GCSE · Chemistry · Edexcel · Spec 1CH0

Dilute/concentrated and weak/strong acids

Vinegar's acid is weak: only about 1 in 100 of its particles splits into ions. Yet a bottle of it can be concentrated. How can both be true?

Chemistry · Acids

Sort the same six acids twice

Pick an acid, then pick its box. When all six are placed, switch to Rule 2 and sort the very same six again.

When this acid is in water, do ALL of its particles split into ions, or only SOME?

Still to sort

Strong acid (0)

Every acid particle splits into ions: it fully dissociates. Hydrochloric, nitric and sulfuric acids are strong.

Where the line is: Strong is about HOW COMPLETELY the particles split, not how much acid there is.

Weak acid (0)

Only some acid particles split into ions: it partially dissociates. Ethanoic, methanoic, citric, phosphoric and carbonic acids are weak.

Where the line is: Weak is about how few particles split, not about how much water there is.

6 of 6 still to sort.

Every acid, when it is dissolved in water, releases hydrogen ions (H⁺). Chemists say it dissociates, or ionises: its particles split up into ions.

Chemistry · Inside the solution

Look inside: a strong acid and a weak acid

View: Strong acid. Showing 1 layer: Hydrochloric acid

View

Switch views to compare

Hydrochloric acid is a strong acid. Every HCl particle has split into a hydrogen ion (H⁺) and a chloride ion (Cl⁻), so not one whole HCl particle is left. That is what 'fully dissociates' looks like.

Chemistry · Acids

Spot the wrong arrow

Write an equation to show each acid dissociating in water. (A single arrow → means a change goes one way only. The reversible arrow ⇌ means it goes both ways.)

A student's answer — which line goes wrong?

Hydrochloric acid is strong: HCl(aq) → H⁺(aq) + Cl⁻(aq)
Sulfuric acid is strong: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
Ethanoic acid is weak: CH₃COOH(aq) → CH₃COO⁻(aq) + H⁺(aq)
All three acids release hydrogen ions, H⁺, when they dissolve in water.

Chemistry · Concentration

Concentration: how much acid in each dm³

the concentration: 5/10. Concentration of sulfuric acid 0.5 mol/dm³. Moles of acid in each dm³ 0.5 mol. Acid particles compared with 0.5 mol/dm³ ×1.0

Concentration of sulfuric acid0.5 mol/dm³Moles of acid in each dm³0.5 molAcid particles compared with 0.5 mol/dm³×1.0

The volume never changes here: it is always 1 dm³ of solution. Only the amount of sulfuric acid dissolved in it changes.

Watch out: Twice as concentrated does not mean twice as strong. Sulfuric acid is a strong acid at 0.5 mol/dm³ and at 1.0 mol/dm³. Every particle fully dissociates either way; at 1.0 there are simply twice as many particles in the same volume.

Predict, then check

Same concentration. Different strength.

Two beakers hold the same volume of acid. One is 0.5 mol/dm³ nitric acid. The other is 0.5 mol/dm³ carbonic acid. Which has the lower pH?

Chemistry · Acids

Weak and concentrated?

A bottle in the prep room is labelled 'Concentrated ethanoic acid'. Ethanoic acid is a weak acid.

Which is closest to what you think right now?

The label must be wrong. Weak means watered down, so a weak acid can't be concentrated.If it's concentrated, it must really be a strong acid.The label is fine. Weak and concentrated answer different questions.It's weak now, but adding more water would make it even weaker.

How sure are you?

I'm sureI think soI'm guessing

The label must be wrong. Weak means watered down, so a weak acid can't be concentrated.

This is a common misconception

That's how we use 'weak' for squash or tea. Chemists borrowed the word and gave it a narrower job: it describes how the acid behaves once it is in water, splitting up only partly. How much water is mixed in is the job of a different word, dilute.

If it's concentrated, it must really be a strong acid.

This is a common misconception

Packing more ethanoic acid into the bottle just gives you more of the same acid, and ethanoic acid only partially dissociates: most of its particles stay whole. More of a weak acid is still a weak acid.

The label is fine. Weak and concentrated answer different questions.

This matches the evidence

Exactly right. The bottle answers two questions: how much acid is packed in (a lot) and how it behaves in water (it only partly splits up).

It's weak now, but adding more water would make it even weaker.

This is a common misconception

Topping it up with water spreads the same acid through more volume, so the label would change to 'dilute'. It would still be ethanoic acid, which only partially dissociates, so it would still be a weak acid.

Two labels, two questions. Strong or weak describes how the acid behaves in water; concentrated or dilute describes how much of it is there. A bottle can honestly say weak and concentrated.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every acid gets asked two separate questions. Sort six acids by one rule, switch the rule, and watch them land somewhere else.

What you need to know

Acids dissociate (ionise) when they are in aqueous solution, dissolved in water, releasing hydrogen ions, H⁺. For example, hydrochloric acid dissociates to form hydrogen ions and chloride ions.
A strong acid fully dissociates to form H⁺ ions in aqueous solution: it is completely ionised. Hydrochloric, nitric and sulfuric acids are strong. The equation uses a single arrow, →.
Sulfuric acid fully dissociates and releases both of its hydrogen ions: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq).
A weak acid only partially dissociates, and the dissociation is reversible, shown with ⇌. Ethanoic, methanoic, citric, phosphoric and carbonic acids are weak. In ethanoic acid only about 1% of the acid particles are ionised.
Concentration is the amount of solute dissolved in a certain volume of solution. Its unit is mol/dm³: the number of moles of acid in each cubic decimetre. 1.0 mol/dm³ sulfuric acid is twice as concentrated as 0.5 mol/dm³.
A concentrated acid has a large number of acid particles per unit volume of water. A dilute acid has fewer acid particles in the same volume.
For a given concentration, a stronger acid contains more H⁺ ions and has a lower pH.

The big picture

Acids release hydrogen ions (H⁺) in water: they dissociate. Every acid solution faces two separate questions. How strong is it? A strong acid (hydrochloric, nitric, sulfuric) fully dissociates, written with →; a weak acid (such as ethanoic, citric or carbonic) only partially dissociates, reversibly, written with ⇌. How concentrated is it? Concentration is the amount of acid in a certain volume, in mol/dm³: concentrated means lots of acid particles per unit volume, dilute means fewer. So an acid can be weak and concentrated. At the same concentration, the stronger acid has more H⁺ ions and a lower pH.

Key points

1Strong or weak: how completely the acid dissociates. Concentrated or dilute: how much acid is in a given volume.

2These are two separate scales, so an acid can be weak and concentrated, or strong and dilute.

3The arrow shows the strength: → for a strong acid, ⇌ for a weak acid.

4Only compare pH by strength when the concentrations are the same. Then the strong acid has more H⁺ ions and the lower pH.

5In chemistry, weak never means watered down. Watered down is dilute.

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Worked example

Problem

Here are three acid solutions. P is 1.0 mol/dm³ ethanoic acid. Q is 0.5 mol/dm³ ethanoic acid. R is 0.5 mol/dm³ hydrochloric acid. (a) Which are strong acids and which are weak? (b) Compare the concentrations of P and Q. (c) Which has the lower pH, Q or R? Explain your answer.

⚠ Watch out

Using weak to mean watered down, the way we do in everyday speech. In chemistry, watered down is dilute. Weak only means the acid partially dissociates into ions, so a weak acid can still be concentrated, and a strong acid can be dilute.

🧠

Memory hook

Strength is how many SPLIT. Concentration is how many FIT. (Split into ions; fit into each dm³.)

✓

Check yourself

Without looking back: say what strong, weak, concentrated and dilute mean for an acid. Then explain why 0.5 mol/dm³ sulfuric acid has a lower pH than 0.5 mol/dm³ citric acid.

Flashcards

(13)

What do acids release when they are dissolved in water?

Hydrogen ions, H⁺. The acid dissociates (ionises) into ions in aqueous solution.

What is a strong acid?

An acid that fully dissociates to form H⁺ ions in aqueous solution. It is completely ionised.

What is a weak acid?

An acid that only partially dissociates to form H⁺ ions in aqueous solution. The dissociation is reversible.

Name three strong acids.

Hydrochloric acid, nitric acid and sulfuric acid.

Name five weak acids.

Ethanoic acid (in vinegar), methanoic acid, citric acid, phosphoric acid and carbonic acid.

What is concentration, and what is its unit?

The amount of solute dissolved in a certain volume of solution. Unit: mol/dm³, the number of moles in each cubic decimetre.

Concentrated or dilute: what's the difference?

A concentrated acid has lots of acid particles per unit volume of water. A dilute acid has fewer acid particles in the same volume.

Which arrow goes in a weak acid's dissociation equation?

The reversible arrow, ⇌, because the dissociation is reversible and only partial. A strong acid gets a single arrow, →.

How many hydrogen ions does each sulfuric acid particle release in water?

Two. Sulfuric acid is strong, so it fully dissociates and releases both: H₂SO₄ → 2H⁺ + SO₄²⁻.

Roughly what fraction of ethanoic acid particles are ionised?

About 1% — roughly 1 particle in every 100.

Can an acid be weak and concentrated at the same time?

Yes. Weak because it only partially dissociates; concentrated because it has lots of acid particles per unit volume.

Two acids have the same concentration. Which has the lower pH?

The stronger one. Same number of acid particles, but more of them have dissociated, so it has more H⁺ ions.

How does 1.0 mol/dm³ sulfuric acid compare with 0.5 mol/dm³?

It is twice as concentrated: twice the moles of acid in each dm³. It is not any stronger.

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Sort the same six acids twice

Spot the wrong arrow

Weak and concentrated?

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