Chemistry Acids, Bases and Salts Notes - · PDF fileChemistry – Acids, Bases and Salts Page 1 of 20 Chemistry Acids, Bases and Salts Notes ... ions as the only positive ion when

Chemistry – Acids, Bases and Salts Page 1 of 20

Chemistry

Acids, Bases and Salts Notes

Focus Area

• Distinguish acids and alkalis by chemical and physical tests

• Distinguish between strong and weak acids and alkalis

• Derive the respective ionic equations for chemical reactions

• Distinguish between the 4 types of oxides and their properties

• Understand alkalinity and acidity of a solution through the colour change of a pH indicator solution

• Describe the correct procedures involved to produce a soluble or insoluble salt or highlight inappropriate means of the salt production

Acid 1. An acid is a substance that forms hydrogen (H

+) ions as the only positive ion when dissolved in water.

2. There are two types of acids: mineral acid (or inorganic acid) and organic acid.

a. Mineral acids are acids that are derived from one or more mineral elements or inorganic matter and can

normally be prepared in the laboratory. Phosphoric acid (H3PO4), sulfuric acid (H2SO4), hydrochloric acid (HCI), carbonic acid (H2CO3) and nitric acid (HNO3) are examples of mineral acids.

b. Organic acids are obtained from plants and animals and are mainly naturally occurring organic compounds

with acidic properties. They include malic acid (C4H6O5) which is found in apples, ethanoic acid (C2H4O2) that can be found in vinegar and citric acid (C6H8O7) that is present in citrus fruits like lime and lemon.

3. Strength of an acid refers to the extent to which an acid molecule dissociates or ionises in an aqueous solution to

form ions.

a. Strong acids are acids that are completely dissociated in water to produce a high concentration of hydrogen ions. Hence, strong acids ionise completely. Strong acids have a pH in the region of about 1. Sulfuric acid, nitric acid and hydrochloric acid are examples of strong acids.

HCI (aq) � H+ (aq) + CI

- (aq)

(dilute hydrochloric acid contains only hydrogen and chloride ions; no HCl molecules are present)

H3PO4 (aq) � 3H+ (aq) + PO4

3- (aq)

b. Weak acids are acids that are partially dissociated in water to produce a low concentration of hydrogen ions. E.g. carbonic acid and ethanoic acid where only a few of its molecules ionises. Weak acids have a pH in the region of about >2.5. Other examples include citric acid and malic acid.

CH3COOH (aq) H+ (aq) + CH3COO

- (aq)

(dilute ethanoic acid contains mainly acid molecules that remain unchanged in the water; very few molecules dissociate to produce hydrogen ions)

C6H8O7 (aq) 3H+ (aq) + C6H5O7

3- (aq)

citric acid citrate ion


4. Concentration refers to the number of acid molecules in unit volume of a solution.

E.g. a 10M ethanoic acid (weak acid) is more concentrated than a 1M hydrochloric acid (strong acid). An acid is said to be either strong or weak. Its strength cannot be changed. The concentration can be changed, i.e. can be increased by adding more acid, or decreased by adding more water.

5. Chemical Properties of Acids

a. Acids react with most metals to form salt and hydrogen gas. (Observation: effervescence or bubbling of a colourless and odourless gas that extinguishes a burning splint with a pop sound, which confirms the presence of hydrogen gas.)

Metal + Acid � Salt + Hydrogen

Fe (s) + H2SO4 (aq) � FeSO4 (aq) + H2 (g) Zn (s) + 2HCl (aq) � ZnCl2 (aq) + H2 (g) Mg (s) + 2HCl (aq) � MgCl2 (aq) + H2 (g)

All metals except copper (Cu) and silver (Ag) react with acid to produce hydrogen.

Lead reacts slowly with dilute hydrochloric acid and dilute sulfuric acid. Initially, the reactions produce a layer of lead (II) chloride or lead (II) sulfate respectively. This layer is insoluble in water and forms a

coating around the metal that prevents further reaction with the acids. b. Acids react with carbonates (CO3

2-) and hydrogen carbonates (HCO3

-) to produce carbon dioxide gas.

(Observation: effervescence of a colourless and odourless gas that forms a white precipitate in limewater [Ca(OH)2], which confirms the presence of carbon dioxide gas.) Precipitate refers to insoluble solid particles produced in a liquid in a chemical reaction.

Carbonate + Acid� Salt + Carbon Dioxide + Water CaCO3 (s) + 2HNO3 (aq) � Ca(NO3)2 (aq) + CO2 (g) + H2O (l)

MgCO3 (s) + H2SO4 (aq) � MgSO4 (aq) + CO2 (g) + H2O (l)

All carbonates react with acids to produce carbon dioxide. This is a chemical test that can be used to confirm the presence of an acid or a carbonate salt.

Hydrogen Carbonate + Acid� Salt + Carbon Dioxide + Water

NaHCO3 + HCl � NaCl + CO2 + H2O

c. Acids react with bases in a reaction called neutralisation to form salt and water. Bases refer to metal oxides or metal hydroxides. (Observation: test-tube feels warm as heat is given off.)

Acid + Base �� Salt + Water H

+ (aq) + OH

- (aq) � H2O (l)

H2SO4 (aq) + CuO (s) � CuSO4 (aq) + H2O (l) H2SO4 (aq)+ 2NaOH (aq) � Na2SO4 (aq) + 2H2O (l)

HCI (aq) + NaOH (aq) � NaCI (aq) + H2O (l)

All bases react with acids to produce water molecules.


Quiz 1a: Deduce the ionic equations: CaCO3 + 2HNO3 � Ca(NO3)2 + CO2 + H2O

Zn + 2HCl � ZnCl2 + H2 Quiz 1b: Are CH3COOH and C6H5COOH acids?

Quiz 1c: Write chemical equations for the following: (a) nitric acid + potassium hydroxide � potassium nitrate + water (b) hydrochloric acid + sodium hydroxide � sodium chloride + water (c) hydrochloric acid + aqueous ammonia � ammonium chloride + water (d) sulfuric acid + calcium hydroxide � calcium sulfate + water


Quiz 1d: Is it possible to determine whether a dilute hydrochloric acid is a stronger acid compared to a very concentrated ethanoic acid? Explain your answer.

6. Physical Properties of Acids

a. have a sour taste. b. turn damp blue litmus paper red, with pH value less than 7, due to the presence of hydrogen ions. c. Acid conducts electricity due to its abundance of mobile hydrogen ions and anions when dissociated in water.

But in other solvents, it would not conduct electricity. For example, hydrogen chloride in methylbenzene: hydrogen chloride does not ionise to produce ions and remains as molecules. Hence, it does not conduct electricity.

d. Acid is a mixture of a covalent compound and water.

Quiz 2a: Why is the above a physical property and not a chemical property of acid?

Quiz 2b: (1) Will a solution of hydrogen chloride in methylbenzene chemically react with metals to liberate hydrogen? (2) Will a solution of hydrogen chloride in methylbenzene chemically react with carbonates to liberate carbon dioxide? (3) Will a solution of hydrogen chloride in methylbenzene chemically react with alkalis?


7. Basicity of an acid refers to the number of hydrogen ions that is dissociated when an acid molecule dissolves in water. a. monobasic acid - hydrochloric acid (HCI) and nitric acid (HNO3)

HCl (aq) � H

+ (aq) +

Cl

- (aq)

HNO3 (aq) � H+ (aq) + NO3

- (aq)

For neutralisation, HX + NaOH � NaX + H2O, where X represents an anion with a charge of –1.

To achieve neutralisation, the ratio of the acid reacting to the base is 1: 1.

b. dibasic acid - sulfuric acid (H2SO4) H2SO4 (aq) � 2 H

+ (aq) + SO4

2- (aq)

Overall equation for complete neutralisation: H2X + 2NaOH � Na2X + 2H2O, where X represents an anion with

a charge of –2.


c. tribasic acid - phosphoric acid (H3PO4) H3PO4 (aq) � 3 H

+ (aq) + PO4

3- (aq)

Overall equation for complete neutralisation: H3X + 3NaOH � Na3X + 3H2O, where X represents an anion with

a charge of –3.


8. Acids are used

a. to remove rust; the industrial process to strip rust from steel is known as pickling where the rusted steel is

dipped into phosphoric acid 2H3PO4 (aq) + 3Fe2O3 (s) � 2Fe3(PO4)2 (aq) + 3H2O (l)

b. as food preservatives and flavor enhancers like in vinegar (ethanoic acid – CH3COOH).

c. for cleaning metals and in leather processing (hydrochloric acid).

d. to make car batteries, manufacture of fertilisers and detergents (sulfuric acid).


Base / Alkali 9. A base is a substance that reacts with an acid to form salt and water only.

10. Bases form a class of chemical substances which include all metal oxides and metal hydroxides. E.g. magnesium

oxide (MgO), copper (II) oxide (CuO), and calcium hydroxide [Ca(OH)2].

a. Soluble bases are called alkalis. An alkali is a substance that forms hydroxide ions (OH-) when dissolved in

water. e.g. sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], aqueous ammonia (NH4OH).

b. Insoluble bases would be unable to produce hydroxide ions in water and thus would not cause pH indicators to change colour.

11. Strength of an alkali refers to the extent to which an alkali molecule dissociates or ionises in an aqueous solution

to form ions.

a. Strong alkalis are alkalis that are completely dissociated in water to produce a high concentration of hydroxide ions. Hence, strong alkalis ionise completely. Strong alkalis have a pH in the region of about 13. Sodium hydroxide, potassium hydroxide (KOH) and calcium hydroxide are examples of strong alkalis.

NaOH (aq) � OH- (aq) + Na

+ (aq)

(dilute sodium hydroxide contains only sodium and hydroxide ions, the ionic lattice structure of the hydroxide breaks down to allow free ions to form)

KOH (aq) � OH

- (aq) + K

+ (aq)

Ca(OH)2 (aq) � 2OH- (aq) + Ca

2+ (aq)

b. Weak alkalis are alkalis that are partially dissociated in water to produce a low concentration of hydroxide ions.

Weak alkalis have a pH in the region of about 11. Aqueous ammonia is the example of a weak alkali.

NH3 (g) + H2O (l) OH- (aq) + NH4

+ (aq)

(aqueous ammonia has most of its molecules remaining unchanged in water; only a small fraction of ammonia molecules dissociate to produce hydroxide ions)

12. Concentration refers to the number of alkali molecules in unit volume of a solution. E.g. a 10M aqueous ammonia (weak acid) is more concentrated than a 1M sodium hydroxide (strong acid). An alkali is said to be either strong or weak. Its strength cannot be changed. The concentration can be changed, i.e. can be increased by adding more alkali, or decreased by adding more water.

13. Chemical Properties of Bases

a. Alkalis and insoluble bases react with acid to form salt and water. (Observation: test-tube feels warm as heat is given off.)

Acid + Base/Alkali �� Salt + Water

H+ (aq) + OH

- (aq) � H2O (l)

H2SO4 (aq) + CaO (s) � CaSO4 (s) + H2O (l)

H2SO4 (aq) + Ca(OH)2 (aq) � CaSO4 (s) + 2H2O (l)

All alkalis and bases react with acids to produce water molecules.


b. Strong Alkalis reacts with ammonium salts to give off ammonia gas. (Observation: colourless and pungent gas is produced and it turns damp red litmus paper blue.)

Alkali + Ammonium Salt (under heating) � Salt + Ammonia/NH3 + Water OH

- (aq) + NH4

+ (aq) � NH3 (g) + H2O (l)

NaOH (aq) + NH4CI (aq) � NaCI (aq) + NH3 (g) + H2O (l) KOH (aq) + NH4CI (aq) � KCI (aq) + NH3 (g) + H2O (l)

All ammonium salt reacts with bases to produce ammonia molecules under gentle warming. This is a chemical

test that can be used to confirm the presence of an alkali or an ammonium salt.

c. Alkalis react with a solution of one metal salt to give metal hydroxide and another metal salt. (Observation: metal hydroxide appears as insoluble white or coloured precipitates.) By observing whether a precipitate is formed, we can identify the metal cation in the salt.

Alkali (with metal ion A)+ Salt (with metal ion B) � Salt (of metal ion A) + Metal Hydroxide (of metal ion B)

2NaOH (aq) + FeSO4 (aq) � Na2SO4 (aq) + Fe(OH)2 (s)

Quiz 3: Given three samples containing either an acid, alkali or water each, describe 2 chemical tests that can be used to identify the substances in each sample. Describe 1 possible physical test that can also be used to distinguish each of the samples.


14. Physical Properties of Bases

a. have a bitter taste and a soapy feel.

b. turns damp red litmus paper blue with a pH value greater than 7 due to the presence of hydroxide ions.

c. conduct electricity in aqueous solutions due to its abundance of mobile hydroxide ions and cations when the base dissolves in water.

d. An alkali is a mixture of ionic compound and water.

15. Plants grow best when the pH value is about 6.5. Calcium hydroxide (slaked lime)/calcium oxide (quicklime) or calcium carbonate can be added if the soil is too acidic.

Quiz 4: Between calcium oxide, calcium hydroxide and calcium carbonate, which is the preferred choice for neutralising an acidic soil? Note that ammonium salts is also used as a fertiliser for plant growth. Explain.

16. Alkalis are used to

a. dissolve dirt and grease (sodium hydroxide), like in eye lotions and household cleaning agents (aqueous

ammonia). b. relieve gastric pain and for making refractory bricks (magnesium oxide).


Indicators and pH Values 17. The pH is a measure of the acidicity/alkalinity of a solution. It is related to the concentration of hydrogen ions or

hydroxide ions present in a solution.

18. The pH scale is a set of numbers from 0 to 14 which is used to indicate whether a solution is acidic, neutral or alkaline.

19. At a temperature of 25oC, the pH scale can be explained as follows:

pH range Concentration of H+ and OH

- in solution Solution is Litmus paper

pH < 7 Concentration of H+ > concentration of OH

- acidic Blue � Red

pH = 7 Concentration of H+ = concentration of OH

- neutral No colour change

pH > 7 Concentration of H+ < concentration of OH

- alkaline Red � Blue

20. pH is defined as the negative logarithm to the base 10 of the hydrogen ion concentration in mol/dm

3. (Not in

Syllabus) pH = - log 10 [H+]

where [H+] is the concentration of H+ ions in the solution

a. For pure water at 25oC, [H+] = [OH-] = 1 x 10

-7 mol/dm

3 � pH = 7

b. Example 1:

[HNO3] = 1 x 10-3 mol/dm

3

pH value = - log1010-3

= 3 because acid is monobasic and a strong acid so all the molecules dissociate to form ions [H+] = [HNO3] = 1 x 10

-3 mol/dm

3

c. Example 2: [CH3COOH] = 1 x 10

-3 mol/dm

3

pH value = - log1010-5

= 5 because acid is a weak acid and only some of the molecules dissociate to form ions [H+] << [CH3COOH]; and [H+] = 10

-5 mol/dm

3

21. A pH indicator is a substance or mixture of substances that when added to the solution gives a different colour

depending on the pH of the solution.

22. Common liquid pH indicators are litmus solution, methyl orange, screened methyl orange and phenolphthalein. Most of these indicators are organic compounds.

23. The pH meter as shown can be used to determine the

pH of a solution. It has a probe that can be calibrated to

determine the concentration of H+ in a solution.

A pH meter


24. Colour changes of various acid-base indicators are summarized below.

Indicator Acid Colour Alkaline Colour pH Range when colour changes

methyl orange red yellow 3.1-4.4

screened methyl orange red green 3.1-4.4

litmus red blue 5.0-8.0

phenolphthalein colourless pink 8.3-10.0

25. Universal indicator is a mixture of indicators that show a variety of colours in accordance to pH values,

depending on the acidity and alkalinity of the solutions tested. It comes in the form of a solution or pH paper.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Red Pink Orange Yellow Green Greenish-blue Blue Violet

Acidic Neutral Alkaline Increasing Acidity Increasing Alkalinity

26. Naturally occurring pH indicators:

a. Many plants or plant parts contain chemicals from the naturally-colored anthocyanin family of compounds. They are red in acidic solutions and blue in basic. Anthocyanins can be extracted with water or other solvents from a multitude of colored plants or plant parts, including leaves of a red cabbage; flowers of geranium, poppy, or rose petals; berries from blueberries, blackcurrant; and stems of a rhubarb. Extracting anthocyanins from household plants, especially red cabbage, to form a crude pH indicator is a popular introductory chemistry demonstration.

b. Litmus, used by alchemists in the Middle Ages and still readily available today, is a naturally occurring pH indicator made from a mixture of lichen (fungus) species. The color changes between red in acid solutions and blue in alkalis.


Types of Oxides 27. Acidic Oxides (behave like an acid)

a. Oxides of non-metals: phosphorous pentoxide (P2O5), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon dioxide (CO2), silicon dioxide (SiO2) etc.

b. Mostly simple covalent molecules (except SiO2 which is a giant covalent molecule). c. React with water to form acids:

carbon dioxide: CO2 (g) + H2O (l) �H2CO3 (aq) (carbonic acid) sulfur dioxide: SO2 (g) + H2O (l) �H2SO3 (aq) (sulfurous acid) sulfur trioxide: SO3 (g) + H2O (l) �H2SO4 (aq) (sulfuric acid)

d. React with bases to form salts (neutralisation) CO2 (g) + 2NaOH (aq)� Na2CO3 (aq) + H2O (l)

28. Basic Oxides (behave like a base) a. Oxides of metals: potassium oxide (K2O), sodium oxide (Na2O), calcium oxide (CaO), iron (II) oxide (FeO) etc. b. Mostly ionic compounds. c. Those that dissolve in water are alkalis, e.g. all Group I oxides are soluble in water and are known as soluble

bases; while CaO is slightly soluble in water. d. React with acids to form salts:

magnesium oxide: MgO (s) + 2HCI (aq) �MgCI2 (aq) + H2O (l) sodium oxide: Na2O (s) + H2SO4 (aq) �Na2SO4 (aq) + H2O (l)

calcium oxide: CaO (s) + 2HNO3 (aq) �Ca(NO3)2 (aq) + H2O (l) copper (II) oxide: CuO (s) + 2HCI (aq) � CuCI2 (aq) + H2O (l)

29. Amphoteric Oxides (can behave like a base or an acid)

a. Oxides of metals that can behave like basic or acidic oxides: zinc oxide (ZnO), aluminium oxide (Al2O3), lead (II) oxide (PbO), lead (IV) oxide (PbO2).

b. Mostly ionic compound. c. React with bases to form complex salts and water. d. React with acids to form salts and water.

Oxide Reacts with base Reacts with acid

ZnO ZnO (s) + 2NaOH (aq) + H2O (l) � Na2Zn(OH)4 (aq) Sodium zincate

ZnO (s) + 2HNO3 (aq) � Zn(NO3)2 (aq) + H2O (l)

Al2O3 Al2O3 (s) + 2NaOH (aq) + 3 H2O (l) � 2 NaAI(OH)4 (aq) Sodium aluminate

Al2O3 (s) + 6HCI (aq) � 2Al2CI3 (aq) + 3H2O (l)

PbO PbO (s)+ 2NaOH (aq)+ H2O (l) � Na2Pb(OH)4 (aq) Sodium plumbate

PbO (s) + H2SO4 (aq) � PbSO4 (s) + H2O (l)

30. Neutral Oxide

a. Oxides of non-metals: carbon monoxide (CO), nitrogen monoxide/nitric oxide (NO), dinitrogen monoxide/nitrous oxide (N2O), water (H2O).

b. Mostly simple covalent molecules. c. Do not react with bases and acids to form salts.


Quiz 5a: Are all non-metal oxides acidic? Explain your answer. Quiz 5b: Are all metal oxides basic? Explain your answer. Quiz 5c: How does the property of the oxides change along Period 3 of the Periodic Table? Quiz 5d: Are all basic oxides soluble in water? Quiz 5e: Are all basic oxides soluble in acids? Quiz 5f: Are all basic oxides and amphoteric oxides soluble in acids? Quiz 5g: Are all acidic oxides and amphoteric oxides soluble in alkalis?


Quiz 5h: Complete and balanced the following equations: Acidic oxides and their chemical reactions (1) SO2 + H2O � (2) CO2 + H2O � (3) SO3 + H2O � (4) SiO2 + H2O � No reaction since silicon dioxide is a macromolecule and does not dissolve

in water (5) SO2 + NaOH � (6) CO2 + KOH � (7) SO3 + LiOH � (8) SiO2 + 2 NaOH � Na2SiO3 + H2O sodium silicate [NOT in Syllabus] Basic oxides and their chemical reactions (1) Na2O + H2O � (2) K2O + H2O � (3) CaO + H2O � (4) MgO + H2O � magnesium hydroxide; highly insoluble (5) Na2O + HCI � (6) K2O + HNO3 � (7) CaO + HNO3 � (8) MgO + H2SO4 �


Basic oxides and their chemical reactions (1) ZnO + H2O � (2) PbO + H2O � (3) Al2O3 + H2O � (4) ZnO + HCI � (5) PbO + HNO3 � (6) Al2O3 + H2SO4 �


Salts 31. A salt is substance formed when one or more hydrogen ions of an acid is replaced by a metallic ion or an

ammonium ion. E.g. the salt sodium chloride (NaCl) is formed when the hydrogen ion of the hydrochloric acid (HCl) is replaced by the sodium ion, a cation (metallic ion) of sodium hydroxide (NaOH). NaOH (aq) + HCl (aq) � NaCl (aq) + H2O (l)

32. The anion of a salt comes from an acid. E.g. Chlorides (Cl

-) can be formed using dilute hydrochloric acid (HCl);

Sulfates (SO42-) can be formed using dilute sulfuric acid (H2SO4);

Nitrates (NO3-) can be formed using dilute nitric acid (HNO3);

Phosphate (PO43-) can be formed using dilute phosphoric acid (H3PO4).

33. The cation of a salt comes from a metal, a carbonate, a base or an alkali.

Possible Reactants Salt Formed

Metal/Carbonate/Base/Alkali Acid

zinc (Zn) hydrochloric acid (HCl) zinc chloride (ZnCl2)

copper (II) carbonate (CuCO3) nitric acid (HNO3) copper (II) nitrate (Cu(NO3)2)

magnesium oxide (MgO) sulfuric acid (H2SO4) magnesium sulfate (MgSO4)

potassium hydroxide (KOH) phosphoric acid (H3PO4) potassium phosphate (K3PO4)

34. Salts can be hydrated or anhydrous.

a. Hydrated salts have water molecules attracted to the crystal lattice ions during the formation of the crystals.

These water molecules that are formed as part of the crystals may affect the shape and colour of the crystals,

and are called the water of crystallisation. Hydrated salts are prepared as follows: heat a solution (to reduce

the amount of solvent) to obtain a saturated solution, before cooling to crystallise the hydrated salt.

Hydrated Salt Salt Formed Ratio of Salt to Water

Molecules Colour of Salt

copper (II) sulfate CuSO4.5H2O 1: 5 blue

cobalt (II) chloride CoCl2.6H2O 1: 6 pink

iron (II) sulfate FeSO4.6H2O 1: 6 green

magnesium sulfate MgSO4.7H2O 1: 7 white

sodium carbonate Na2CO3.10H2O 1: 10 white

calcium sulfate CaSO4.2H2O 1: 2 white

b. Anhydrous salt comprises crystals that have lost their water of crystallisation. Anhydrous salt can be obtained

by drying the hydrated salt crystals. This can be done by warming the crystals over a steam bath, or

evaporation to dryness. The anhydrous salt of copper (II) sulfate is white, after it loses its water of

crystallisation.


35. Solubility of Salts in Water (must be determined to allow us to choose a suitable method for preparing the salt).

Salt Type Soluble Insoluble

Group I Salts (sodium and potassium)

all -----

ammoniums all ----

nitrates all ----

chlorides all others silver chloride (AgCl) and lead (II) chloride (PbCl2)

sulfates all others barium sulfate (BaSO4),

lead (II) sulfate (PbSO4) and calcium sulfate (CaSO4 - only slightly soluble)

iodides all others silver iodide (AgI) and

lead (II) iodide (PbI2)

carbonates ammonium, sodium, potassium all others

hydroxides ammonium, sodium, potassium,

barium. all others

36. The chemical reactions to obtain salts are as follows:

Means Challenges

Solid-Acid Reaction

Metal-Salt Displacement Reaction

Metal + Salt Solution � Salt + Metal of salt solution

• Soluble salts except Group I and ammonium salts.

• Excess solid reactants (namely metal, insoluble base or carbonate) are added so that all the acid is completely used up, leaving behind the salt solution and excess solid reactants. Otherwise, the salt produced will be contaminated with the acid.

• Filter to remove excess (unreacted) metal/base/ carbonate that is added as residue.

• Salt solution is obtained as filtrate.

• Obtain salt through heating � crystallisation or filtration � drying by evaporation/filter paper.

Metal + Acid � Salt + Hydrogen

Insoluble Base + Acid � Salt + Water

Insoluble Carbonate + Acid�Salt + Carbon

dioxide + Water

Titration

Alkali + Acid � Salt + Water

Soluble Carbonate + Acid � Salt + Water + Carbon

dioxide

• Soluble salts of Group I and ammonium only.

• Titration is first done with an indicator to determine the volume of alkali and acid required for the complete reaction so that salt and water are the only products (the reactants will then be completely used up).

• 2nd

titration is to be done with the known volume of acid and alkali but without the indicator.

• Salt is obtained by crystallisation or drying.

Precipitation

Reaction between Two Aqueous Solutions

• Salt must be insoluble.

• Two aqueous solutions are chemically reacted to precipitate the insoluble salt. One of the reagents provides the cation, while the other provides the anion of the salt.

• Normally, a nitrate solution is added with an acid or Group I salts to precipitate the salt. (All nitrates, acids and Group I salts are soluble)

• Two solutions can be mixed together in any proportion.

• Salt is obtained by filtration or drying.


37. Preparation Means: 38. Collection of Salts:

a. Salts like CuSO4.5H2O, ZnSO4.7H2O have water of crystallisation. Hence most salts are collected by means of crystallisation rather than by direct evaporation.

b. Salts collected must be wiped with filter paper and cleaned with distilled water to remove contaminants that stay on the surface of the crystals.

Precipitation Method

• Mix solutions of two soluble salts. An insoluble salt is precipitated.

• Filter the mixture to obtain the insoluble salt as the residue.

• Wash the residue with distilled water.

• Dry the residue with filter paper.

No

Titration Method

• Pipette a fixed volume of an alkali into a conical flask.

• Add an indicator to the alkali in the flask.

• Put an acid in a burette.

• Titrate the alkali with the acid until reaction is completed. Record the volume of the acid used.

• Repeat titration without indicator.

• Put the same volume of the alkali in the flask, then add known volume of the acid to the alkali.

• Heat and cool solution to crystallise the salt.

• Filter crystallised salt and dry the salt with filter paper.

Yes

No

Solid-Acid Reaction

• Add excess metal/base/carbonate to acid until no more reaction. (warm acid for a quicker chemical reaction)

• Filter to remove the excess insoluble metal/base/carbonate as residue.

• Heat and cool solution to crystallise salt.

• Filter crystallised salt and dry the salt with filter paper.

• Carbonate and acid reactions are preferred; since the carbonates chosen are usually not soluble.

• Except for aluminium or iron (III) salt: aluminium oxide or iron (III) oxide are to be used since aluminium or iron (III) carbonate do not exist.

Is the salt a Group I or NH4

+ salt?

Solubility of the salt to be

produced

There is no need to heat and concentrate the solution for crystallisation. The salt that is formed cannot dissolve in water at room temperature. The salt can be washed with large amounts of distilled water and dried in an oven. The salt has no water of crystallisation since water is vapourised in the oven and hence, is not destroyed on strong heating.

There is a need to add indicator because both the reactants (acid and alkali) and the products (salt and water) are colourless solutions. There is a need to repeat the titration without adding the indicator because the indicator added would contaminate the salt formed.

Yes

Excess solid is added to ensure all acid has reacted. When effervescence stops or when solid reactants no longer react, this indicates that the reaction has ended. Solution is heated to form a saturated solution. Solution is cooled to form crystals. Do not heat the crystals to dryness or they will lose water of crystallisation.


39. Selection of reagents: a. Precipitation

Insoluble Salt Formula Reagents Chemical equation

silver chloride AgCl aqueous AgNO3 and aqueous

HCl/NaCl/KCl

AgNO3(aq) + HCl(aq) � AgCl(s) + HNO3(aq) AgNO3 + NaCl � AgCl + NaNO3

lead (II) chloride PbCl2 aqueous Pb(NO3)2 and

aqueous HCl/NaCl/KCl

Pb(NO3)2 + 2HCl � PbCl2 + 2HNO3

Pb(NO3)2 + 2NaCl � PbCl2 + 2NaNO3

lead (II) sulfate PbSO4

aqueous Pb(NO3)2 and

aqueous

H2SO4/Na2SO4/K2SO4

Pb(NO3)2 + H2SO4 � PbSO4 + 2HNO3

Pb(NO3)2 + Na2SO4 � PbSO4 + 2NaNO3

calcium carbonate CaCO3

aqueous Ca(NO3)2 /CaCl2 and

aqueous

H2CO3/Na2CO3/K2CO3

Ca(NO3)2 + H2CO3 � CaCO3 + 2HNO3

Ca(NO3)2 + Na2CO3 � CaCO3 + 2NaNO3

b. Titration (i) Acid + Alkali

Soluble Salt Formula Reagent Chemical equation

ammonium nitrate NH4NO3 aqueous ammonia and aqueous HNO3 NH3 + HNO3 � NH4NO3

potassium sulfate K2SO4 aqueous KOH and aqueous H2SO4 2KOH(aq) + H2SO4 (aq) � K2SO4

(aq) + 2H2O (l)

(ii) Acid + Soluble Carbonate/Alkali


potassium nitrate KNO3 aqueous K2CO3 /KOH and

aqueous HNO3

K2CO3 (aq) + 2HNO3 (aq) � 2KNO3 (aq) +

CO2 (g) + H2O (l)

sodium sulfate Na2SO4 aqueous Na2CO3 /NaOH and

aqueous H2SO4 Na2CO3 + H2SO4 � Na2SO4 + CO2 + H2O

sodium chloride NaCl aqueous Na2CO3 /NaOH and

aqueous HCl Na2CO3 + 2HCl � 2NaCl + CO2 + H2O

c. Solid-Acid (i) Insoluble Carbonate + Acid


zinc chloride ZnCl2 solid zinc carbonate and

aqueous HCl

ZnCO3 (s) + 2HCl (aq) � ZnCl2 (aq) + 2H2O (l) + CO2 (g)

calcium nitrate Ca(NO3)2 solid calcium carbonate and

aqueous HNO3

CaCO3 + 2HNO3 � Ca(NO3)2 +

2H2O + CO2

silver nitrate AgNO3 solid silver carbonate and

aqueous HNO3

Ag2CO3 + 2HNO3 � 2AgNO3 + H2O

+ CO2

magnesium sulfate MgSO4 solid magnesium carbonate and

aqueous H2SO4

MgCO3 + H2SO4 � MgSO4 + H2O +

CO2


(ii) Metal + Acid


zinc chloride ZnCl2 zinc and aqueous HCl Zn (s) + 2HCl (aq) � ZnCl2 (aq)

+ H2 (g)

calcium nitrate Ca(NO3)2 calcium and aqueous HNO3 Ca + 2HNO3 � Ca(NO3)2 + H2

magnesium sulfate MgSO4 magnesium and aqueous H2SO4 Mg + H2SO4 � MgSO4 + H2

(ii) Insoluble Base + Acid


zinc chloride ZnCl2 solid zinc oxide and aqueous

HCl

ZnO (s) + 2HCl (aq) � ZnCl2 (aq) + 2H2O (l)

magnesium nitrate Mg(NO3)2 solid magnesium oxide and

aqueous HNO3

MgO + 2HNO3 � Mg(NO3)2 +

H2O

aluminium sulfate Al2(SO4)3 solid aluminium oxide and

aqueous H2SO4

Al2O3 + 3H2SO4 � Al2(SO4)3 +

3H2O

Quiz 6: Explain how the following dry salt crystals can be obtained by stating the chemical reagents required and the means of obtaining pure and dry crystals.

• copper (II) sulfate

• zinc sulfate

• sodium nitrate

• barium sulfate

• lead(II) chloride


Quiz 7: Comment on the following statements and explain whether they are true or false.

a. To produce all Group I salts and ammonium salts, only the titration method can be used. b. To produce all nitrate salts, only the titration method can be used. c. To the use precipitation method to produce a salt XY (where X is a metal and Y is non-metal/s), we can always use a solution of X nitrate and a solution of Group I Y.

Quiz 8. State 2 possible means of obtaining calcium chloride. For each method, explain how the salt can be obtained.

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Chemistry Acids, Bases and Salts Notes - · PDF fileChemistry – Acids, Bases and Salts Page 1 of 20 Chemistry Acids, Bases and Salts Notes ... ions as the only positive ion when