Strong and weak acids and bases
The key misconceptions that students have with acids and alkalis are that:
1. Acids can burn and eat material away:
Students think of acids as active agents that damage skin and other materials. The idea develops in young children, who learn to think of acids as “dangerous”. Acids are not perceived as being particulate, but rather continuous matter with special properties.
2. Neutralisation means an acid breaking down
Rather than considering neutralisation as a reaction between an acid and an alkali, students perceive this as removing acid properties. The alkali may stop the action of an acid, or alternatively the acid may break down.
3. A base/alkali inhibits the burning properties of an acid
Students tend to meet acids in formal education well before alkalis, so ideas about these chemicals are relatively under-developed. Although dilute alkalis are in fact more corrosive than dilute acids, students’ perceptions are that they have no corrosive properties, instead acting to or inhibit acids “eating away” other material.
4. Hydrogen ions are present in acids, but acids remain molecular in solution
That hydrogen ions are responsible for acidic behaviour is relatively well-known amongst students. However, a common model for acid behaviour seems to be that hydrogen ions remain in a molecule and “swap partners” or are “displaced” from this molecule by reaction with an alkali or metal.
Teachers need to be aware of students’ difficulties with these acid/base reactions. Students’ problems may arise because acids and alkalis both look like water. Reacting them together needs precision and some way of knowing that neutralisation is complete, so an indicator is required. Addition of this extra chemical adds and extra layer of “mystery”.
In understanding strong and weak acids and alkalis, students need to consider what is happening at the particle level more deeply. They also learn that over time, different theories have been used to define what acids and alkalis are and what is happening at the particle level. It is not surprising that students find these concepts challenging: it took scientists hundreds of years to work out what acids and bases are and how they work!
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.
Links and Resources
This activity can be used with students after they have understood the concepts of 'strength' and 'concentration'. It provides the opportunity for students to compare the properties of a weak organic acid with the common laboratory acids they are more familiar with from earlier in the course.
A more unusual comparison of strong and weak acids is given in Classic chemistry demonstrations (experiement 10): the rates and extent of reaction of hydrochloric acid and ethanoic acid are compared approximately by looking at the heights of froth produced with calcium carbonate.
Titrations are an important form of chemical analysis which can be used to determine the concentrations of either an acid or an alkali. This titration, which can be carried out by students, involves the most commonly used laboratory acid and alkali.
If this is the first time that students have been introduced to titrations they will need some support with using the burette and pipettes - especially using the pipette fillers.
A balanced equation for the reaction must be known also, so that the mole ratio between the acid and alkali in the reaction (the stoichiometry) can be taken into account in the calculation. The balanced equation for this reaction involves a 1:1 ratio of acid to alkali which makes the calculations straightforward.
Investigations to find out how pH changes during neutralisation of an alkali with an acid can be carried out. Titration curves can be plotted by dripping the alkali into the acid at a steady rate and using a pH data logger to record the change in pH with time.This could be extended to show how the titration curve varies between weak and strong acids and bases. Students would expect the pH to change gradually because they do not appreciate the log values involved. The first activity in this list may help to get across this point.
Indicators can also be investigated to identify their colour changes. Use this to explain why you should use single indicators to determine end points rather than mixed indicators, such as universal indicator.
Many single indicators are available for specific pH ranges, such as litmus, phenolphthalein and (screened) methyl orange. Volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator. For example:
• strong acid + strong alkali: any acid base indicator
• strong acid + weak alkali: methyl orange indicator
• weak acid + strong alkali: phenolphthalein.
Students can carry out this titration between a strong acid and a strong base, measuring the pH and temperature after the addition of each aliquot of acid. Plotting the results reveals that the highest temperature and the neutral point coincide. Molecular models can be used to reinforce the reaction between the acid and base, showing that water and a salt are formed.
In discussion, students can be led towards the notion that formation of bonds in water is a source of the energy. Varying the acid to dibasic and tribasic reveals that more energy is released – this can be directly related to the equations for the reactions.
This activity can be used to develop students practical skills, It is an alternative to the earlier titration of sodium hydroxide and hydrochloric acid but does not require as large a quantity of solutions. It might also be an alternative if burettes are not available.
This activity, neutralising a range of different indigestion tablets with hydrochloric acid, provides students with the opportunity to plan and carry out an investigation to compare which is the best value for money. It is important to stress to students that indigestion tablets are only used to neutralise 'excess' stomach acid. The natural pH of the stomach needs to be acidic in order for the digestive enzymes to work. This could lead to a wider discussion of optimum pH or temperature for some reactions to take place.
An alternative investigation could be carrying out a titration to determine whether vinegar sold in supermarkets is being watered down .
This experiment can be used as an extension activity. It provides an opportunity for students to practice calculations and apply what they have learnt. The context could be related to applications of quantitative chemistry used in analytical laboratories to assess the constituents of manufactured chemicals.