GCSE Chemistry Notes: Preparation of a salt by an acid-alkali neutralisation reaction
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6a. Making a soluble salt by neutralising a
soluble acid and
an alkali (a soluble base)
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Method (a) Making a salt by neutralising a soluble acid with a soluble base (alkali) - neutralisation reaction (this page)
Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level & ~US grades 8, 9, 10 school science courses for ~14-16 year old science students for national examinations in chemistry topics including acids bases alkalis salts preparations reactions
6. METHODS of MAKING SALTS - salt preparation procedures
6a. A Method of Making a Water Soluble Salt - NEUTRALISATION
The ionic theory of neutralisation is described and explained in section 2.
Salt solubility affects the method you choose to make a salt, the table below will help you decide on the method
6a. A Method of Making a Water Soluble Salt
One important point is to recognise that all the reactants are soluble here, which is why you need a titration procedure to work out how much of the acid is to be added to a given volume of alkali.
e.g. the hydroxide of an alkali metal like sodium hydroxide, potassium hydroxide or ammonia solution (wrongly called )ammonium hydroxide. Steps (1) to (3) below is called a titration.
Typical common soluble bases (alkalis) used for preparing soluble salts:
NaOH sodium hydroxide, KOH potassium hydroxide and some soluble carbonates
Typical examples shown by the word and symbol equations below include ...
METHOD (a) procedure for making a soluble salt by neutralising a soluble base (alkali) with an acid.
Don't forget to wear safety glasses or goggles when doing this preparation.
You need to know the exact amount of acid to just neutralise completely the alkali (soluble base).
(1) A known volume of acid is pipetted into a conical flask and universal indicator added. The acid is titrated with the alkali from the burette.
(2) The acid is added until the indicator turns green, pH 7 neutral. This means all the acid has been neutralised to form the salt. I've illustrated the method using universal indicator BUT it isn't that accurate an indicator for titrations.
You should use a more precise indicator like phenolphthalein or methyl orange. I didn't repeat all the titration details here again, I've just kept to the basic ideas and description, but there lots of detailed examples on another page in the calculation sections (more examples - diagrams, descriptions of titration procedures, calculations)
(3) The volume of alkali needed for neutralisation is then noted, this is called the endpoint volume. (1)-(3) are repeated with both known volumes mixed together BUT without the contaminating indicator, such as phenolphthalein or methyl orange.
(4) The solution is transferred to an evaporating dish and heated to partially evaporate the water causing crystallisation or can be left to very slowly evaporate - which tends to give bigger and better crystals.
(5) The residual liquid can be decanted away and the crystals can be carefully collected and dried by 'dabbing' with a filter paper OR the crystals can be collected by filtration and dried (as above).
See also the preparation of ammonium salts using this method.
Extra guidance notes
GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids
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