6a. Making a soluble salt by neutralising an 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

6. METHODS of MAKING SALTS - salt preparation procedures

6a. A Method of Making a Water Soluble Salt - NEUTRALISATION

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.

Salt solubility affects the method you choose to make a salt, the table below will help you decide on the method

A solubility guide for salts and other compounds

Information required to decide on the method used to prepare a salt

salts and other compounds

solubility?

common salts of sodium, potassium and ammonium ions usually soluble in water
common sulfates (sulphates) usually quite soluble except for calcium sulfate (slightly soluble), lead sulfate and barium sulfate are both insoluble
common chlorides (similar rule for bromides and iodides) usually soluble except for insoluble lead(II) chloride and silver chloride
common nitrates all soluble
common carbonates most metal carbonates are insoluble apart from sodium & potassium carbonate.  Ammonium carbonate is also soluble
common hydroxides most metal hydroxides are insoluble apart from soluble sodium, potassium and ammonium hydroxide

6a. A Method of Making a Water Soluble Salt

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

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

NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(l)

 

top sub-indexsodium hydroxide + sulphuric acid ==> sodium sulphate + water

2NaOH(aq) + H2SO4(aq) ==> Na2SO4(aq) + 2H2O(l)

 

potassium hydroxide + sulphuric acid ==> potassium sulphate + water

2KOH(aq) + H2SO4(aq) ==> K2SO4(aq) + 2H2O(l)

 

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

NaOH(aq) + HNO3(aq) ==> NaNO3(aq) + 2H2O(l)

 

ammonia + nitric acid ==> ammonium nitrate

NH3(aq) + HNO3(aq) ==> NH4NO3(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

Na2CO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + CO2(g)

 

(c) doc b More examples of neutralization equations are given in section 4.

soluble salt preparation from soluble base-acid neutralisationMETHOD (a) procedure for making a soluble salt by neutralising a soluble base (alkali) with an acid.

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. 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

Note (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.

volumetric apparatus

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

 


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