Acids, Alkalis and Bases
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Transcript of Acids, Alkalis and Bases
Acids, Alkalis and
Bases
Learning ObjectivesBy the end of the lesson, you should
be able to:
a) Give the definition of a strong and weak acid, and the properties of it.
b) Identify the products of reactions between Acid and metals, carbonates, bases.
c) Give the definition of a strong and weak Alkali, and the properties of it.
d) Identify the products of reactions between Bases and Ammonia salts
e) State the reactivity series and colour changes of each indicators.
Acids An acid is a substance which produces
hydrogen ions as the only positive ions when it is dissolved in water.
E.g. HCl molecules dissolve in water to give hydrogen ion and chloride ion.
[Examples] Strong Acids
Common Name Chemical Name Chemical Formula
Hydrochloric Acid
Hydrogen Chloride
HCl
Nitric Acid Hydrogen Nitrate
HNO3
Sulfuric Acid Hydrogen Sulfate H2SO4
Strength of an Acid The strength of an acid depends on its degree
of dissociation/ ionization in water to form hydrogen ions (H+).
A strong acid is one that ionizes completely in water to produce hydrogen ions (H+). There
are no molecules left. The solution contains a high concentration of ions : Good electrical
conductor.
HCl (aq) → H+ (aq) + Cl– (aq)
A weak acid is one that ionizes partially in water to produce few hydrogen ions (H+). Most of
the acid molecules remain as molecules. The solution contains a low concentration of ions :
Poor electrical conductor.
Properties of AcidsAn acid, when dissolved in water, forms a
colourless solution. Solution of acids have the following properties:
a) Sour taste
b) Change the colour of indicators (E.g. Turns blue litmus paper red)
c) Contains hydrogen ions and conduct electricity
d) Reacts with metals, carbonates and bases.
*Note: Acids only behave as acids when they are dissolved in water.
Reaction with metalsAcids react with metals to produce a salt and
hydrogen
E.g. 2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)
Observations:
a) Effervescence observed. Colourless, odourless gas evolved which extinguished a light wooden splint with a “pop” sound.
b) Grey solid reduced in size (when in excess). Grey solid dissolves to form a colourless solution.
c) Heat is liberated.
Reaction with carbonatesAcids react with carbonates to produce a salt,
carbon dioxide and water
E.g. Na2CO3 (s) + H2SO4 (aq) → Na2SO4 (aq)+
CO2 (g)+ H2O (l)
Observations:
a) Effervescence observed. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate.
b) White solid dissolves to form a colourless solution.
Reaction with basesAcids react with bases (metal oxides and
hydroxides) to produce a salt and water
E.g. MgO (s) + H2SO4 (aq) → MgSO4 (aq)+ H2O (l)
Observations:
a) White solid dissolves to form a colourless solution.
b) Heat is evolved. No visible change.
Uses of acids
Sulfuric Acid: Ethanoic Acid:
To make Detergents and Fertilizers
To preserve food
In car batteries as battery acid
treatment of the sting of box jellyfish
Oil refining, wastewater processing, and chemical synthesis.
Used in photographic films
IndicatorsIndicators Observations
Litmus paper An acid turns blue litmus paper red
Methyl Orange Used to identify strong acidsColour change at pH 4Strong acids turn methyl orange redAlkalis turn methyl orange yellow
Phenolphthalein Used to identify strong alkalisColour change at pH 9Strong alkali turn phenolphthalein pinkPhenolphthalein remains colourless in acid.
Universal Indicator
In neutral solutions (pH 7) , universal indicator remains green. Strong acid: redStrong alkali: violet
Bases and AlkalisDefinition of base:
Bases are the oxides or hydroxides of metal that reacts with an acid to form salt and
water only.
Definition of alkali:
An alkali is a metal hydroxide which is soluble in water and produces hydroxide ions (OH–)
in water. All alkalis are bases but not all bases are alkalis.
Insoluble base:
Copper (II) oxide, aluminium oxide, zinc oxide, zinc hydroxide.
Soluble base/alkali:
Sodium oxide, Potassium oxide, Sodium hydroxide, Potassium hydroxide,
Ammonium Hydroxide
The strength of an alkali depends on its degree of dissociation/ ionization in water to
form hydroxide ions (OH–) . The properties are due to hydroxide ions.
A strong alkali is one that ionizes completely in water to produce hydroxide ions (OH–) . There
are no molecules left. The common strong alkalis are sodium hydroxide and potassium
hydroxide.
KOH(aq) → K+ (aq) + OH– (aq)
A weak alkali is one that ionizes partially in water to produce few hydroxide ions (OH–). Most of the alkali molecules remain as molecules.
Strength of an alkali
Properties of basesa) Alkalis have a soapy feel and bitter taste
b) Alkalis can change the colour of indicators (e.g. turn red litmus paper blue)
c) Bases can react with acids to form salt and water only- Neutralization
Example:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
Reaction with Ammonium SaltsBases react with ammonium salts to produce
a salt, ammonia and water.
E.g. NaOH (aq) + NH4Cl (s) → NaCl (aq) + NH3 (g) + H2O (l)
Observations:
a) A colourless, pungent gas is evolved, that turns damp red litmus paper blue.
Uses of bases and alkalisMagnesium Hydroxide:
- In toothpaste to neutralize the acids on our teeth produced by bacteria
Ammonia Solution:- Used for cleaning glass windows
THE END