Acids and Bases IB Syllabus > Acids & bases > pH scale 

These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

8.3 - Strong and weak acids and bases


8.3.1: Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity. AIM 8: Although weakly acidic solutions are relatively safe, they still cause damage over long periods of time. Students could consider the effects of acid deposition on limestone buildings and living things.


Strong and weak acids

It is important to understand, and differentiate between, the terms 'concentration' and 'strength' when referring to acids (or bases)

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'. The concentration of an acid (or any other solute ) is measured in moles/litre (moldm-3).

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 completely breaks apart to give ions in solution (100% dissociation) whereas a weak acid only slightly dissociates in solution (perhaps less than 1%)

Strong acid

HCl + H2O H+(aq) + Cl-(aq)
0%   100%

Weak acid

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

Strong and weak acids are defined by the ease with which they lose (or donate) hydrogen ions (protons).

A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H+ (aq) ions from water.

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 dissociate to form more H+ 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 pH values.

A weak base is the same as this, only it accepts protons (H+ ions) and so produces OH- ions from water rather than H3O+.


8.3.2: State whether a given acid or base is strong or weak. Students should consider hydrochloric acid, nitric acid and sulfuric acid as examples of strong acids and carboxylic acids and carbonic acid (aqueous carbon dioxide) as weak acids. Students should consider all group 1 hydroxides and barium hydroxide as strong bases - ammonia and amines as weak bases.


Strong acids showing dissociation

Strong acids dissociate 100% into ions in solution.

Hydrochloric acid

HCl + H2O H+(aq) + Cl-(aq)
0%   100%

Nitric acid

HNO3 + H2O H+(aq) + NO3-(aq)
0%   100%

Sulphuric acid

H2SO4 + H2O 2H+(aq) + SO42-(aq)
0%   100%

Weak acids showing dissociation

Weak acids only partially dissociate into ions. This does not mean that they do not 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.

For example the reaction of ethanoic acid with sodium hydroxide.

The sodium hydroxide reacts with the available hydrogen ions removing them from the right hand side of the equilibrium below. The equilibrium then responds by dissociating some 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 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).

Ethanoic acid

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

Carbonic acid

CO2 + H2O 2H+(aq) + CO32-(aq)
>> 99%   << 1%

Note: Carbonic acid can be considered to be the acid that makes carbonates by reaction with bases. However, is really a solution of carbon dioxide in water. It is slightly dissociated into ions by reaction with water. You may see the formula given for carbonic acid as H2CO3 - this is merely another way of expressing the carbon dioxide solution, the actual molecule of carbonic acid H2CO3 has no real existence

Dissociation demo in strong and weak acids (hydrochloric acid and hydrofluoric acid)

Strong base showing dissociation

Strong bases fully dissociate to give ions in solution. Weak bases only partially dissociate.

The ion responsible for basicity is the hydroxide ion OH-(aq)

Sodium hydroxide

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

Weak base showing dissociation

Ammonia

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

Here it can be seen that the ammonia causes the water molecules to break apart by removing a hydrogen ion (proton) from the water molecule leaving an excess of OH- ions in the solution.

The ammonium ions NH4+(aq) in the solution have no effect on the acidity or basicity, they are merely spectators.

If however, strong base is added to the equilibrium then this adds hydroxide ions to the right hand side and the ammonia equilibrium will be pushed to the left hand side. Free ammonia molecules will be produced in an even greater excess than before. This forms the basis of the preparation of ammonia in the laboratory. Sodium or calcium hydroxide is added to an ammonium salt and the mixture heated. Ammonia gas is produced. (The heating is needed to prevent the ammonia gas dissolving in the water as ammonia is very soluble)


8.3.3 - Distinguish between strong and weak acids and bases and determine the relative strengths of acids and bases using experimental data.


Differentiation between weak and strong acids and bases

Methods for differentiation

pH measurement

If the concentration of the acid is known then measurement of the pH using a pH meter (or similar) will tell us whether or not an acid is strong or weak.


Example: A 0.1 M solution of hydrochloric acid has a pH value of 1.00, while a 0.1 M solution of ethanoic acid has a pH value of 2.87

Comparison of the two pH values shows us that the hydrochloric acid is completely dissociated into ions whereas the ethanoic acid is only partially dissociated into ions


Neutralisation enthalpy

Any strong acid will release approximately - 57 kJ/mol of energy on neutralisation with strong base.

This energy release is due to the formation of water molecules from the H+ ions from the acid reacting with the OH- ions from the base.

H+(aq) + OH-(aq) --> H2O(l)

When a weak acid is neutralised some of the energy is needed to dissociate the molecules to make the hydrogen ions available for neutralisation. The result is a value for neutralisation enthalpy lower than - 57 kJ/mol (numerically lower, not more negative!)

Note that as the weak acid is in equilibrium as the hydrogen ions are 'mopped up' by the base the equilibrium shifts to the right hand side to make more until eventually all of the acid is able to react. This does, however, absorb energy as bonds are being broken to dissociate the weak acid.

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

Conductivity

Any solution's ability to conduct electricity is conditioned by the concentration of ions it contains. A strong acid has more ions than a weak one, and so it's solution will be a better electrical conductor than a weak acid. The same goes for strong/weak bases.

The conductivity may be measured using a power pack and two graphite electrodes connected to an ammeter. The apparatus is assembled and current values measured for a given voltage setting. The strong acids pass more current than the weak acids for the same voltage.

Strong acids : HCl, HNO3, H2SO4. - good conductors - large value for current passing

Weak acids : CH3COOH, H2CO3. - poor conductors - low value for current passing

Strong bases : group 1 hydroxides (ie NaOH etc), or lower group 2 hydroxides Ba(OH)2. - good conductors

Weak bases : NH3, CH3CH2NH2. - poor conductors

Reaction rates

Similarly, the rate of reaction will reveal the strength of an acid. The rate of a chemical reaction is usually proportional to the concentration of the reactants.

As it is the hydrogen ions that are reacting the concentration of these ions at any one time will be less for a weak acid of the same concentration than for a strong acid.

Example: The reaction between magnesium metal and 0.1 M acids (ethanoic acid and hydrochloric acid)

The hydrochloric acid had a hydrogen ion concentration of 0.1 mol/litre. These hydrogen ions can react directly with the magnesium producing hydrogen gas. There will be collisions between the magnesium atoms and the hydrogen ions.

The ethanoic acid has fewer free hydrogen ions in the solution. It has a hydrogen ion concentration of 0.0013 mol/litre consequently there will be fewer collisions between hydrogen ions and magnesium atoms and hence the reaction will be slower.

Experimentally both reaction rates can be determined by measuring the volume of gas released against time and plotting a graph. The slope of the graph will give the reaction rate at that time. (see rates of reaction)

Teacher- Resources

Powerpoint: The pH scale