IB Colourful Solutions in Chemistry

Strong and weak acids and bases are characterised by their chemical and physical properties.

Syllabus reference R3.1.6

Reactivity 3.1.6 - Strong and weak acids and bases differ in the extent of ionization.

Recognize that acid–base equilibria lie in the direction of the weaker conjugate.

Guidance

HCl, HBr, HI, HNO₃ and H₂SO₄ and are strong acids, and group 1 hydroxides are strong bases.
The distinction between strong and weak acids or bases and concentrated and dilute reagents should be covered.

Tools and links

Reactivity 2.3 - How would you expect the equilibrium constants of strong and weak acids to compare?
Reactivity 1.1 - Why does the acid strength of the hydrogen halides increase down group 17?
Tool 1, Inquiry 2 - What physical and chemical properties can be observed to distinguish between weak and strong acids or bases of the same concentration?

Concentration
Strength
Bases
Neutralisation
Worked examples

Concentration

It is important to understand, and differentiate between, the terms 'concentration' and 'strength' when referring to acids (or bases). Unfortunately, these two terms are often used interchangeably when dealing with acids and bases, leading to confusion.

The term ' Concentration' means the number of moles of a dissolved substance per litre (mol dm^-3) of aqueous solution. It is a measure of the dilution of the acid (or base). A more concentrated solution contains more moles of solute.

A concentrated acid may be either weak or strong depending on its nature. Similarly, a strong acid may be concentrated or dilute, depending on the amount of water with which it is mixed.

Note: The term 'acid concentration' may cause confusion if there is more than one hydrogen atom which can be released from the acid molecule in solution.

For example, a 1.0 mol dm^-3 solution of sulfuric acid (H₂SO₄) actually has a hydrogen ion concentration of greater than 1.0 mol dm^-3, as there are two ionisations that take place:

First dissociation of sulfuric acid

H₂SO₄ → HSO₄^- + H⁺

Second dissociation of sulfuric acid

HSO₄^- ⇋ SO₄^2- + H⁺

The second ionisation does not go 100% to the right hand side, so the molarity of the hydrogen ions lies somewhere between the molarity of the sulfuric acid and 2 x the molarity of the sulfuric acid.

The term 'concentrated' may also be used to indicate acid that has a high a molarity as feasibly possible. Approximate values are shown below for the concentrations normally commercially available. The percentage values are in weight/volume (w/v):

Concentrated Reagents	Density g/cm³	Molarity (M)	Volume in cm³ required to make 1dm³ of 1M solution
Ethanoic acid - 99.5%	1.05	17.4	57.5
Hydrochloric acid - 36%	1.18	11.65	85.8
Hydrochloric acid - 32%	1.16	10.2	98.0
Hydrofluoric acid - 40%	1.13	22.6	44.2
Nitric acid - 70%	1.42	15.8	63.3
sulfuric acid 98%	1.84	18.4	54.3

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Strength

'Strength' refers to the degree of ionisation or dissociation of the acid or base.

Acidity is caused by the presence of hydrogen ions in the solution. If ANY acid has a high concentration then this will consequently increase the hydrogen ion concentration making the acid "stronger in layman's terms". When chemists refer to strong and weak acids they are referring to the degree with which the acid molecules break apart to give ions in aqueous solution (dissociation).

A strong acid, when placed in water, will ionise/dissociate virtually 100%, producing H₃O⁺ ions from water.

Strong acid

HCl + H₂O ⇋ H₃O⁺(aq) + Cl^-(aq)
0% ⇋ 100%

A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution. An equilibrium is established, and so when some of the H₃O⁺ ions produced by a weak acid react, Le Chatelier's principle means that more of the acid will react to form H₃O⁺ ions.

This means that, given an equal number of moles of acid, they will be neutralized by the same amount of strong base, but their solutions will have different concentrations of H+ ions and hence different pH values.

Weak acid

CH₃COOH + H₂O ⇋ H⁺(aq) + CH₃COO^-(aq)
approx 99% ⇋ approx 1%

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Bases

A strong base dissociates 100% in solution releasing OH- ions.

Sodium hydroxide

NaOH + H₂O ⇋ Na⁺(aq) + OH^-(aq)
0% ⇋ 100%

A weak base accepts protons (hydrogen ions) from the water and in doing so, leaves some free OH^- ions in solution. The stronger the base the higher the concentration of free hydroxide ions in the solution.

Ammonia

NH₃ + H₂O ⇋ NH₄⁺(aq) + OH^-(aq)
> 99% ⇋ < 1%

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Neutralisation

Strong acids dissociate 100% into ions in solution, whereas weak acids only partially dissociate into ions. This does not however mean that weak acids fail to react fully with bases. Le Chatelier's principle tells us that if we remove one of the components from one side of an equilibrium, the equilibrium responds by making more.

Question How is the equation for a neutralisation reaction affected by the degree of dissociation of the acid or the base?

The stoichiometry of any reaction is unaffected by the strength of the acid or base.

For example the reaction of ethanoic acid with sodium hydroxide.

CH₃COOH + NaOH → CH₃COONa + H₂O

Sodium hydroxide reacts with the available hydrogen ions removing them from the right hand side of the ethanoic acid equilibrium below:

CH₃COOH ⇋ CH₃COO^- + H⁺

and

NaOH + H⁺ → Na⁺ + H₂O

The equilibrium then responds by dissociating more ethanoic acid molecules to make more hydrogen ions. These then react with the sodium hydroxide and get removed from the equilibrium. The equilibrium makes more hydrogen ions by dissociating even more ethanoic acid, and so on until all the ethanoic acid has been neutralised by the sodium hydroxide.

The net result is that all the ethanoic acid reacts with the sodium hydroxide (if there is enough sodium hydroxide).

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Worked examples

Q841-01 Consider a weak acid HA dissolved in water.

HA(aq) + H₂O(l) ⇋ H₃O⁺(aq) + A-(aq)

Which of the following statements is/are correct?

I. A-(aq) is a much stronger base than H₂O(l)
II. HA dissociates only to a very small extent in aqueous solution.
III. The concentration of H₃O⁺ is much greater than the concentration of HA(aq)

I, II and III
II and III only
I and II only
I and III only

Answer

The weak acid equilibrium lies to the left hand side, thus hydrogen ions are held to A- more tightly than they are to water molecules. Hence A-(aq) is a much stronger base (proton acceptor) than H₂O(l). I is true

As a weak acid, HA only partially dissociates. II is true

As explained above the equailibrim lies to the left hand side therefore the concentration of HA is greater than the concentration of H₃O⁺. III is false.

Correct response I and II only

Q841-02 Which compound will produce a solution with a pH of greater than 7 when added to water?

NaHCO₃(s)
SiO₂(s)
SO₃(g)
CH₃CO₂H(aq)

Answer

Of the compounds shown, the only basic substance is sodium hydrogen carbonate, NaHCO₃(s).

Q841-03 When the following 0.10 mol dm^-3 solutions are arranged in order of increasing pH (lowest first) what is the correct order?

NH₃(aq), NaOH(aq), HCl(aq), CH₃COOH(aq)
NaOH(aq), NH₃(aq), CH₃COOH(aq), HCl(aq)
HCl(aq), CH₃COOH(aq), NH₃(aq), NaOH(aq)
HCl(aq), CH₃COOH(aq), NaOH(aq), NH₃(aq)
NaOH(aq), NH₃(aq), HCl(aq), CH₃COOH(aq)

Answer

Hydrochloric acid is a strong acid whereas ethanoic acid is weak. Sodium hydroxide is a strong base while ammonia is a weak base. The correct order of increasing pH is HCl(aq), CH₃COOH(aq), NH₃(aq), NaOH(aq)

Q841-04 When the following 1.0 mol dm^-3 solutions are listed in increasing order of pH (lowest first) what is the correct order?

HNO₃ < H₂CO3 < NH₃ < Ba(OH)₂
NH₃ < Ba(OH)₂ < H₂CO₃ < HNO₃
Ba(OH)₂ < H₂CO₃ < NH3 < HNO₃
HNO₃ < H₂CO₃ < Ba(OH)₂ < NH₃

Answer

Strong acids include nitric, hydrochloric and sulfuric acids. Barium hydroxide is a strong base whereas ammonia is weak. The correct order of increasing pH is HNO₃ < H₂CO3 < NH₃ < Ba(OH)₂

Q841-05 Four flasks labelled A, B, C and D each contain equal volumes of hydrochloric acid of different concentrations. When equal volumes of 1M sodium hydroxide are added to each flask the pH values below are produced.

Flask	A	B	C	D
pH	1	5	7	13

Which flask contained the most concentrated hydrochloric acid initially?

Answer

More concentrated = lower pH for an acid, hence flask A contains the most concentrated acid.

Q841-06 Aqueous solutions of each of the following have a concentration of 0.100 mol dm^-3. Which has the highest pH?

HCl
CH₃COOH
NaOH
NH₃

Answer

The highest pH means the weakest acid, or conversely the strongest base.

In this case NaOH is the strongest base.

Q841-07 Separate 20.0 cm³ solutions of a weak acid and a strong acid of the same concentration are titrated with NaOH solution. Which will be the same for these two titrations?

I. Initial pH
II. pH at equivalence point
III. Volume of NaOH required to reach the equivalence point

I only
III only
I and II only
II and III only

Answer

The initial pH is different as the acids are weak and strong. The pH at the equivalence point is different, however the same volume of NaOH is required to reach the equivalence point. Response B

Q841-08 Four aqueous solutions I, II, III and IV are listed below:

I. 0.100 mol dm^-3 HCl
II. 0.010 mol dm^-3 HCl
III. 0.100 mol dm^-3 NaOH
IV. 0.010 mol dm^-3 NaOH

What is the correct order of increasing pH of these solutions?

Answer

A dilute acid has a higher pH than a more concentrated acid, therefore I < II. Conversely a more concentrated base has a higher pH than a dilute base, thus IV < III.

The correct order of increasing pH is I < II < IV < III

Q841-09 Solutions of hydrochloric acid (HCl(aq)) and ethanoic acid (CH₃COOH(aq)) of the same concentration reacted completely with 5.0 g of calcium carbonate in separate containers. Which of the following statements is correct?

CH₃COOH(aq) reacted slower because it has a lower pH than HCl(aq)
A smaller volume of CO₂(g) was produced with CH₃COOH(aq) than with HCl(aq).
A greater volume of CO₂(g) was produced with CH₃COOH(aq) than with HCl(aq).
The same volume of CO₂(g) was produced with both CH₃COOH(aq) and HCl(aq).

Answer

Both of the acids are the same concentration and volume they have the same number of moles and will produce the same volume of gas when reacting with the same mass of calcium carbonate. Response D

Q841-10 A household cleaner contains aqueous ammonia. A 2.447g sample of the cleaner is diluted with water to 20.00 cm³. This solution requires 28.51 cm³ of 0.404 mol dm^-3 sulfuric acid to reach the equivalence point.

a) Write a balanced equation for the reaction between sulfuric acid and ammonia.

b) Calculate the moles of sulfuric acid required for this reaction and the moles, mass and percentage mass of ammonia in the household cleaner.

Answer

Balanced equation for the reaction between the weak base ammonia and the strong acid sulfuric acid:

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

From the titration, moles of sulfuric acid = 0.02851 x 0.404 = 0.0115 moles of sulfuric acid

From the equation this is equivalent to 2 x 0.0115 = 0.0230 moles of ammonia

Hence the mass of ammonia in the original sample = 0.0230 x 17 = 0.392 g

Hence the percentage by mass in the original sample = 0.392/2.447 x 100% = 16%

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