, H₂PO₄⁻, acting as a base with water.
H₂PO₄⁻(aq) + H₂O(l) ⇌ H₃PO₄(aq) + OH⁻(aq). H₂PO₄⁻ gains a proton (base); water loses one (acid).
6Write an equation for H₂PO₄⁻ acting as an acid with water.
H₂PO₄⁻(aq) + H₂O(l) ⇌ HPO₄²⁻(aq) + H₃O⁺(aq).
7Is every Arrhenius base also a Brønsted-Lowry base? Explain using NaOH.
Yes. NaOH supplies OH⁻, and OH⁻ accepts a proton (OH⁻ + H₃O⁺ → 2 H₂O), so the OH⁻ ion is a Brønsted-Lowry base. The reverse is not true: NH₃ is a Brønsted-Lowry base but not an original Arrhenius base.
More practice: warm-up
1In one phrase, what is a Brønsted-Lowry acid?
2In one phrase, what is a Brønsted-Lowry base?
3What particle is transferred in every Brønsted-Lowry acid-base reaction?
4What does the word amphiprotic mean?
Able to act as either an acid (proton donor) or a base (proton acceptor).
5Name the ion formed when water accepts a proton.
More practice: on your own
1Identify the acid and base, and explain your reasoning: HCOOH(aq) + CN⁻(aq) ⇌ HCOO⁻(aq) + HCN(aq).
HCOOH becomes HCOO⁻: it lost one H and its charge went from 0 to −1, so HCOOH is the acid. CN⁻ becomes HCN: it gained one H and its charge went from −1 to 0, so CN⁻ is the base.
2Write the product formed when each species accepts one proton: (a) F⁻ (b) HPO₄²⁻ (c) NH₃ (d) H₂O.
Add H and add +1 to the charge: (a) HF (b) H₂PO₄⁻ (c) NH₄⁺ (d) H₃O⁺.
3Write the product formed when each species donates one proton: (a) H₂SO₄ (b) HSO₄⁻ (c) NH₄⁺ (d) H₂O.
Remove H and subtract 1 from the charge: (a) HSO₄⁻ (b) SO₄²⁻ (c) NH₃ (d) OH⁻.
4Which of these can be amphiprotic: Cl⁻, HS⁻, NH₄⁺, HCO₃⁻? Explain.
HS⁻ and HCO₃⁻. Each still has an H to donate and a negative charge with lone pairs to accept a proton. Cl⁻ has no H to donate (it can only accept), and NH₄⁺ has no lone pair (it can only donate).
5The reaction HCl(g) + NH₃(g) → NH₄Cl(s) occurs in air. Explain why it is an acid-base reaction under one theory but not the other.
It is a proton transfer from HCl to NH₃, so it fits Brønsted-Lowry theory. It does not fit the original Arrhenius theory because there is no water, so no H⁺(aq) or OH⁻(aq) is produced.
Challenge
1Sodium hydride, NaH, reacts with water: NaH(s) + H₂O(l) → Na⁺(aq) + OH⁻(aq) + H₂(g). The hydride ion is H⁻. Identify the Brønsted-Lowry acid and base and explain.
Water donates a proton (H₂O → OH⁻), so water is the acid. The hydride ion H⁻ accepts that proton, forming H₂ (H⁻ + H⁺ → H₂), so H⁻ is the base. It has a lone pair (two electrons) that bonds to the proton.
2Write the three successive proton-transfer equations for citric acid, a triprotic acid you can write as H₃Cit, with water. Which species are amphiprotic?
H₃Cit + H₂O ⇌ H₃O⁺ + H₂Cit⁻; H₂Cit⁻ + H₂O ⇌ H₃O⁺ + HCit²⁻; HCit²⁻ + H₂O ⇌ H₃O⁺ + Cit³⁻. H₂Cit⁻ and HCit²⁻ are amphiprotic: each can donate a proton (shown) and each can accept one (going back to the species before it). Water is amphiprotic too.
3Explain why the second ionization of sulfuric acid (HSO₄⁻ → SO₄²⁻) happens to a much smaller extent than the first (H₂SO₄ → HSO₄⁻).
In the second step the proton must leave an ion that is already negative. The negative HSO₄⁻ ion attracts the positive proton more strongly than the neutral H₂SO₄ molecule does, so it gives the proton up less readily. (The first step is essentially complete; the second is only partial.)
Quiz
1. Which statement defines a Brønsted-Lowry base?
2. In HSO₄⁻(aq) + NH₃(aq) ⇌ SO₄²⁻(aq) + NH₄⁺(aq), which species is the Brønsted-Lowry acid?
3. Which species is amphiprotic?
4. What is formed when the carbonate ion, CO₃²⁻, accepts one proton?
5. Why does the reaction HCl(g) + NH₃(g) → NH₄Cl(s) show a limit of the Arrhenius theory?
6. In CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq), water acts as
Big idea (E3.6): in Brønsted-Lowry theory an acid is a proton donor and a base is a proton acceptor, and every acid-base reaction is a proton transfer. This definition does not need water, and it explains why substances such as ammonia and carbonate are bases.
How to help: Write a reaction on paper and ask your teen to circle the particle that loses an H (the acid) and box the particle that gains one (the base). Ask them to check the charges: the acid's charge goes down by one and the base's goes up by one. The red cabbage activity needs an adult for the knife and hot water; keep it to food items only and never use cleaning products.
Common difficulties:
- Thinking acids and bases are fixed labels. Water, HCO₃⁻ and H₂PO₄⁻ can be either, depending on their partner.
- Forgetting to change the charge when adding or removing H⁺ (writing CO₃²⁻ + H⁺ as HCO₃²⁻).
- Removing the wrong hydrogen from acetic acid: only the H on the –COOH group is acidic, so CH₃COOH becomes CH₃COO⁻.
- Assuming the theory replaces Arrhenius entirely. Arrhenius acids and bases are still Brønsted-Lowry acids and bases; the new theory simply covers more cases.