CHEMISTRY FORM 4

CHAPTER 8 : SALTS

8.1 SYNTHESISING SALTS

A salt is a compound formed when the hydrogen ion, H+ from an acid is replaced by a metal ion or an ammonium ion, NH4+

ExamplesHCl + NaOH NaCl + H2O2HNO3 + Zn Zn(NO3)2 + H2

H2SO4 + MgCO3 MgSO4 + H2O + CO2 Complete the table below

MetalSulphate saltChloride saltNitric AcidCarbonate saltIon(From H2SO4)(From HCl)(From HNO3)(From H2CO3)Na+

K+

Zn2+

Mg2+

Ca2+

Fe2+

Fe3+

Cu2+

NH4+

Ba2+

Al3+

Pb2+

Ag2+

INSOLUBLE SALT

1Solubility of salt.

All K+ , Na+ and NH4+ salts are soluble All nitrate salts are soluble

All carbonate are soluble except K2CO3, Na2CO3 and (NH4)2CO3 All Sulphate salts are soluble except BaSO4 , CaSO4, and PbSO4 All chloride salts are soluble except AgCl2 and PbCl2

The preparation of salt on its solubility of the salt and the nature of the reaction.

Preparation of salt

METHOD 1SolubleMETHOD 2

METHOD 3

Insoluble

K+ , Na+ NH4+

Acid + Alkali Salt + H2O

Titration of acid and alkali Other than K+ , Na+NH4+ salts

Acid + basic oxide Salt + water Acid + metal Salt + Hydrogen gasAcid + Metal Carbonate Salt + Water + CO2 Mix two solution containing cations and anions of insoluble salts Stir using glass rod Filter using filter funnel Rinse the residue with distilled water

Dry the residue with filter paper

Add acid to the alkali until it is neutral with the aid of an indicator

Repeat the procedure without using an indicator

Add basic oxide fillings/metals/metal carbonate to the acid and heat it until some of it no longer dissolves. (This mean all acid has reacted with the basic oxide/metal/metal carbonate)

Filter to move excess solid residue

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- Gently heat the filtrate to obtain a saturated solution (one-third of its original volume remains)

- Cool the hot saturated solution to allow it to crystallize

- Filter and dry the crystals by using filter paper

TRY THIS 1

Write all possible chemical equation to prepare soluble salts and chemical equation for insoluble salt

*******KEY*******

S = SOLUBLE SALT IS = INSOLUBLE SALT

SALTSOLUBILITYCHEMICAL EQUATIONZinc Chloride

Sodium

Nitrate

Silver Chloride

Copper(II)

Sulphate

Lead(II)

Sulphate

Aluminium

Nitrate

Lead(II)

Chloride

Magnesium

Nitrate

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Potassium

Chloride

Lead(II)

Nitrate

Barium

Sulphate

Ammonium

Sulphate

Numerical problem involving stoichiometry reactions in the preparation of saltsA student prepare copper(II) nitrate, Cu(NO3)2 by reacting copper(II) oxide, CuO with 100 cm3 of 1.5 mol dm-3 nitric acid, HNO3. Calculate the mass of copper(II) oxide, CuO needed to react completely with the acid. [RAM : Cu,64 ; O,16]

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8.2 SYNTHESISING QUALITATIVE ANALYSIS OF SALTS

Colour of Salts

SOLID

AQUEOUS

SOLID/METAL OXIDE

White salt

Colourless

Green

Insoluble

Light Green

Light Green

Blue

Blue

Brown

Brown

Black

Insoluble

Yellow when hot

Insoluble

White when cold

Brown when hot

Insoluble

Yellow when cold

Confirmatory test for several gases

GAS

COLOURSMELL

EFFECT ONREAGENTCONFIRMATORY TEST

DAMPUSED

LITMUS

PAPER

Hydrogen

ColourlessOdourless

No effectLightedBring a lighted splinter to the

splintermouth of test tube. It gives

pop soundOxygen

ColourlessOdourless

No effectGlowingBring a glowing splinter to the

Splintermouth of test tube. The

glowing splinter lights upCarbon

ColourlessOdourless

Blue redLime waterBubble the gas produce intoDioxide

lime water. Lime water will

turns chalky/milky/cloudyAmmonia

ColourlessPungent

Red blueConcentratedDip a glass rod into

Smell

HClconcentrated HCl. White

fumes is releasedChlorine

GreenishPungent

Blue red--

yellowSmell

white

Nitrogen

BrownPungent

Blue red--

Dioxide

Smell

Sulphur

ColourlessPungent

Blue redAcidifiedTurns acidified potassiumdioxide

Smell

potassiummanganate (VII) solution from

manganate(VII)purple to colourless

solution

Hydrogen

ColourlessPungent

Blue redConcentratedDip a glass rod intochloride

Smell

ammonia, NH3Concentrated ammonia, NH3,

aqueouswhite fumes is released

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Action of heat on salts

a) Most compound which decomposes leave behind oxides

Saltmetal oxide+gas

(residue identification

(gas identification

refers to specific cations

refers to anion)

only)

b)

Types of gas

Colourless

released

residue

Can predict the

Can predict theidentify of anions

identify of cationspresence in salt

presence in salt

c) Test of carbon dioxide gas liberated when the carbonate salt is heated strongly

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d) Write down the chemical equation for the reactions that occur.

IonCarbonate Salt

Nitrate Salt

Is not decomposed by heat

Decompose into O2 gas and metal nitrate

K+

Na+

Ca2+

Decompose into metallic oxide and CO2

Decompose into metal oxide, NO2 and O2

Mg2+

Al3+

Zn2+

Fe2+

Fe3+

Pb2+

Cu2+

Sulphate salts usually do not decompose

Chloride salts do not decompose except NH4Cl

NH4Cl (s)NH3 (g) + HCl (g)[sublimation]

Confirmatory test for anions

Unknown Salt Solution

+ H2SO4 dilute

+ dilute+HNO3

+ HNO3

+ FeSO4

acid+ AgNO3

+ Ba(NO3)2

+ H2SO4 conc

Gas turns____________________________

limewaterPrecipitate

precipitate________

__________formed

formed

__

Confirmatory test for cationsThe presence of cation can be detected by using two common bench alkalis namely _____________________________ solution

_____________________________ solution Write the ionic equation for the reaction of cations and sodium hydroxide

The function of the alkalis is to produce a precipitate that is

Metal hydroxideMETAL + HYDROXIDE ION METAL HYDROXIDE

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IonIonic equation for the reaction of cations and sodium hydroxideAluminium ionAl3+ + 3OH- Al(OH)3Lead (II) Ion

Zinc Ion

Magnesium Ion

Calcium Ion

Copper (II) Ion

Ferrum (II) Ion

Ferrum (III) Ion

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NH4+

Ca2+ , Fe2+ ,,Fe3+ ,

Zn2+ , Al3+, Pb2+

Cu2+ , Mg2+

No

Formation of

Formation of white

Precipitate

precipitation &

precipitate & then

Non-soluble in

soluble in excess

excess

+NaOH a little

and then in

excess

+NH4OH a little

and then in

excess

NH4+, Ca2+

Fe2+ ,,Fe3+, Al3+

Zn2+ Cu2+

Pb2+, Mg2+

No

Formation of

precipitate is

Precipitate

precipitation &

formed & dissolve

Not dissolve in

in excess

excess

Anion to be test:

Examples

CO32- SO42- Cl- NO3-

Anion

Test

Observation

CO32-+ HClGas form white

precipitate with lime

water

SO42-+ HClWhite precipitate is

Cl-+ BaCl2/ Ba(NO3)2formed

+ HNO3White precipitate is

NO3-+ Ag(NO3)2formed

+ H2SO4 (Dil)Brown ring is formed

+ FeSO4

+ H2SO4 (conc)

Fe2+ ,Fe3+

K2Fe(II)(CN)6Light BlueBlue

precipitateprecipitate

K2Fe(III)(CN)6Dark BlueBrown

PrecipitateSolution

KSCNNo ChangeRed solution

CATIONS TEST

+ HCl, white precipitate

occurs and dissolved in

Fe2+ Fe3+

hot water

Confirmatory test

Pb2+

+ KI, yellow precipitate

ION TEST

NH4+

occurs and dissolved in

hot water

+ Nessler reagent, brown precipitate is form

ANION TEST

Pre-Test

Confirmatory test

Gas evolved from the

reaction with salt

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Test for cations

CationsTest with a small amount of NaOH solutionTest with a small amount of NH4OH until in

until in excessexcessAl3+White precipitate soluble in excess. NaOHWhite precipitate, insoluble in excess NH4OH

solutionsolutionPb2+White precipitate, soluble in excess NaOHWhite precipitate, insoluble in excess NH4OH

solutionsolutionZn2+White precipitate, soluble in excess NaOHWhite precipitate, soluble in excess NH4OH

solutionsolutionMg2+White precipitate, insoluble in excess NaOHWhite precipitate, insoluble in excess NH4OH

solutionsolutionCa2+White precipitate, insoluble in excess NaOHNo precipitate

solution

Cu2+Blue precipitate, insoluble in excess NaOHBlue precipitate, soluble in excess NH4OH

solutionsolutionFe2+Green precipitate, insoluble in excess NaOHGreen precipitate, insoluble in excess NH4OH

solutionsolutionFe3+Brown precipitate, insoluble in excessBrown precipitate, insoluble in excess NH4OH

NaOH solutionsolutionNH4+No precipitateNo precipitate

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