Having H in a formula does not make an acid, and having OH does not make a base. CH₄ carries four hydrogens and CH₃OH carries an OH, yet neither gives anything to water. The first acid-base theory asks just one question: what does it release in water?
1. H⁺ in water makes an acid, OH⁻ makes a base
For Arrhenius, an acid is a compound containing hydrogen that gives hydrogen ions (H⁺) in aqueous solution: HCl, HNO₃, H₂SO₄, CH₃COOH. A base is a compound that gives hydroxyl ions (OH⁻) in aqueous solution: NaOH, KOH, Ca(OH)₂, NH₄OH.
The test is what ends up as ions in water, not what is written in the formula. Not every compound containing hydrogen is an acid: CH₄ is not an acid, and CH₃OH is not a base, because it has an OH but gives no OH⁻. Sort the formulas into three boxes below.
sort themwhat it gives in water
H⁺ or OH⁻?
Pick up a formula, then drop it in a box. Ask: in water does it give H⁺, give OH⁻, or give nothing?
Right. Next one.
Not yet. Remember, an OH in the formula does not guarantee OH⁻, and an H does not guarantee H⁺.
All in place. CH₄ gives no H⁺ in water, and CH₃OH has an OH but gives no OH⁻, so both stay outside.
2. Neutralisation: always the same heat
When an acid and a base meet, H⁺ and OH⁻ join to make water: H⁺ + OH⁻ → H₂O. This is neutralisation. When a strong acid and a strong base neutralise, about 57.34 kJ of heat is released per mole of water formed. Whether the acid is HCl or HNO₃ and the base is NaOH or KOH, the reaction underneath is always the same H⁺ + OH⁻ → H₂O, so the value is the same too.
So the sum is short: heat released = moles of water formed × 57.34 kJ/mol. 0.05 mol HCl and 0.05 mol NaOH give 0.05 mol water, and the heat is 0.05 × 57.34 = 2.867 kJ. Whichever side runs short fixes the moles of water. Check it on the calculator; why choosing H₂SO₄ changes the count of H⁺ is in the exam box at the end.
calculatorcount moles of water
heat = n × 57.34 kJ
The sum uses the book's 57.34 kJ/mol; it is not a measurement of yours.
3. Ka and Kb measure strength
How strong an acid is gets measured by an equilibrium constant. For HA ⇌ H⁺ + A⁻ it is Ka = [H⁺][A⁻]/[HA]. For a base, BOH ⇌ B⁺ + OH⁻, it is Kb = [B⁺][OH⁻]/[BOH].
A larger Ka means a strong acid, such as HCl; a smaller Ka means a weak one, such as CH₃COOH. A larger Kb means a strong base, such as KOH and NaOH; a smaller Kb means a weak one, such as NH₄OH.
4. Where the theory stops
The successes first: the theory explains acid-base reactions in aqueous solution, and why an aqueous solution is acidic or basic. But it stops at four places.
Limit 1: away from water, that is, in non-aqueous solutions, the theory fails. Limit 2: it cannot explain why aqueous solutions of CuSO₄, FeSO₄, ZnSO₄ and AlCl₃ are acidic and why that of Na₂CO₃ is basic.
Limit 3: it cannot explain a reaction in the gaseous state, NH₃ + HCl → NH₄Cl. Limit 4: it cannot explain the basic character of metal oxides (CaO), NH₃, amines and aniline, because they have no OH group. Now drop each example into the box of its limit.
limit and examplefour limits
Where the theory stops
Pick up an example, then drop it in the box of its limit.
Right. Next one.
Not there. Ask: is it away from water, in a salt solution, in the gas state, or a basic character with no OH group?
All four limits matched: away from water, salt solutions, the gas state, and basic character without OH.
The next theory, Brønsted-Lowry, picks up exactly these gaps.
5. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
According to Arrhenius an acid is a compound that, in aqueous solution-
A compound that gives OH⁻ is a base, not an acid.
Right. An Arrhenius acid gives H⁺ in aqueous solution.
Accepting an electron pair defines a Lewis acid, not an Arrhenius acid.
Accepting a proton is a Brønsted-Lowry base, not an acid.
In the second, recall the definition.
check yourselfpick one answer
Question 2
If the Ka of an acid is large, the acid is-
Ka measures acid strength; a large Ka means the compound is an acid, not neutral.
A weak acid has a small Ka, like CH₃COOH.
Right. A large Ka means more ionisation, so a strong acid (like HCl).
Ka measures acid behaviour; the measure for a base is Kb.
Read the last one slowly.
check yourselfpick one answer
Question 3
How much heat is released when 0.2 mol HNO₃ and 0.2 mol KOH fully neutralise in aqueous solution? (57.34 kJ per mole of water)
2.867 kJ is the heat for 0.05 mol of water; here the water is 0.2 mol.
57.34 kJ is the heat for 1 mol of water; here 0.2 mol of water forms.
5.734 kJ is the heat for 0.1 mol of water; here the water is 0.2 mol.
Right. Water is 0.2 mol; heat = 0.2 × 57.34 = 11.468 kJ.
The whole note on a few cards.
cut from the notetap to turn over
Six cards
QuestionWhat is an Arrhenius acid? Give examples.
AnswerA compound containing hydrogen that gives H⁺ in aqueous solution; for example HCl, HNO₃, H₂SO₄, CH₃COOH.
QuestionWhat is the heat of neutralisation of strong acids and bases and why is it always the same?
AnswerAbout 57.34 kJ per mole of water; because in every case the reaction is H⁺ + OH⁻ → H₂O.
QuestionHow do Ka and Kb show the strength of an acid or base?
AnswerLarger Ka: strong acid (HCl), smaller: weak (CH₃COOH); larger Kb: strong base (KOH, NaOH), smaller: weak (NH₄OH).
QuestionAre CH₄ and CH₃OH Arrhenius acids or bases?
AnswerNo; not every compound with hydrogen is an acid: CH₄ is not an acid and CH₃OH is not a base.
QuestionWhose basic character is not explained by the Arrhenius theory and why?
AnswerMetal oxides (CaO), NH₃, amines and aniline; because they have no OH group.
QuestionWhat is the concentration of H⁺ in 0.5 M H₂SO₄ solution, and why?
Answer1 M; each H₂SO₄ molecule gives two H⁺ (H₂SO₄ → 2H⁺ + SO₄²⁻), so 0.5 × 2 = 1 M.