STUDYSMART CHEMISTRY FORM 4 CHAPTER 8 : SALTS 8.1 Synthesising Salts 8.2 Synthesising qualitative analysis of salts 8.3 Practising to be systematic and meticulous when carrying out activities
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+ Examples HCl + NaOH NaCl + H2O 2HNO3 + Zn Zn(NO3)2 + H2 H2SO4 + MgCO3 MgSO4 + H2O + CO2 Complete the table below Metal Ion
Sulphate salt Chloride salt (From H2SO4) (From HCl)
Nitric Acid (From HNO3)
Carbonate salt (From H2CO3)
Na+ K+ Zn2+ Mg2+ Ca2+ Fe2+ Fe3+ Cu2+ NH4+ Ba2+ Al3+ Pb2+ Ag2+ INSOLUBLE SALT 1 Study Smart www.studysmart.page.tl
Solubility 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 3
METHOD 1
Soluble
K+ , Na+ NH4+
Other than K+ , Na+ NH4+ salts
Acid + Alkali Salt + H2O Titration of acid and alkali - Add acid to the alkali until it is neutral with the aid of an indicator - Repeat the procedure without using an indicator
Insoluble
METHOD 2
Acid + basic oxide Salt + water Acid + metal Salt + Hydrogen gas Acid + 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 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 2
2 - 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
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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 SALT Zinc Chloride
SOLUBILITY
CHEMICAL EQUATION
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 salts A 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 White salt Green Light Green Blue Brown Black Yellow when hot White when cold Brown when hot Yellow when cold
AQUEOUS Colourless Insoluble Light Green Blue Brown Insoluble Insoluble
SOLID/METAL OXIDE
Insoluble
Confirmatory test for several gases GAS COLOUR SMELL EFFECT ON DAMP LITMUS PAPER
REAGENT USED
CONFIRMATORY TEST
Hydrogen
Colourless
Odourless
No effect
Lighted splinter
Oxygen
Colourless
Odourless
No effect
Glowing Splinter
Carbon Dioxide
Colourless
Odourless
Blue red
Lime water
Ammonia
Colourless
Pungent Smell
Red blue
Concentrated HCl
Chlorine
Greenish yellow Brown
Pungent Smell Pungent Smell Pungent Smell
Blue red white Blue red
-
Bring a lighted splinter to the mouth of test tube. It gives “pop” sound Bring a glowing splinter to the mouth of test tube. The glowing splinter lights up Bubble the gas produce into lime water. Lime water will turns chalky/milky/cloudy Dip a glass rod into concentrated HCl. White fumes is released -
-
-
Blue red
Turns acidified potassium manganate (VII) solution from purple to colourless
Pungent Smell
Blue red
Acidified potassium manganate(VII) solution Concentrated ammonia, NH3 aqueous
Nitrogen Dioxide Sulphur dioxide
Hydrogen chloride
Colourless
Colourless
Dip a glass rod into Concentrated ammonia, NH3, white fumes is released
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Action of heat on salts a) Most compound which decomposes leave behind oxides Salt metal oxide + gas (residue identification (gas identification refers to specific cations refers to anion) only) b) Types of gas released
Can predict the identify of anions presence in salt
Colourless residue
Can predict the identify of cations 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. Carbonate Salt Nitrate Salt Is not decomposed by heat Decompose into O2 gas and metal nitrate
Ion K+ Na+ Ca2+
Decompose into metallic oxide and CO2
Decompose into metal oxide, NO2 and O2
2+
Mg
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
+ dilute acid Gas turns limewater __________
+HNO3 + AgNO3
+ H2SO4 dilute + FeSO4 + H2SO4 conc
+ HNO3 + Ba(NO3)2
__________ Precipitate formed
__________ precipitate formed
________ ________ __
Confirmatory test for cations a) The presence of cation can be detected by using two common bench alkalis namely i) _____________________________ solution ii) _____________________________ solution b) 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 hydroxide METAL + HYDROXIDE ION METAL HYDROXIDE 7 Study Smart www.studysmart.page.tl
Ion Aluminium ion
Ionic equation for the reaction of cations and sodium hydroxide
Al3+ + 3OH- Al(OH)3
Lead (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+ , Cu2+ , Mg2+ No Formation of Precipitate precipitation & Non-soluble in excess
Zn2+ , Al3+, Pb2+ Formation of white precipitate & then soluble in excess
K2Fe(II)(CN)6 K2Fe(III)(CN)6 KSCN
+NaOH a little and then in excess
Fe2+ Light Blue precipitate Dark Blue Precipitate No Change
Fe2+
NH4+, Ca2+ No Precipitate
Fe2+ ,,Fe3+, Al3+ Pb2+, Mg2+ Formation of precipitation & Not dissolve in excess
SO42ClNO3-
Fe3+
Pb2+ ,
Confirmatory test
precipitate is formed & dissolve in excess
ION TEST
Blue precipitate Brown Solution Red solution
NH4+
+ KI, yellow precipitate occurs and dissolved in hot water
Anion to be test: CO32- SO42- Cl- NO3-
Examples Anion CO32-
Zn2+ Cu2+
Fe3+
+ HCl, white precipitate occurs and dissolved in hot water
CATIONS TEST +NH4OH a little and then in excess
,
Test + HCl
+ HCl + BaCl2/ Ba(NO3)2 + HNO3 + Ag(NO3)2 + H2SO4 (Dil) + FeSO4 + H2SO4 (conc)
Observation Gas form white precipitate with lime water White precipitate is formed White precipitate is formed Brown ring is formed
+ 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 Cations Al3+ Pb2+ Zn2+ Mg2+ Ca2+ Cu2+ Fe2+ Fe3+ NH4+
Test with a small amount of NaOH solution until in excess White precipitate soluble in excess. NaOH solution White precipitate, soluble in excess NaOH solution White precipitate, soluble in excess NaOH solution White precipitate, insoluble in excess NaOH solution White precipitate, insoluble in excess NaOH solution Blue precipitate, insoluble in excess NaOH solution Green precipitate, insoluble in excess NaOH solution Brown precipitate, insoluble in excess NaOH solution No precipitate
Test with a small amount of NH4OH until in excess White precipitate, insoluble in excess NH4OH solution White precipitate, insoluble in excess NH4OH solution White precipitate, soluble in excess NH4OH solution White precipitate, insoluble in excess NH4OH solution No precipitate Blue precipitate, soluble in excess NH4OH solution Green precipitate, insoluble in excess NH4OH solution Brown precipitate, insoluble in excess NH4OH solution No precipitate
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