Acids, Bases and Salt Preparations

We can think of neutralisation reactions in terms of proton transfer, with an acid ‘giving away’ its proton to a base. Neutralisation reactions can be used to produce different salts.

 
 

Proton transfer

Acids are a source of hydrogen ions (H+) which can be neutralised by the hydroxide ions (OH-) released by alkalis. In essense, what is happening is that a hydrogen ion is being transferred from the acid to the base. In chemistry, we refer to hydrogen ions as protons and we call this behaviour proton transfer. The acid is a proton donor, releasing a proton which is accepted by a base.

Reactions of acids with metals, bases and carbonates

You need to know what is formed when an acid reacts with either a metal, a base or a carbonate. Make sure you know each of the general equations in coloured boxes.

Magnesium oxide is a base which can neutralise acids. It’s often used as a treatment for neutralising excess stomach acid.

Magnesium oxide is a base which can neutralise acids. It’s often used as a treatment for neutralising excess stomach acid.

Bases

A base is a substance that can neutralise acids. Alkalis are bases which are soluble in water. The following substances can act as bases:

  • metal oxides, such as calcium oxide

  • metal hydroxides, such as magnesium oxide

  • ammonia

Solubility rules

The method for preparing a salt depends on whether it is soluble or insoluble. You need to learn the general rules for predicting the solubility of ionic compounds in water, which are included below:

  • Salts containing sodium, potassium and ammonium are always soluble.
  • All nitrates are soluble.
  • Chlorides are soluble EXCEPT silver chloride and lead chloride .
  • Sulfates are soluble EXCEPT lead sulfate, barium sulfate and calcium sulfate.
  • Carbonates are insoluble EXCEPT for those containing sodium, potassium and ammonium.
  • Hydroxides are insoluble EXCEPT for those containing sodium, potassium and ammonium. Calcium hydroxide is slightly soluble.

Preparing a soluble salt from an insoluble base

Soluble salts can be prepared by reacting an acid with an insoluble base (usually a metal oxide or metal hydroxide). The method is as follows:

  • Heat the acid in a water bath to speed up the rate of reaction.

  • Once the acid is warm, add the insoluble base. Add the solid base bit by bit, until there is some unreacted solid left over (it is in excess).

  • The acid and base will have reacted to produce a soluble salt and water. Filter to remove the excess unreacted base to leave a solution containing only the salt and water.

  • Evaporate off the water by gently heating the solution. Allow to cool and the soluble salt will crystallise.

  • Filter the solid salt, wash and dry.

For example, let’s say we want to prepare crystals of the soluble salt magnesium nitrate. To do this I’d use nitric acid and add an excess of solid magnesium oxide to form magnesium nitrate and water. I’d then need to remove the excess magnesium oxide by filtration, cool to allow crystals of magnesium nitrate to form and evaporate to remove the water. I’d then filter, wash and dry the crystals.

The acid that you will use will depend on what salt you want:

  • Hydrochloric acid produces chloride salts

  • Sulfuric acid produces sulfate salts

  • Nitric acid produces nitrate salts

Preparing a soluble salt using an acid and an alkali

Acids react with alkalis to form a salt and water, so we can also use this reaction to prepare a soluble salt. This time we can’t use an excess of one reactant since both of the things we are reacting are solutions and cannot be separated by filtration. Therefore, we need to carry out a titration to work out exactly how much of each reactant is needed so that there is none left over at the end of the reaction. The steps are:

  • Carry out a titration to work out the precise volumes of acid and alkali needed for neutralisation to occur.

  • Mix the appropriate volumes of acid and alkali so that they react completely and we are left with a solution containing only salt and water.

  • Evaporate off the water by gently heating the solution. Allow to cool and the soluble salt will crystallise.

  • Filter the solid salt, wash and dry.

Preparing an insoluble salt using two soluble salts

Preparing an insoluble salt is referred to as a precipitation reaction because the solid insoluble salt is called a precipitate. Depending on the insoluble salt you want to make, you just need to select the right soluble salts using the solubility rules discussed earlier on this page.

Silver chloride forms a white precipitate. Image: BBC

Silver chloride forms a white precipitate. Image: BBC

Let’s say we want to form the insoluble salt silver chloride. We need to select a soluble chloride salt - the only one we can’t choose is lead chloride. So anything like magnesium chloride, sodium chloride, potassium chloride etc will work. Next, we need to select a soluble salt containing silver. We know that all nitrates are soluble so let’s choose silver nitrate. We can then prepare the insoluble silver chloride using the following method:

  • Dissolve magnesium chloride in a test tube containing water and do the same for the silver nitrate.

  • Once they have both dissolved, we can add the contents of both test tubes to a beaker and mix to stir.

  • A precipitate of silver chloride should form.

  • Remove the precipitate by filtration.

  • Wash the precipitate to remove any solution on the surface and dry in an oven.

Did you know..

Ingesting silver can turn the skin and even the whites of the eyes blue. In the early 1900s, a man treated himself for syphilis by dissolving silver into nitric acid and evaporating the solution. The silver nitrate salt he ingested turned his skin blue and he even made a living as the ‘Blue Man’ in a freak show.

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