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Changing models of acids and bases
How the definition of an acid changed from Lavoisier's oxygen theory to Davy's replaceable hydrogen, Arrhenius's ions in water and the Brønsted–Lowry proton-transfer model, with the evidence that overturned each one and the limitations of each model, plus the Lewis model as an extension.
The word "oxygen" comes from Greek words meaning "acid-maker". Antoine Lavoisier named the element in the 1770s because he was sure oxygen was what made a substance acidic. He was wrong, and the name has been a fossil of his mistake ever since. The story of acids and bases is a story of good models being replaced by better ones as new evidence turns up, which is exactly how the HSC wants you to discuss it.
Why definitions change
A scientific model is kept as long as it explains the observations and makes useful predictions. When reliable evidence appears that it can't explain, the model is modified or replaced. Each acid–base model below explained more than the one before, and each has limitations you need to be able to state with an example. The HSC questions on this dot point usually ask you to explain a limitation or evaluate a model, so learn one clear piece of evidence for each.
Lavoisier: acids contain oxygen (1776)
Lavoisier burned non-metals such as sulfur, phosphorus and carbon in oxygen and dissolved the oxides in water. The solutions were all acidic. He concluded that oxygen is the essential element in every acid.
Limitations: hydrochloric acid (HCl) and hydrogen sulfide (H₂S) are acidic but contain no oxygen. Many oxides, such as CaO, Na₂O and MgO, contain oxygen but are basic. The model also said nothing about bases. Its value was that it was the first attempt to explain acidity by composition rather than by properties.
Davy: acids contain replaceable hydrogen (about 1810–1815)
Humphry Davy showed that "muriatic acid" (HCl) contained only hydrogen and chlorine, and that other acids without oxygen existed. He proposed that hydrogen is the essential element. Justus von Liebig refined this in 1838: an acid is a compound containing hydrogen that can be replaced by a metal, which matched the reaction of acids with metals to give salts and hydrogen.
Limitations: plenty of hydrogen compounds, such as CH₄ and NH₃, aren't acids, and the model couldn't explain why. It couldn't explain differences in acid strength, and it still gave no definition of a base beyond "something that neutralises an acid".
Arrhenius: ions in water (1887)
Svante Arrhenius studied the electrical conductivity of solutions and proposed that some substances dissociate (ionise) into ions in water. He defined:
- an acid as a substance that ionises in water to produce hydrogen ions, H⁺: HCl(aq) → H⁺(aq) + Cl⁻(aq)
- a base as a substance that dissociates in water to produce hydroxide ions, OH⁻: NaOH(aq) → Na⁺(aq) + OH⁻(aq)
- neutralisation as H⁺(aq) + OH⁻(aq) → H₂O(l), so acid + base → salt + water.
Strengths: it explained why solutions of acids and bases conduct; it explained strength as the extent of ionisation (strong acids ionise fully, weak acids partly); and it explained why every strong acid–strong base pair has the same enthalpy of neutralisation (6C). Arrhenius won the 1903 Nobel Prize in Chemistry for this work.
Limitations:
- It only applies to aqueous solutions. It can't describe reactions in other solvents or in the gas phase, such as NH₃(g) + HCl(g) → NH₄Cl(s).
- It can't explain bases that contain no hydroxide, such as ammonia and sodium carbonate.
- It predicts that every salt is neutral, but solutions of NH₄Cl are acidic and solutions of CH₃COONa and Na₂CO₃ are basic.
- It ignores the solvent. HCl is a strong acid in water but a weak acid in pure ethanoic acid, and doesn't ionise at all in benzene.
- The free H⁺ ion (a bare proton) doesn't exist in water: it is always bonded to water molecules, written as H₃O⁺.
Brønsted–Lowry: proton transfer (1923)
Johannes Brønsted in Denmark and Thomas Lowry in England independently proposed that acid–base reactions are proton transfers:
- an acid is a proton (H⁺) donor
- a base is a proton acceptor.
$$\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)} \qquad \mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$$
In the first reaction water is a base; in the second it is an acid. This model solves every Arrhenius problem: ammonia is a base because it accepts a proton; the gas-phase NH₃ + HCl reaction is a proton transfer; NH₄⁺ is an acid because it can donate a proton to water; the ethanoate ion is a base because it can accept one; and the solvent matters because the solvent's ability to accept or donate protons decides how far the reaction goes. It also introduces conjugate pairs and amphiprotic species, which you'll use in 6E.
Limitations: it requires a proton to be transferred. It can't explain substances that behave as acids without having a proton to donate, such as BF₃, AlCl₃ and SO₃, or reactions such as CaO(s) + SO₃(g) → CaSO₄(s), where an acidic oxide neutralises a basic oxide with no proton involved.
- Start with Lavoisier. How many of the eight observations does the model explain? Which one does it get right?
- Step through Davy and Arrhenius. How many does each explain? Read the reason the sim gives for ammonia under Arrhenius.
- Choose Brønsted–Lowry. Which two observations can it still not explain, and what do they have in common?
- Turn on "Compare all theories". Which is the only observation every model from Davy onwards explains?
Beyond the syllabus: the Lewis model
In the same year as Brønsted and Lowry, Gilbert Lewis proposed a broader definition based on electron pairs: a Lewis base donates a lone pair of electrons to form a coordinate bond, and a Lewis acid accepts one. NH₃ + BF₃ → F₃B–NH₃ is a Lewis acid–base reaction with no proton involved. Every Brønsted–Lowry base is a Lewis base (it uses a lone pair to accept a proton), so the Lewis model includes the Brønsted–Lowry model as a special case. The HSC focuses on Brønsted–Lowry because proton transfer explains everything in aqueous chemistry that you'll be asked to calculate.
Explain one strength and one limitation of the Arrhenius model, using a chemical equation in each case.
Test yourself
Which observation was the strongest evidence against Lavoisier's model of acids?
An acid with no oxygen directly contradicts the claim that oxygen is essential to acidity.
Which reaction is an acid–base reaction under the Brønsted–Lowry model but not the Arrhenius model?
A proton moves from HBr to NH₃ in the gas phase, with no water or OH⁻. The zinc reaction is redox.
A strength of the Arrhenius model was that it
The model was built on conductivity data: acids and bases form ions in water.
Which reaction can't be explained by the Brønsted–Lowry model?
No proton is transferred between CaO and SO₃, but it is a Lewis acid–base reaction.
Label the ammonia and hydrogen chloride demonstration.
Put the models in the order they were proposed.
Arrhenius defined a base as a substance that produces ions in water. His model could not explain why is a base. In the Brønsted–Lowry model an acid is a proton and a base is a proton . The H⁺ ion in water is better written as the ion, H₃O⁺.
Evaluate the Arrhenius and Brønsted–Lowry models of acids and bases. In your answer, refer to at least two chemical reactions and include equations.
Arrhenius defined an acid as a substance that ionises in water to produce H⁺ (HCl(aq) → H⁺(aq) + Cl⁻(aq)) and a base as one that produces OH⁻ (NaOH(aq) → Na⁺(aq) + OH⁻(aq)). This explained why these solutions conduct, explained acid strength as the extent of ionisation, and explained why the enthalpy of neutralisation of every strong acid with every strong base is about −57 kJ/mol, since the reaction is always H⁺ + OH⁻ → H₂O. However, the model is limited to aqueous solutions and to bases containing OH⁻. It can't explain why ammonia is a base, or why NH₃(g) + HCl(g) → NH₄Cl(s) is a neutralisation, and it predicts that all salts are neutral, though NH₄Cl solution is acidic.
The Brønsted–Lowry model defines acids as proton donors and bases as proton acceptors. It explains ammonia as a base, NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), and the gas-phase reaction as a proton transfer from HCl to NH₃. It explains acidic salts, NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq), and the role of the solvent. Its limitation is that it requires a proton, so it can't explain acids such as BF₃ or the reaction CaO(s) + SO₃(g) → CaSO₄(s). Overall the Brønsted–Lowry model is more useful because it explains everything the Arrhenius model does plus reactions outside water and the behaviour of salts, although even it is not complete.