GCSE · Chemistry · Edexcel · Spec 1CH0

Balanced ionic equations

Mix hydrochloric acid with sodium hydroxide solution and four different ions are floating about. Only two of them do anything. Can you tell which?

Chemistry · Ionic equations

Who's reacting, and who's just watching?

Pick an ion, then pick its box. The test: is it the same ion before and after, and still dissolved?

Still to sort

Takes part: goes in the ionic equation (0)

It changes: it ends up joined into water or into a solid.

Where the line is: If the ion ends up as part of a new substance, it takes part.

Spectator: stays dissolved, left out (0)

The same ion, still dissolved, before and after.

Where the line is: A spectator takes no direct part in the reaction, so it is cancelled out of the equation.

8 of 8 still to sort.

Two mixtures, eight ions. Decide which ions actually change and which just sit there dissolved.

Watch out: A spectator isn't 'changed a little'. It takes no direct part at all, so it is left out.

Chemistry · Ionic equations

What belongs in an ionic equation?

Hydrochloric acid reacts with sodium hydroxide solution. The ions in the mixture are H⁺, Cl⁻, Na⁺ and OH⁻.

Which of these is closest to what you think the ionic equation should show?

All four ions go in, because the equation should show everything that is in the mixture.Spectator ions get changed a bit during the reaction, which is why they are crossed out.Spectator ions are left out because they aren't actually in the mixture.Only H⁺ and OH⁻ go in, because they are the ones that change. Na⁺ and Cl⁻ are spectators and are left out.

How sure are you?

I'm sureI think soI'm guessing

All four ions go in, because the equation should show everything that is in the mixture.

This is a common misconception

It feels safer to list everything. But an ionic equation includes only the species that change. Na⁺ and Cl⁻ are the same before and after, so they are left out, and listing them would hide the actual change.

Spectator ions get changed a bit during the reaction, which is why they are crossed out.

This is a common misconception

Spectators are not changed at all. They take no direct part and stay dissolved, so there is nothing to show. That is exactly why they are cancelled.

Spectator ions are left out because they aren't actually in the mixture.

This is a common misconception

They are in the mixture, dissolved in the solution. They are left out because they don't change, not because they are missing.

Only H⁺ and OH⁻ go in, because they are the ones that change. Na⁺ and Cl⁻ are spectators and are left out.

This matches the evidence

Yes. Spectator ions take no direct part in the reaction and stay dissolved, so the ionic equation includes only the ions that change: H⁺ and OH⁻ forming water.

Spectators are in the mixture and unchanged, so they get cancelled. The ionic equation shows only what changes.

Worked example · lead iodide

Problem

Aqueous potassium iodide is added to aqueous lead nitrate. Solid lead iodide forms and potassium nitrate solution remains. Write the balanced ionic equation with state symbols.

Chemistry · Reaction balancer

Make the lead iodide equation balance

Tap + or − under a particle to change its number. Start at 1 of each and watch the I count.

Reactants

Pb²⁺(aq)

I⁻(aq)

Products

PbI₂(s)

Pb1→1✓

I1→2✗

Conservation check: count atoms on both sides.Not yet.

PbI₂ holds two I, so two I⁻ are needed on the left. The counter checks atoms; you check charge. Left: 2+ and two 1− total 0. Right: the solid is neutral, 0. Atoms and charge both balance.

Tap + or − under each molecule to set its coefficient.

Watch out: The 2 goes in front of I⁻ (two iodide ions). It never goes inside the formula PbI₂.

Chemistry · Ionic equations

Your turn: neutralisation

Hydrochloric acid reacts with sodium hydroxide solution. Write the ionic equation with state symbols.

Write the full balanced equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)The acid releases H⁺(aq). The alkali, a soluble base, releases OH⁻(aq) when it reacts with the acid.
missing step

Which line is step 2?

Split the aqueous compounds into ions: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)Split the aqueous compounds into ions: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H⁺(aq) + OH⁻(aq)Split the aqueous compounds into ions: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)Split the aqueous compounds into ions: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → NaCl(s) + H₂O(l)

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

An ionic equation shows only the species that actually change in a reaction in solution.
Spectator ions take no direct part and stay dissolved, so they are left out of the ionic equation.
Have a goMia says: 'A spectator ion isn't even in the beaker, that's why it gets left out.' Is she right?
No. A spectator is in the mixture, dissolved. It is left out because it doesn't change.
Spectator ions stay dissolved in the solution; being left out is about taking no direct part, not about being absent.
To write one: balance the full equation, split aqueous compounds into ions, then cancel the spectator ions.
State symbols (s), (l), (g) and (aq) show each substance's physical state, and which substances change.
Atoms must balance in an ionic equation, and so must charge.
Have a goA student's equation has every type of atom matching on both sides, but the charge is 2+ on the left and 0 on the right. Is it balanced?
No. Charge has to balance as well as atoms.
Matching atoms is only half of the check; an ionic equation must balance in charge too.
Acids release H⁺(aq). Alkalis are soluble bases that release OH⁻(aq) when they react with acids.
In neutralisation, H⁺(aq) and OH⁻(aq) react to form water: H⁺(aq) + OH⁻(aq) → H₂O(l).
With hydrochloric acid and sodium hydroxide solution, the sodium and chloride ions are the spectators.
A precipitation reaction has aqueous reactants and at least one solid product.
Have a goYou mix two aqueous solutions and write out the products. Which state symbol would show that a precipitate has formed?
(s): precipitation gives at least one solid product.
The aqueous reactants are the starting point; it is a solid product, (s), that makes it a precipitation reaction.
For potassium iodide and lead nitrate, potassium and nitrate ions are spectators; Pb²⁺ and I⁻ form solid lead iodide.

The big picture

An ionic equation shows only the species that change in a reaction in solution. Spectator ions take no direct part and stay dissolved, so they are cancelled. You write the full balanced equation, split the aqueous compounds into ions, cancel the spectators and keep what remains, checking that atoms and charge balance. Acid plus alkali gives H⁺(aq) + OH⁻(aq) → H₂O(l); potassium iodide with lead nitrate gives Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).

Key points

1Keep only what changes: spectator ions are cancelled out of an ionic equation.

2The ionic equation must balance in atoms and in charge, with state symbols on every species.

3Acid with alkali: the ions that form the salt are spectators, leaving H⁺(aq) + OH⁻(aq) → H₂O(l).

4Precipitation: lead and iodide ions join as solid lead iodide, Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).

5Quick test for a spectator: the same ion, still dissolved, before and after.

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Worked example

Problem

Show that H⁺(aq) + OH⁻(aq) → H₂O(l) is balanced in both atoms and charge.

⚠ Watch out

Writing every ion from the mixture into the ionic equation, or thinking spectators are changed in some small way. Spectator ions take no direct part and stay dissolved, so they are cancelled out. Only the species that change stay in.

🧠

Memory hook

Same before, same after, still dissolved? Spectator, so cross it out. What is left is the change, so check its atoms and its charge.

✓

Check yourself

Cover the page. Pick one ion from each mixture and decide whether it is a spectator. Say how you know.

Flashcards

(12)

What does an ionic equation show?

Only the species that actually change in a reaction in solution.

What is a spectator ion?

An ion that takes no direct part in the reaction and stays dissolved. It is left out of the ionic equation.

What is the quick test for a spectator ion?

Is it the same ion, still dissolved, before and after? If yes, it is a spectator.

What are the four steps for writing an ionic equation?

Write the full balanced equation. Split the aqueous compounds into ions. Cancel the spectator ions. What remains is the ionic equation.

What must balance in an ionic equation?

The atoms of each type, and the charge.

What do the state symbols (s), (l), (g) and (aq) mean?

Solid, liquid, gas, and aqueous (dissolved in water). They show each substance's physical state.

Which ion do acids release in solution?

The hydrogen ion, H⁺(aq).

What are alkalis, and which ion do they release?

Alkalis are soluble bases. When they react with acids they release hydroxide ions, OH⁻(aq).

What is the ionic equation for neutralisation?

H⁺(aq) + OH⁻(aq) → H₂O(l)

Which ions are the spectators when hydrochloric acid reacts with sodium hydroxide solution?

Sodium ions and chloride ions: the ions that form the salt.

What is a precipitation reaction?

A reaction with aqueous reactants and at least one solid product.

Which ions are the spectators when aqueous potassium iodide reacts with aqueous lead nitrate?

Potassium ions and nitrate ions. The lead ions and iodide ions form the solid lead iodide.

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Who's reacting, and who's just watching?

What belongs in an ionic equation?

Make the lead iodide equation balance

Your turn: neutralisation

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