7.1 Acids and Bases

Acid
■ Definition of acid (Arrhenius theory)

	►	Chemical compound which dissociates (ionizes) to produce hydrogen ion, H+ (hydroxonium ion, H3O+) when dissolved in water.
	►	Example of acid: Acid Equation (hydrogen ion, H+) Hydrochloric acid ○ HCl(g) $\stackrel{H_{2}O}{\to }$H+(aq) + Cl-(aq) ○ HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq) Nitric acid ○ HNO3(aq) $\stackrel{H_{2}O}{\to }$H+(aq) + NO3-(aq) ○ HNO3(aq) + H2O(l) → H3O+(aq) + NO3-(aq) Sulphuric acid ○ H2SO4(aq) $\stackrel{H_{2}O}{\to }$2H+(aq) + SO42-(aq) ○ H2SO4(aq) + 2H2O(l) → 2H3O+(aq) + SO42-(aq)

■ Definition of acid (Bronsted and Lowry)

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Proton donor (H+) when dissolved in water.

■ Basicity of acid

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The number of hydrogen ions, H+,which can be produced by one molecule of acid. Basicity Example Monoprotic acid ○ One molecule of an acid dissociates in water producing one hydrogen ion, H+ HCl(g) $\stackrel{H_{2}O}{\to }$H+(aq) + Cl-(aq) Diprotic acid ○ One molecule of an acid dissociates in water producing two hydrogen ion, H+ H2SO4(aq) $\stackrel{H_{2}O}{\to }$2H+(aq) + SO4-(aq)

■ This video contains information on the definition of acid and base.

Meaning of base and alkali
■ Definition of base

	►	Chemical compound that can react with acid to form salt and water only. ○ Base + Acid → Salt + Water
	►	All oxide and hydroxide metals are bases.
	►	Example: ○ Magnesium oxide : MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ○ Copper hydroxide: Cu(OH)2(s) + 2HNO3(aq) → Cu(NO3)2(aq) + 2H2O(l)
	►	Not all bases can dissolve in water. A base that dissolves in water is called an alkali. Base Solubility Sodium oxide Soluble in water, Thus sodium hydroxide is called an alkali. Na2O(s) + H2O(l) → 2NaOH(aq) Copper hydroxide Insoluble in water. Thus, copper hydroxide is a base but not an alkali.

■ Definition of alkali( Arrhenius theory)

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A compound which produces hydroxide, OH- , when dissolved in water.

■ Alkali solution

	►	Contains hydroxide ion, OH-, which move freely.
	►	Hydroxide ion, OH- which are free to move in water enable an alkali to show its properties. Alkali Ionization of base in water Sodium hydroxide NaHO(s) $\stackrel{H_{2}O}{\to }$Na+(aq) + OH-(aq) Barium hydroxide Ba(OH)2(s)$\stackrel{H_{2}O}{\to }$Ba2+(aq) + 2OH-(aq)
	►	Alkali is a proton receiver.

Application of acids and alkalis in daily life
■ Examples of acids and their uses:

	►	Sulphuric acid is used in making of detergents, paints fertilizers and as an electrolyte in lead-acid accumulators.
	►	Hydrochloric acid is used to clean metals before electroplating in the industry.
	►	Methanoic acid is used to coagulate the latex in rubber industry.
	►	Ethanoic acid is used to make vinegar.
	►	Carbonic acid is used to make fizzy drinks
	►	Nitric acid is used to make fertilizers, plastics and explosive substances.

■ Examples of bases and their uses:

	►	Ammonia is used to make cleaning agents, fertilizers, nitric acids and keep latex in liquid form.
	►	Magnesium hydroxide is used to make gas mixtures and gastric tablets and toothpaste.
	►	Calcium hydroxide is used to make cement, lime water and neutralize acidity of soil.
	►	Sodium hydroxide is used to make detergents, soaps, fertilizers and bleaching agents.

■ This video contains information on the applications of acids and alkalis.

Role of water to show properties of acids
■ Role of water in the formation of hydrogen ions

	►	In the presence of water, ○ an acid will ionise to form hydrogen ion, H+ ○ shows the acidic properties.
	►	Example: hydrochloric acid Hydrochloric acid in organic solvent ○ Exist as neutral covalent molecules. ○ Do not dissociate to form hydrogen ion, H+. ○ Do not show any acidic property. Hydrochloric acid in water ○ Dissociate in water to form hydrogen ion, H+ which are free to move. ○ Show the properties of acids.

■ This video contains information on the role of water to show properties of acids and alkalis

Laboratory Activity 7.1.1: Role of water to show properties of acids

Role of water to show properties of alkalis
■ Role of water in the formation of hydroxide ions

	►	In the presence of water, ○ an alkali will dissociate to form ion, OH- ○ shows the alkalinity properties.
	►	Example: Ammonia Ammonia in organic solvent ○ Ammonia in organic solvent exists in molecules form. ○ Do not dissociate to form hydroxide ion, OH-. ○ Do not show any alkaline property. Ammonia in water ○ Dissociate in water to form hydroxide ion, OH- which are free to move ○ Show the properties of alkalis.
	►	Example: Calcium hydroxide Dry calcium hydroxide Calcium hydroxide aqueous ○ Dry calcium hydroxide is in the solid state, Ions are not present to move. ○ Do not dissociate to form hydroxide ion, OH-. ○ Do not show any alkaline property. ○ Dissociate in water to form hydroxide ion, OH- which are free to move. ○ Show the properties of alkali.

Laboratory Activity 7.1.2: Role of water to show properties of alkalis

Properties of an acid
■ Physical properties

	►	Tastes sour
	►	Corrosive
	►	Turns blue litmus paper red
	►	Has pH value < 7
	►	Conduct electricity

■ Chemical properties

	►	Reacts with a reactive metal to release hydrogen gas, H2. Example: Hydrochloric acid reacts with magnesium to release hydrogen gas. ○ Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
	►	Reacts with a metal carbonate to release carbon dioxide, CO2. Example: Hydrochloric acid reacts with calcium carbonated to release carbon dioxide gas. ○ CaCo3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
	►	Reacts with a base to form salt and water. Example: Hydrochloric acid reacts with magnesium oxide to from magnesium chloride and water. ○ MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
	►	Neutralises an alkaline solution to form salt and water ONLY. Example: Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. ○ HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

■ This video contains information on the properties of acid and bases

Laboratory Activity 7.1.3: Properties of acid

Properties of an alkali
■ Physical properties

	►	tastes bitter and feels smooth
	►	Corrosive
	►	turns red litmus paper blue
	►	Has pH value > 7
	►	Conduct electricity

■ Chemical properties

	►	React with an ammonium salt to produce salt, water and ammonia gas. Example: Sodium hydroxide reacts with ammonium chloride to produce sodium chloride, water and ammonia gas. ○ NaOH(s) + 2NH4Cl(aq) → NaCl(aq) + H2O(l) + NH3(g)
	►	Neutralises an acid to form a salt and water ONLY. Example: Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water. ○ NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

✍ Worked-example 7.1(a)
Choose true [✔] or false [✘] for the statement given.

Statement	Answer
Hydrogen ion, H+ and hydroxonium ions, H3O+ are not the same.	✘
All alkalis are bases but not all bases are alkalis.	✔
An acid is a proton donor and alkali is a proton receiver.	✔
All bases contain the hydroxide ion OH-.	✘
Acid and bases need water to show their acidic and alkaline properties respectively.	✔

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