Strong & weak acids & bases

8.4 Strong and weak acids and bases (1 hour)

Pause for thought

This sub-topic is really a very basic introduction to the differences between strong and weak as applied to acids and bases. Where it can get slightly tricky is when students are asked to determine the relative strengths of acids and bases from experimental data. This data might be conductivity measurements or more likely the extent of dissociation in water. Standard Level students should understand about acid (and base) dissociation constants as equilibrium constants are covered in the previous topic on equilibrium. but they are not expected to know specifically about pK_a and pK_b. It would not be unreasonable to give Standard Level students the values of K_a for two different weak acids and ask them which is the strongest.

The problem is that Section 21 in the IB data booklet is specifically referred to in the 'Guidance' section of this sub-topic 8.4 (see below). This Section lists the values for the strengths of common acids and bases, but it only gives them as pK_a or pK_b values even though pK_aand pK_b are not on the Standard Level programme.

Some examples are given below:

Either you will need to explain to Standard Level students how pK_a relates to K_a (and how pK_b relates to K_b) or you will need to convert the pK_a and pK_b values to K_a and K_b yourself to give to students or find them in other data sources.

Nature of science

As a result of improved instrumentation advanced analytical techniques have meant that the relative strengths of different acids and bases can be quantified. This has enabled trends and discrepancies (patterns and anomalies) to be found and explained at the molecular level. Further evidence that weak acids exist in equilibria can be obtained from experiments and models

Learning outcomes

After studying this topic students should be able to:

Understand:

Strong acids and bases differ to weak acids and bases in the extent to which they are ionized in aqueous solution.
The conductivity of solutions of strong acids and bases is higher than the conductivity of solutions of weak acids and bases with the same concentration.
Strong acids are good proton doors and have weak conjugate bases.
Strong bases are strong proton acceptors and have weak conjugate acids.

Apply their knowledge to:

Distinguish between strong and weak acids and bases in terms of the different rates of their reactions with metals, metal oxides, metal hydroxides, metal hydrogen carbonates and metal carbonates and in terms of their electrical conductivities when the concentrations of their solutions are equal.

Clarification notes

The terms dissociation and ionization are interchangeable.

A list of weak acids and bases can be found in Section 21 of the data booklet.

International-mindedness

Nothing is listed on the syllabus under this heading.

Teaching tips

This is a relatively straightforward topic to teach as students really only need to grasp the fact that a strong acid is completely dissociated into its ions in water whereas a weak acid is only slightly dissociated. It is then obvious that a strong acid will be a better conductor of electricity than a weak acid with the same concentration.

Make sure they can write the equation for the dissociation of both strong and weak acids in water. For a strong acid use a single arrow and for a weak acid use a reversible arrow. It does not matter whether hydrogen ions are expressed as H₃O⁺(aq) or H⁺(aq) in water - the IB tends to use H⁺(aq) in the programme and in exams but H₃O⁺(aq) is equally acceptable. Students should also be able to write the equilibrium expression for weak acids so that they can compare K_a values.

Weak and strong bases follow a similar logic. For the equation for their reaction with water students must now include water on the reactant side so that OH^–(aq) can be shown as a product, i.e.

NH₃(aq) + H₂O(l) ⇄ NH₄⁺(aq) + OH^–(aq)

Students will need to understand that a strong acid readily loses a proton so its conjugate base will be weak as it will not easily regain the proton. A similar logic can be applied to strong bases which have weak conjugate acids. It can be useful to give them water and alcohol as an example. Water is actually slightly more dissociated than ethanol even though both are extremely weak. This means that the ethoxide ion will be an even stronger base than the hydroxide ion.

Finally, make sure that students understand that when it comes to a titration the same volume of a particular concentration of sodium hydroxide will be required to react with equal volumes of equimolar solutions of either a strong or a weak acid. This is so even though the hydrogen ion concentration in solution will be less for the weak acid. This of course is because more acid will dissociate as the H⁺(aq) ions are neutralised in order to restore the position of equilibrium but students often do not realise this. What will be different is the amount of heat evolved.

Study guide

Page 61

Questions

For ten 'quiz' multiple choice questions with the answers explained see MC test: Strong & weak acids & bases.

For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Strong & weak acid & bases questions.