Preparing a soluble salt by neutralisation

An acid and an alkali react to form a soluble salt in solution. In this experiment, students produce ammonium sulfate from the reaction between ammonia and sulfuric acid. They can then recover this salt by crystallisation.

This is a well-tried standard class experiment, which should take no more than 30 minutes. If reagent solutions are provided in ready-measured quantities in small labelled bottles, the experimental work can start immediately.

Most classes should be able to perform this experiment as a class experiment but, if there are real doubts about safe behaviour or adequate manipulative skills while solutions are being heated, then student-aided demonstrations of these may be more sensible.

Equipment

Apparatus

• Eye protection
• Beaker, 100 cm³
• Glass stirring rod, 15 cm
• Evaporating basin, 50–100 cm³ capacity
• Crystallising dish
• Bunsen burner
• Tripod
• Heat resistant mat
• Pipeclay triangle or gauze

Chemicals

• Sulfuric acid, 0.5 M (IRRITANT), 20 cm³ (in a small bottle)
• Ammonia solution, 1 M, 50 cm³ (in a small bottle)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Dilute sulfuric acid, H₂ SO₄ (aq), (IRRITANT at concentration used) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.
• Ammonia solution, NH₃ (aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.

Procedure

Stage 1

1. Add 20 cm³ of 0.5 M sulfuric acid to the 100 cm³ beaker. Add 1 M ammonia solution slowly with stirring, about 5 cm³ at a time, until the solution smells faintly of ammonia even after stirring. (Students should be reminded how to smell chemicals safely.)

Stage 2

2. Transfer the solution to an evaporating basin and place on a tripod with a pipeclay triangle or gauze.
3. Heat the solution gently over a medium Bunsen flame so that water boils steadily.
4. When about half the water has boiled away, take a drop of the hot solution on the end of a glass rod and let it cool. If the drop crystallises on cooling, the solution is ready for the next stage; if it does not crystallise, keep boiling and repeat the testing until the drop does crystallise on cooling. Do not boil dry.

Stage 3

5. Pour the hot solution carefully into a crystallising dish.
6. Set it aside to cool, with a label attached giving the name of the salt being prepared and the names of the group.
7. Leave the crystallising dish in a warm place, safe from interference, until it has produced a good crop of crystals.
8. If necessary, filter the solution, collect the crystals from the filter paper onto a paper towel, and allow to dry.

Teaching notes

There may be problems associated with younger students heating evaporating dishes perched on tripods, and with lifting hot apparatus off a hot tripod after heating. If there is any doubt about the safety of this step, you should lift each evaporating dish down onto the heat resistant mat first, before the students pour into the crystallising dish.

It is very important not to allow the students to boil the solution dry. If they do, the ammonium sulfate starts to decompose and release corrosive and toxic fumes into the laboratory.

The concentration of ammonia solution used is below that necessary for carrying a warning label. Nevertheless it is sensible to remind students to keep their faces away from the bottle of ammonia when it is open, and to replace the lid on the bottle whenever it is not in use.

The balanced equation for the reaction is only relevant for older students:

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Otherwise, a simple word equation is sufficient.

If a comparison with other acid-alkali neutralisations is made, then the equation in which ammonia solution is represented as NH₄ OH(aq) could be used, also showing water as one of the products of neutralisation:

2NH₄OH(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq) + 2H₂O(l)

Questions for students

The following questions relate to stage 3 of the procedure:

1. What substance do you think these crystals are? Give a reason for your answer.
2. Write word and symbol equations for the reaction you have done.