Thursday, 22 January 2015

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) H2 OH+(aq) + Cl-(aq)
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
Nitric acid
HNO3(aq) H 2OH+(aq) + NO3-(aq)
HNO3(aq) + H2O(l) → H3O+(aq) + NO3-(aq)
Sulphuric acid
H2SO4(aq) H2O2H+(aq) + SO42-(aq)
H2SO4(aq) + 2H2O(l) → 2H3O+(aq) + SO42-(aq)
■ Definition of acid (Bronsted and Lowry)

Proton donor (H+) when dissolved in water.
■ Basicity of acid

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) H2 OH+(aq) + Cl-(aq)
Diprotic acid
One molecule of an acid dissociates in water producing two hydrogen ion, H+
H2SO4(aq) H2O2H+(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)

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) H2 ONa+(aq) + OH-(aq)
Barium hydroxide Ba(OH)2(s)H 2OBa2+(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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