Chemistry Acids, Bases and Salts Notes - SmartLab

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) +

3-

H3PO4 (aq) 3H (aq) + PO4 (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) citric acid

+

3-

3H (aq) + C6H5O7 (aq) citrate ion

Chemistry – Acids, Bases and Salts Page 1 of 20

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. 2-

-

b. Acids react with carbonates (CO3 ) 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) +

2-

H2SO4 (aq) 2 H (aq) + SO4 (aq) Overall equation for complete neutralisation: H2X + 2NaOH Na2X + 2H2O, where X represents an anion with a charge of –2. To achieve neutralisation, the ratio of the acid reacting to the base is 1: 2. c.

tribasic acid - phosphoric acid (H3PO4) +

3-

H3PO4 (aq) 3 H (aq) + PO4 (aq) Overall equation for complete neutralisation: H3X + 3NaOH Na3X + 3H2O, where X represents an anion with a charge of –3. To achieve neutralisation, the ratio of the acid reacting to the base is 1: 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) 2+ Ca(OH)2 (aq) 2OH (aq) + Ca (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. -

+

OH (aq) + NH4 (aq) NH3 (g) + H2O (l) (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. Chemistry – Acids, Bases and Salts Page 6 of 20

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. o

19. At a temperature of 25 C, the pH scale can be explained as follows: pH range pH < 7 pH = 7 pH > 7

+

-

Concentration of H and OH in solution

Solution is

Litmus paper

+

-

acidic

Blue Red

+

-

neutral

No colour change

+

-

alkaline

Red Blue

Concentration of H > concentration of OH Concentration of H = concentration of OH Concentration of H < concentration of OH

3

20. pH is defined as the negative logarithm to the base 10 of the hydrogen ion concentration in mol/dm . (Not in Syllabus) pH = - log 10 [H+] where [H+] is the concentration of H+ ions in the solution o

-7

3

a. For pure water at 25 C, [H+] = [OH-] = 1 x 10 mol/dm pH = 7 b. Example 1: -3 3 [HNO3] = 1 x 10 mol/dm -3 pH value = - log1010 = 3 because acid is monobasic and a strong acid so all the molecules dissociate to form ions -3 3 [H+] = [HNO3] = 1 x 10 mol/dm c.

Example 2: -3 3 [CH3COOH] = 1 x 10 mol/dm -5 pH value = - log1010 = 5 because acid is a weak acid and only some of the molecules dissociate to form ions -5 3 [H+] << [CH3COOH]; and [H+] = 10 mol/dm

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 Chemistry – Acids, Bases and Salts Page 9 of 20

24. Colour changes of various acid-base indicators are summarized below. Indicator methyl orange screened methyl orange litmus phenolphthalein

Acid Colour red red red colourless

Alkaline Colour yellow green blue pink

pH Range when colour changes 3.1-4.4 3.1-4.4 5.0-8.0 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 ZnO Al2O3 PbO

Reacts with base ZnO (s) + 2NaOH (aq) + H2O (l) Na2Zn(OH)4 (aq) Sodium zincate Al2O3 (s) + 2NaOH (aq) + 3 H2O (l) 2 NaAI(OH)4 (aq) Sodium aluminate PbO (s)+ 2NaOH (aq)+ H2O (l) Na2Pb(OH)4 (aq) Sodium plumbate

Reacts with acid ZnO (s) + 2HNO3 (aq) Zn(NO3)2 (aq) + H2O (l) Al2O3 (s) + 6HCI (aq) 2Al2CI3 (aq) + 3H2O (l) 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 +

H 2O

(3)

SO3 +

H 2O

(4)

SiO2 +

H 2O

(5)

SO2 + NaOH

(6)

CO2 + KOH

(7)

SO3 + LiOH

(8)

SiO2 + 2 NaOH

No reaction since silicon dioxide is a macromolecule and does not dissolve in water

Na2SiO3 + H2O sodium silicate [NOT in Syllabus]

Basic oxides and their chemical reactions (1) Na2O + H2O (2)

K2O +

H 2O

(3)

CaO +

H2O

(4)

MgO +

H2 O

magnesium hydroxide; highly insoluble

HCI

K2O +

HNO3

(7)

CaO +

HNO3

(8)

MgO +

H2SO4

(5)

Na2O +

(6)


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); 2Sulfates (SO4 ) can be formed using dilute sulfuric acid (H2SO4); Nitrates (NO3 ) can be formed using dilute nitric acid (HNO3); 3Phosphate (PO4 ) 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 Metal/Carbonate/Base/Alkali Acid zinc (Zn) hydrochloric acid (HCl) copper (II) carbonate (CuCO3) nitric acid (HNO3) magnesium oxide (MgO) sulfuric acid (H2SO4) potassium hydroxide (KOH) phosphoric acid (H3PO4)

Salt Formed zinc chloride (ZnCl2) copper (II) nitrate (Cu(NO3)2) magnesium sulfate (MgSO4) 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 Group I Salts (sodium and potassium) ammoniums nitrates

Soluble

Insoluble

all

-----

all all

chlorides

all others

sulfates

all others

------silver chloride (AgCl) and lead (II) chloride (PbCl2) barium sulfate (BaSO4), lead (II) sulfate (PbSO4) and calcium sulfate (CaSO4 - only slightly soluble) silver iodide (AgI) and lead (II) iodide (PbI2) all others

all others

iodides carbonates

ammonium, sodium, potassium ammonium, sodium, potassium, barium.

hydroxides

all others

36. The chemical reactions to obtain salts are as follows: Means Metal-Salt Displacement Reaction Metal + Salt Solution Salt + Metal of salt solution Metal + Acid Salt + Hydrogen Insoluble Base + Acid Salt + Water Insoluble Carbonate + AcidSalt + Carbon dioxide + Water

• • •

Alkali + Acid Salt + Water

• •

Soluble Carbonate + Acid Salt + Water + Carbon dioxide

•

• •

• Reaction between Two Aqueous Solutions

• • • • •

Challenges Solid-Acid Reaction 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.

Titration 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). nd 2 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 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: Solubility of the salt to be produced

No

• • • •

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.

Yes

Is the salt a Group I or + NH4 salt?

• Yes

• • •

• • 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.

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.

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.

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.

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. Chemistry – Acids, Bases and Salts Page 17 of 20

39. Selection of reagents: a.

Precipitation Insoluble Salt

Formula

silver chloride

AgCl

lead (II) chloride

PbCl2

lead (II) sulfate

PbSO4

calcium carbonate

CaCO3

Reagents

Chemical equation

aqueous AgNO3 and aqueous HCl/NaCl/KCl aqueous Pb(NO3)2 and aqueous HCl/NaCl/KCl aqueous Pb(NO3)2 and aqueous H2SO4/Na2SO4/K2SO4 aqueous Ca(NO3)2 /CaCl2 and aqueous H2CO3/Na2CO3/K2CO3

AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) AgNO3 + NaCl AgCl + NaNO3 Pb(NO3)2 + 2HCl PbCl2 + 2HNO3 Pb(NO3)2 + 2NaCl PbCl2 + 2NaNO3 Pb(NO3)2 + H2SO4 PbSO4 + 2HNO3 Pb(NO3)2 + Na2SO4 PbSO4 + 2NaNO3 Ca(NO3)2 + H2CO3 CaCO3 + 2HNO3 Ca(NO3)2 + Na2CO3 CaCO3 + 2NaNO3

b. Titration (i) Acid + Alkali Soluble Salt

Formula

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)

Reagent

(ii) Acid + Soluble Carbonate/Alkali Soluble Salt Formula potassium nitrate

KNO3

sodium sulfate

Na2SO4

sodium chloride

NaCl

Chemical equation

Reagent

aqueous K2CO3 /KOH and aqueous HNO3 aqueous Na2CO3 /NaOH and aqueous H2SO4 aqueous Na2CO3 /NaOH and aqueous HCl

Chemical equation K2CO3 (aq) + 2HNO3 (aq) 2KNO3 (aq) + CO2 (g) + H2O (l) Na2CO3 + H2SO4 Na2SO4 + CO2 + H2O Na2CO3 + 2HCl 2NaCl + CO2 + H2O

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

Formula

zinc chloride

ZnCl2

calcium nitrate

Ca(NO3)2

silver nitrate

AgNO3

magnesium sulfate

MgSO4

Reagent

Chemical equation

solid zinc carbonate and aqueous HCl solid calcium carbonate and aqueous HNO3 solid silver carbonate and aqueous HNO3 solid magnesium carbonate and aqueous H2SO4

ZnCO3 (s) + 2HCl (aq) ZnCl2 (aq) + 2H2O (l) + CO2 (g) CaCO3 + 2HNO3 Ca(NO3)2 + 2H2O + CO2 Ag2CO3 + 2HNO3 2AgNO3 + H2O + CO2 MgCO3 + H2SO4 MgSO4 + H2O + CO2 Chemistry – Acids, Bases and Salts Page 18 of 20

(ii) Metal + Acid Soluble Salt

Formula

Reagent

Chemical equation

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 Soluble Salt Formula zinc chloride

ZnCl2

magnesium nitrate

Mg(NO3)2

aluminium sulfate

Al2(SO4)3

Reagent solid zinc oxide and aqueous HCl solid magnesium oxide and aqueous HNO3 solid aluminium oxide and aqueous H2SO4

Chemical equation ZnO (s) + 2HCl (aq) ZnCl2 (aq) + 2H2O (l) MgO + 2HNO3 Mg(NO3)2 + H 2O 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.


Chemistry Acids, Bases and Salts Notes - SmartLab

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