01. A systematic approach

What do the symbols of a chemical equation show?

Outcomes:

> Organise information into a suitable table of results

> Follow instructions to carry out a practical investigation

> Create chemical equations for reactions carried out

> Make assumptions based on observed evidence

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Word equations of the reaction between two ionic compounds will reveal that the cations have changed their relationship to the anions. This type of reaction is called a double displacement reaction.

As an example, consider the reaction between solutions of potassium iodide and lead(II) nitrate:

Activity:

Organisation skills: Keep an organised and logical system of information files/notebooks.

You will be provided with solutions of six ionic compounds in dropper bottles.

To test whether a pair of solutions reacts, place a few drops (no more than ten) of each in the same cavity of a spotting tile. If a compound is slightly soluble, it may precipitate when the solutions mix but then dissolve again. Therefore, leave the mixture for about 10 minutes before recording the presence and colour of any precipitates.

Safety: Wear safety goggles and avoid contact with skin.

Materials and equipment

- 10 cm³ samples of dilute (0.1 M)
  - copper(II) sulfate (aq)
  - sodium carbonate (aq)
  - lead(II) nitrate (aq)
  - potassium hydroxide (aq)
  - barium chloride (aq)
  - iron(II) sulfate (aq)
- spotting tile(s)
- rinse bottle with water
- beaker for waste

Method

1. Design a table to enable you to record the results of systematic tests between each of the solutions and the other five.
2. After testing a set of solutions, rinse the waste mixtures into the beaker. At the end of the activity, dispose of this substance in the laboratory's 'heavy metals waste' bottle.

Analysing results

1. If a pair of solutions formed a precipitate, write the word equation for the reaction.
2. State whether there were any anions that appeared not to form precipitates.
3. Identify where the elements forming cations are located on the periodic table.
4. If a precipitate formed, suggest the identity of the solid product.
5. Can you suggest any general patterns or trends?

The ionic solid produced in a precipitation reaction is very different in its properties from ions in aqueous solutions.

Solubility rules for ionic compounds

Precipitation is a natural occurrence. Most minerals including limestone cave formations and the 'scale' that forms inside electric kettles, are precipitates. Bones and teeth, corals, bird eggs, and mollusc shells are vital examples of precipitation products in living organisms. As you develop familiarity with the 'solubility rules' for common ions, you will recognise this chemistry is everywhere, every day.

DISCUSS

1. State evidence that new ionic compounds were created.
2. Suggest whether the results of these reactions could be reversed using separation techniques based on physical properties.
3. Safety concerns are very important to the practice of science.
  1. Identify which of the solutions you used included 'heavy metals'.
  2. Suggest why you should not dispose of solutions containing heavy metals down the sink.
4. Suggest how chemical and physical reactions resulting in changes of state might be distinguished.
5. Predict what you might observe if you could measure the electrical conductivity of ions in a solution during a precipitation reaction.
6. Evaluate the claim that precipitation reactions are not chemical reactions at all, because the only involve a change in phase of a pair of ions. As the ionic equation reveals, no new substances are made - there is merely a change in phase.²

Citations:

¹“To Celebrate 20,000 Subscribers, Here's the Reaction of Lead Nitrate and Potassium Iodide! • r/Chemicalreactiongifs.” Reddit, www.reddit.com/r/chemicalreactiongifs/comments/14b36q/to_celebrate_20000_subscribers_heres_the_reaction/. Accessed 16 Apr. 2017.

²Termaat, Annie. “Chapter 5.” Chemistry for the IB MYP 4 & 5, Hodder Education, London, 2016, pp. 100–101.

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