Salts & its preparation

Chapter 10

Reactions of acids(usually these are method)s used for the preparation of salts

Acids + Metals → Salt + H2

Acids + Carbonates → Salt + CO2 + H2O

Acids + Bases → Salt + H2O

Two other methods include: Precipitation Titration Easy !

How do salts form?Defining salts

Acids + Metals → Salt + H2

HCl + Ca → CaCl2 + H2 (not balanced)

2HCl + Ca → CaCl2 + H2 (balanced)

2HCl + Ca → CaCl2 + H2

(a chloride salt)

(hydrogen has been replaced by a metal)

What are the other types of salts that can form?

Acid Salt

HCl NaCl

HNO3 NaNO3

H2SO4 Na2SO4 or NaHSO4 (acid salt)

H2SO3 Na2SO3 or NaHSO3 (acid salt)

H2CO3 Na2CO3 or NaHCO3 (acid salt)

Acid salts can undergo a further reaction with the alkali to form a “true” salt.

+ NaOH

chloride

nitrate

sulphate

sulphite

carbonate

Ac

id S

alts

co

nta

in H

+

Non-salts

Non-salts do not have an acidic hydrogen that can be removed.

Common examples are:

1. Oxides

2. Hydroxides O2−

No acidic Hpossible

OH−

Why prepare salts?... Uses…

They have many uses…. Fertilizers

e.g. ammonium nitrate NH4NO3

e.g. potassium nitrate KNO3

Food e.g. flavouring, fillers, etc…

Industrial e.g. modifying properties of cement mixtures,

paint formulations, fillers in plastics, inks and medical industry, etc…

Solubility table

Nitrates All soluble

SulphatesAll soluble except C,L,BCalcium, Lead, BariumMnemonics: Chinese-Language B

ChloridesAll soluble except S,M,LSilver, Mercury, LeadMnemonics: Senior Minister Lee

CarbonatesAll insoluble except S,P,ASodium, Potassium, Ammonium

Hydroxides All insoluble except S,P,A + Ba + Ca

• All Group I & Ammonium compounds are

soluble

Other solubilities….

Salt precipitationinsoluble

MCB

Acid + + +

MCB

insolubleM,C,B

soluble

soluble M,C,B

Titration(acid+alkali)

Acid + excess

of 2 soluble aqueous solutions

Salt solution

Pre

par

atio

n o

f S

alts

Crystallize

Method

Method

Method

What must we know before determining a method of preparing them?

E.g. if we want to prepare calcium sulphate (plaster of paris) or calcium chloride, how can we do it?

1st – determine if the salt to be prepared is soluble or insoluble? (if insoluble, precipitation → )

2nd – if soluble, determine the acid required.

3rd – determine if substance (M,C,B) being added to the acid is soluble or insoluble?

4th – determine the method (if M,C,B is soluble →titration

or if M,C,B is insoluble → addition in excess)

Method 1Precipitation

Simply mix two (2) aqueous solutions together to form the insoluble salt.

BaNO3(aq) + Na2SO4(aq) → BaSO4(s) + NaNO3(aq)

insoluble

The precipitate (ppt) is the solid that comes out of solution

Easy !

Method 2Titration

Simply mix two (2) aqueous solutions of known volume together (for complete reaction) to form the soluble salt.

Usually between an acid and a base.

e.g.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Is this possible to titrate?

2HCl(aq) + (NH4)2CO3(aq) → 2NH4Cl(aq) + CO2(g) + H2O(l)

Simple?

Method 2Titration

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

soluble salt

Steps:

1. Titrate acid & base (with indicator)

2. Determine exact quantities that will completely neutralise the acid & alkali.

3. Titrate acid & base again (without indicator)

4. Salt formed can then be crystallised out in pure form.

Acid + excess (M,C,B) → salt + (H2O) + (gas) + excess MCB

Crystallisation:1. Filter the mixture (to remove excess MCB) & obtain

salt solution.2. Evaporate the salt solution to half its volume (by

direct heat or steam bath) until it is saturated/very concentrated.

3. Allow the saturated solution to cool (by air or ice-bath)

4. Filter the crystals (metal salt) that appear on cooling.

soluble salt

Method 3 – most commonly asked. Reactions of acids with Metals, insol Carbonates & insol Bases Tough?

N2004 P2 A6

a. For each salt, suggest the name of the missing reagent and briefly describe how to obtain the solid product from the reaction mixture.

i. Salt to be made: lithium chloride

Reagent 1: dilute hydrochloric acid

Reagent 2: ???

I could obtain solid lithium chloride by:

_________________________________

N2004 P2 A6

a. For each salt, suggest the name of the missing reagent and briefly describe how to obtain the solid product from the reaction mixture.

ii. Salt to be made: barium sulphate

Reagent 1: aq. potassium sulphate

Reagent 2: ???

I could obtain solid barium sulphate by:

_________________________________

N2004 P2 A6

a. For each salt, suggest the name of the missing reagent and briefly describe how to obtain the solid product from the reaction mixture.

iii. Salt to be made: blue CuSO4 crystals

Reagent 1: dilute sulphuric acid

Reagent 2: ???

I could obtain blue CuSO4 crystals by:

_________________________________

N2004 P2 A6

b. Ammonium sulphate can be made by reacting aqueous ammonia with dilute sulphuric acid.

2NH3(aq) + H2SO4(aq) → (NH4)2SO4 (aq)

Calculate the mass of ammonium sulphate that can be made from 51 g ammonia.

2NH3(aq) + H2SO4(aq) → (NH4)2SO4 (aq)

Mr of NH3 = 17

Mr of (NH4)2SO4 = 132

According to the equation,

2 X 17 g of NH3 produces 132 g of (NH4)2SO4

Therefore,

51 g of NH3 produces of (NH4)2SO4

▬► 198 g of (NH4)2SO4

51 g Mass?

g 132 34

51

A sample of hydrated copper(II) sulphate, CuSO4.5H2O, was heated in the apparatus shown below.

On gentle heating, the blue crystals changed to a white solid P and a neutral liquid Q was collected in the tube cooled by cold water. Name the white solid and the neutral liquid.

[2]

Cambridge Qn

Cambridge Qn On stronger heating, the white solid P changed to

a black solid R and a gas was evolved which dissolved in liquid Q to form solution S.

The indicator paper turned red as solution S was formed.

Addition of aqueous barium chloride to solution S gave a white precipitate. Name the white precipitate Give the formulae of the two ions present in

solution S. Name the black solid R. [4]

P gas Colourlesssolution

Which could P be?1.Calcium oxide2.Aluminium sulphate3.Copper(II) carbonate4.Zinc carbonate

Hence, identify all the other unknowns.

+Excess HNO3

White pptExcess NaOH

NaOH

ppt dissolves

N2003 P2 B11

A toilet cleaner contains the acid salt, sodium dihydrogen phosphate, NaH2PO4.

a. Explain why sodium dihydrogen phosphate is both an ‘acid’ and a ‘salt’.

b. Sodium dihydrogen phosphate can be made by reacting sodium hydroxide with phosphoric acid, H3PO4.i. Write an equation for the formation of sodium

dihydrogen phosphate.

ii. Suggest the formula of two other salts formed from sodium hydroxide and phosphoric acid.

N2003 P2 B11

c. Explain why sulphuric acid (pH=1) behaves as a strong acid but ethanoic acid (pH=4) behaves as a weak acid.

d. Describe an experiment, other than measuring pH, that you could carry out to show that sulphuric acid is a strong acid but ethanoic is a weak acid.

State what measurements you would make and what results you would expect.

J2002 P2 A1

CuSO4 KCl K2Cr2O7 KI

KMnO4 MgSO4 NH3 ZnSO4

All the above are in aqueous solution form.Which of them are: Alkaline? Orange in colour? Used to test for oxidising agents? Used to test for sulphur dioxide? Reacts with acidified lead(II) nitrate to give

a yellow precipitate?