6a. Making a soluble salt by neutralising a soluble acid and an alkali (a soluble base)

Doc Brown's Chemistry GCSE/IGCSE/O level Science-Chemistry Revision Notes

The pH scale of acidity and alkalinity, acids, alkalis, salts and neutralisation

6. Methods of making salts

6a. Making a soluble salt by neutralising an acid and an alkali (soluble base)

Method (a) Making a salt by neutralising a soluble acid with a soluble base (alkali) - neutralisation reaction (this page)

Method (b) preparing a salt by reacting an acid with a metal or with an insoluble base - oxide, hydroxide or carbonate

Method (c) Preparing an insoluble salt by mixing solutions of two soluble compounds

Method (d) Making a salt by directly combining its constituent elements

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

See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index

6a. METHOD (a) Neutralising a soluble acid with a soluble base (alkali) to give a soluble salt

 The ionic theory of neutralisation is described and explained in section 2.

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 ...

sodium hydroxide + hydrochloric acid ==> sodium chloride + water

NaOH_(aq) + HCl_(aq) ==> NaCl_(aq) + H₂O_(l)

sodium hydroxide + sulfuric acid ==> sodium sulfate + water

2NaOH_(aq) + H₂SO_4(aq) ==> Na₂SO_4(aq) + 2H₂O_(l)

 

potassium hydroxide + sulfuric acid ==> potassium sulfate + water

2KOH_(aq) + H₂SO_4(aq) ==> K₂SO_4(aq) + 2H₂O_(l)

sodium hydroxide + nitric acid ==> sodium nitrate + water

NaOH_(aq) + HNO_3(aq) ==> NaNO_3(aq) + 2H₂O_(l)

 

ammonia + nitric acid ==> ammonium nitrate

NH_3(aq) + HNO_3(aq) ==> NH₄NO_3(aq)

 

potassium hydroxide + hydrobromic acid ==> potassium bromide + water

KOH_(aq) + HBr_(aq) ==> KBr_(aq) + H₂O_(l)

 

sodium carbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide

Na₂CO_3(s) + 2HCl_(aq) ==> 2NaCl_(aq) + H₂O_(l) + CO_2(g)

 

More examples of neutralization equations are given in section 4.

(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.

You can use any volumes as long as they are the same ratio from the initial titration.

All that's left in solution is the salt.

(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.

This evaporation can be done safely with an electric heater of a hot water bath. Then hot concentrated solution is left to cool and crystallise.

(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.

AND also more examples - diagrams, descriptions of titration procedures

Extra guidance notes

(i) You can put the acid in the burette and the alkali in the flask.

(ii) Parts (1) to (3) are known specifically as an acid-base (alkali) titration, and the general method is known as a volumetric titration by which it possible to find out exactly what volume ratios are needed for neutralisation.

So knowing one concentration, you can calculate the other.

(iii) Concentration calculations are on calculations pages sections 11. and 12.

(iv) Apparatus used: (1) pipette and conical flask; (2)-(3) burette and conical flask; (4) evaporating (crystallising) dish, bunsen burner, tripod and gauze; (5) filter paper.

(v) Other indicators e.g. phenolphthalein can be used instead (pink alkaline, colourless acid).

(vi) The burette and pipette are both used for the accurate measurement of volume.

(vii) The pH changes in this preparation are described in section 7

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LINKS

• Water of crystallisation is dealt with in section 8.

• Salt solubility affects the method you choose to make a salt and so  section 8. contains tables of information-data on salt solubility which will help you decide on the method to prepare a salt.

• Multiple choice revision quizzes and other worksheets

• GCSE/IGCSE foundation-easier multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE higher-harder multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE Structured question worksheet on Acid Reaction word equations and symbol equation questions

•  Word equation answers and symbol equation answers)

• GCSE/IGCSE word-fill worksheet on Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE matching pair quiz on Acids, Bases, Salts and pH

• See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index

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