Hydrogen Ions and Neutralisation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.31 — Acids as a source of hydrogen ions and alkalis as a source of hydroxide ions in solution
• 2.32 — Alkalis neutralise acids

Specification section 2(f) Acids, alkalis and titrations — also taught in Uses of Metals, and Alloys, Acids, Alkalis and the pH Scale, Titrations, Solubility Rules and Proton Transfer.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The previous page classified acids without ever saying what one is. This page answers that, and the answer is remarkably simple. AN ACID IS A SUBSTANCE THAT PRODUCES HYDROGEN IONS, H⁺, WHEN DISSOLVED IN WATER. That is what hydrochloric, sulfuric and nitric acid have in common, and it is the only thing they have in common.

Look at their formulae and the pattern is visible. HCl, H₂SO₄, HNO₃ — every one begins with hydrogen, and in solution each releases that hydrogen as H⁺ ions. The rest of the molecule differs completely from one acid to another and turns out to be almost irrelevant to the acidity.

An ALKALI is the mirror image: A SUBSTANCE THAT PRODUCES HYDROXIDE IONS, OH⁻, WHEN DISSOLVED IN WATER. Sodium hydroxide, potassium hydroxide and calcium hydroxide all end in OH, and in solution each releases OH⁻ ions.

Think of it like every football team having a goalkeeper

Teams differ in every visible way — colours, formations, players, tactics — and yet every one of them has exactly one goalkeeper, and that shared feature is what makes them all football teams rather than an assortment of groups in matching clothes. Hydrochloric, sulfuric and nitric acid look nothing alike and behave alike, and the hydrogen ion is the goalkeeper. It also explains something the previous page could only record: the pH scale is not measuring 'acidity' as a vague quality but the CONCENTRATION of that one ion, which is why a single number works for every acid there is.

That is the payoff of the ionic definition, and it is worth stating explicitly. pH measures the concentration of hydrogen ions. A low pH means many hydrogen ions; pH 7 means the hydrogen and hydroxide ions are balanced; a high pH means many hydroxide ions and very few hydrogen ions. The scale that looked arbitrary on the previous page is measuring one specific thing.

Statement 2.32 — what happens when an acid meets an alkali

1. The acid solution contains H⁺ ions, and the alkali solution contains OH⁻ ions.
2. When they are mixed, the H⁺ ions and the OH⁻ ions REACT TOGETHER to form water.
3. The ionic equation is H⁺ + OH⁻ → H₂O, and it is the same for every acid–alkali neutralisation there is.
4. As the hydrogen ions are used up the pH rises towards 7, and when they are exactly balanced by the hydroxide ions the solution is NEUTRAL.
5. The other ions present — the chloride from the acid and the sodium from the alkali, say — take no part at all. They are SPECTATOR IONS, and they remain in solution as a dissolved SALT.
6. So a neutralisation always produces a salt and water, whichever acid and alkali were used.

So hydrochloric acid + sodium hydroxide → sodium chloride + water, or HCl + NaOH → NaCl + H₂O. Strip out the spectators and what is left is H⁺ + OH⁻ → H₂O, with the sodium and chloride ions unchanged throughout.

The general word equation is worth learning because it never varies: ACID + ALKALI → SALT + WATER. What changes from one reaction to another is only which salt, and that is decided by which acid and which metal were involved — hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.

Naming the salt, and writing both equations

Sulfuric acid reacts with potassium hydroxide. Name the salt formed, write the word equation, and give the ionic equation for the neutralisation.

1. The metal comes from the alkali: potassium. So the salt's first name is potassium.
2. The second part comes from the acid: sulfuric acid gives SULFATES. So the salt is potassium sulfate.
3. Word equation: sulfuric acid + potassium hydroxide → potassium sulfate + water.
4. Balanced symbol equation: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O — two hydroxides are needed because sulfuric acid supplies two hydrogen ions.
5. The ionic equation is H⁺ + OH⁻ → H₂O, exactly as for every other neutralisation. The potassium and sulfate ions are spectators.

Answer: Potassium sulfate and water — and the ionic equation is the same one as always.

The fact that the ionic equation never changes is the single most useful idea on this page. Thousands of different acid–alkali reactions exist and every one of them is, at the level of what is actually reacting, the same reaction. The salt that appears is simply whatever spectator ions were left over.

One point of vocabulary to be exact about, because it is examined. A BASE is any substance that neutralises an acid — metal oxides, metal hydroxides and ammonia all qualify. An ALKALI is a base that DISSOLVES IN WATER. So every alkali is a base, and most bases are not alkalis: copper oxide neutralises acid perfectly well but does not dissolve, so it is a base and not an alkali.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.31 — the definitions to learn (the key ideas matter, not the exact wording)

Acid

A substance that produces hydrogen ions, H⁺, when dissolved in water.

Alkali

A substance that produces hydroxide ions, OH⁻, when dissolved in water.

Base

Any substance that neutralises an acid — metal oxides, metal hydroxides and ammonia.

Spectator ion

An ion present during a reaction but unchanged by it, taking no part in the chemistry.

H⁺ + OH⁻ → H₂O

Conditions: The ionic equation for EVERY acid–alkali neutralisation. The other ions are spectators and form the salt.

The three acids and the salts they give

• HYDROCHLORIC acid, HCl → CHLORIDES
• SULFURIC acid, H₂SO₄ → SULFATES
• NITRIC acid, HNO₃ → NITRATES
• The metal part of the salt's name comes from the ALKALI or base
• General equation: ACID + ALKALI → SALT + WATER, without exception

Naming the salt from an acid and an alkali

1. Take the METAL from the alkali or base — that is the first word of the salt's name.
2. Take the ending from the ACID — chloride, sulfate or nitrate.
3. Write the word equation: acid + alkali → salt + water.
4. Balance the symbol equation, remembering that sulfuric acid supplies TWO hydrogen ions.
5. The ionic equation is always H⁺ + OH⁻ → H₂O.

A base against an alkali

A. A BASE is any substance that neutralises an acid. Copper oxide is a base, and it does not dissolve in water.

B. An ALKALI is a base that DISSOLVES in water. Every alkali is a base; most bases are not alkalis.

Model answer [4 marks]

Explain what all acids have in common, and what happens when an acid is neutralised by an alkali. [4]

All acids produce hydrogen ions, H⁺, when they are dissolved in water, and it is these hydrogen ions that make a solution acidic. All alkalis produce hydroxide ions, OH⁻, when dissolved in water. When an acid is mixed with an alkali the hydrogen ions react with the hydroxide ions to form water, according to the ionic equation H⁺ + OH⁻ → H₂O. The remaining ions from the acid and the alkali take no part in the reaction and stay in solution as a salt, so a neutralisation always produces a salt and water.

Model answer [3 marks]

Nitric acid reacts with sodium hydroxide. Name the salt formed and write the word equation. [3]

The salt formed is sodium nitrate. The metal part of the name, sodium, comes from the alkali sodium hydroxide, and the ending, nitrate, comes from nitric acid. The word equation is: nitric acid + sodium hydroxide → sodium nitrate + water.

Model answer [3 marks]

Explain why the pH of an acid rises towards 7 as an alkali is added to it. [3]

The pH scale measures the concentration of hydrogen ions in the solution, and an acidic solution has a high concentration of them. As the alkali is added, its hydroxide ions react with the hydrogen ions to form water, so the concentration of hydrogen ions steadily falls. A lower concentration of hydrogen ions means a higher pH, so the pH rises towards 7, reaching it when the hydrogen and hydroxide ions have been exactly balanced.

Not this: Different acids neutralise alkalis by different reactions, which is why they produce different salts.

This: Every acid–alkali neutralisation is the SAME reaction: H⁺ + OH⁻ → H₂O. The different salts are simply the spectator ions that were left over and took no part at all.

Mark-losing trap. An ACID gives H⁺ ions; an ALKALI gives OH⁻ ions. Both definitions specify 'in water'.

Mark-losing trap. The ionic equation is ALWAYS H⁺ + OH⁻ → H₂O, whichever acid and alkali are used.

Mark-losing trap. Every alkali is a base, but most bases are NOT alkalis — an alkali is a base that dissolves.

Mark-losing trap. Sulfuric acid supplies TWO hydrogen ions, so it needs two hydroxides to balance.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which ion do all acids produce when dissolved in water?
2. Grade 7 · Name [2 marks] — Name the salt formed when sulfuric acid reacts with sodium hydroxide.
3. Grade 8 · Write [2 marks] — Write the ionic equation for the neutralisation of nitric acid by potassium hydroxide.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of what happens, in terms of ions, when hydrochloric acid is neutralised by sodium hydroxide solution.
5. 9+ · Analyse [6 marks] — A student says: 'Copper oxide neutralises acid, so copper oxide must be an alkali.' Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who showed hydrogen, not oxygen, was the common factor, and the one whose vocabulary made this explanation sayable. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who showed hydrogen, not oxygen, was the common factor

The definition on this page is the answer to a question Davy forced open. Lavoisier had declared oxygen the essential component of every acid, and Davy's demonstration that hydrochloric acid contains none of it destroyed that. In the same work he pointed towards hydrogen as the thing all acids actually share — the correct answer, arrived at by removing the wrong one. He is the right person to ask what it takes to dismantle a definition proposed by the most authoritative chemist of the previous generation, and how you know what to put in its place.

• “What did chemists think made a substance an acid?”
• “How did hydrochloric acid disprove that?”
• “What did you think acids actually had in common?”
• “How do you replace a definition once you have destroyed it?”
• “Why did the wrong definition survive so long?”

Michael Faraday — the one whose vocabulary made this explanation sayable

This page explains acidity in terms of ions, and the word and the concept in its modern working form are Faraday's. His electrolysis experiments showed that when an acid solution conducts, hydrogen appears specifically at the negative electrode, which means whatever is carrying it must be POSITIVELY charged and mobile. That is the hydrogen ion, observed decades before anyone could say what a charge was made of. He is the right figure for how an explanation can be established experimentally before the underlying particle is known.

• “Why does hydrogen appear at one electrode and not the other?”
• “What does that tell you about what is carrying it?”
• “How did you know the amounts were in fixed ratios?”
• “Why did you choose the words ion, anion and cation?”
• “How much did you think you were still missing?”

Carry on through the course

• Previous lesson: Acids, Alkalis and the pH Scale
• Next lesson: Titrations
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification