Acids and Bases
Acids
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An acid is a substance which when dissolves in water produces hydrogen ions (𝐻 + ) as the only positively charged
ions.
An acid can also be defined as a proton donor.
Acids have a sour taste, turn blue litmus red and give solutions with pH values below 7.
As hydrogen ions are responsible for the properties of acids, an acid that is not dissolved in water does not show
these properties.
Some commonly used acids are hydrochloric acid(𝐻𝐶𝑙), sulfuric acid (𝐻2 𝑆𝑂4 ), and nitric acid(𝐻𝑁𝑂3 ).
Types of Acids
(a) Mineral Acids
Mineral acids are also called inorganic acids. They are acids that are prepared from the minerals obtained from
the earth.
Generally all mineral acids are strong acids.
e.g. Hydrochloric acid(𝐻𝐶𝑙), Sulfuric acid (𝐻2 𝑆𝑂4 ), Nitric acid(𝐻𝑁𝑂3 ), and Phosphoric acid(𝐻3 𝑃𝑂4 ).
(b) Organic Acids
Organic acids are naturally occurring acids and are found in organic matter of living things.
They are prepared from organic sources such as plants.
Generally organic acids are weak acids.
e.g. Methanoic acid (𝐻𝐶𝑂𝑂𝐻), Ethanoic acid (𝐶𝐻3 𝐶𝑂𝑂𝐻) and Propanoic acid(𝐶2 𝐻5 𝐶𝑂𝑂𝐻).
Chemical Properties of Acids
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Dilute acids react with metals that lie above hydrogen in the reactivity series. The reaction produces salt and
hydrogen gas. e.g.
(a) Zinc + dilute hydrochloric acid → zinc chloride + hydrogen
𝑍𝑛(𝑆) + 𝐻𝐶𝑙(𝑎𝑞) → 𝑍𝑛𝐶𝑙2 (𝑎𝑞) + 𝐻2 (𝑔)
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Metals such as copper and silver do not react with dilute acids as they are unreactive.
Although lead lies above hydrogen in the reactivity series, it appears to be unreactive in dilute hydrochloric acid
and dilute sulfuric acid. This is due to the formation of a layer of insoluble salt around the metal. The layer
prevents contact between the acid and the metal, therefore causing the reaction to end prematurely
Acids react with carbonates (and hydrogen carbonates) to produce salt, water and carbon dioxide. e.g.
(a) Calcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide
𝐶𝑎𝐶𝑂3 (𝑆) + 𝐻2 𝑆𝑂4 (𝑎𝑞) → 𝐶𝑎𝑆𝑂2 (𝑎𝑞) + 𝐻2 𝑂(𝑙) + 𝐶𝑂2 (𝑔)
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Acids react with bases to form salt and water. e.g.
(a) Aluminium oxide + dilute hydrochloric acid → Aluminium chloride + water
𝐴𝑙2 𝑂3 (𝑆) + 𝐻𝐶𝑙(𝑎𝑞) → 𝐴𝑙𝐶𝑙3 (𝑎𝑞) + 𝐻2 𝑂(𝑔)
(b) Sodium hydroxide + dilute hydrochloric acid → sodium chloride + water
𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐻𝐶𝑙(𝑎𝑞) → 𝑁𝑎𝐶𝑙(𝑎𝑞) + 𝐻2 𝑂(𝑙)
Ionization of acids
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Ionization is the process of forming ions. An ion is a charged particle.
When acids dissolve in water, they produce hydrogen ions 𝐻 + as the only positively charged ions.
e.g. 𝐻2 𝑆𝑂4 (𝑎𝑞) → 2𝐻 + (𝑎𝑞) + 𝑆𝑂4 2− (𝑎𝑞)
𝐶𝐻3 𝐶𝑂𝑂𝐻(𝑎𝑞) ⇌ 𝐻 + (𝑎𝑞) + 𝐶𝐻3 𝐶𝑂𝑂 − (𝑎𝑞)
Acid Strength and Concentration
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Strength of an acid is the measure of its ability to produce hydrogen ions.
It can also be defined as the ability to donate protons.
A strong acid fully ionizes in water to form 𝐻 + ions. Such acids include hydrochloric acid, sulfuric acid and
phosphoric acid. e.g. 𝐻𝐶𝑙(𝑎𝑞) → 𝐻 + (𝑎𝑞) + 𝐶𝑙 − (𝑎𝑞)
A weak acid partially ionizes in water. The partial dissociation is represented in an equation with a ⇌ symbol.
Examples of weak acids include carboxylic acids, such as ethanoic acid(𝐶𝐻3 𝐶𝑂𝑂𝐻).
e.g. 𝐶𝐻3 𝐶𝑂𝑂𝐻(𝑎𝑞) ⇌ 𝐻 + (𝑎𝑞) + 𝐶𝐻3 𝐶𝑂𝑂 − (𝑎𝑞)
The concentration of an acid depends on the amount of acid dissolved in water. Dissolving a small amount of acid
in water gives a dilute acid solution, while dissolving a large amount of acid in water gives a concentrated acid
solution.
Basicity of an acid
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Basicity of an acid is the number of moles of hydrogen ions 𝐻 + produced from one mole of an acid.
e.g.
(a) 𝐻𝐶𝑙 is a Monobasic acid, because it produces one Hydrogen ion. (𝐻𝐶𝑙(𝑎𝑞) → 𝐻 + (𝑎𝑞) + 𝐶𝑙 − (𝑎𝑞) )
(b) 𝐻2 𝑆𝑂4 is a Dibasic acid, because it produces two Hydrogen ions. (𝐻2 𝑆𝑂4 (𝑎𝑞) → 2𝐻 + (𝑎𝑞) + 𝑆𝑂4 2− (𝑎𝑞) ).
(c) 𝐻3 𝑃𝑂4 is a Tribasic acid, because it produces three Hydrogen ions. (𝐻3 𝑃𝑂4 (𝑎𝑞) → 3𝐻 + (𝑎𝑞) + 𝑃𝑂4 3− (𝑎𝑞) ).
BASES
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A base is a metal oxide or hydroxide that reacts with acids to produce salt and water.
Bases have a bitter taste, turn red litmus blue, and give solutions with pH values above 7.
Some bases dissolve in water to produce 𝑂𝐻 − ions. These bases are known as alkalis.
An alkali is a substance which when dissolved in water produces the hydroxide ions, OH- , as the only negatively
charged ions.
Examples of alkalis include:
(a) Sodium hydroxide(𝑁𝑎𝑂𝐻) Known as Caustic Soda
(b) Calcium hydroxide(𝐶𝑎(𝑂𝐻)2 ) Known as Lime Water
(c) Potassium Hydroxide (𝐾𝑂𝐻) Known as Caustic Potash
(d) Ammonium Hydroxide (𝑁𝐻4 𝑂𝐻) Known as Aqueous Ammonia
Examples of Insoluble Bases Include:
(a) Copper (II) oxide(𝐶𝑈𝑂)
(b) Zinc oxide (𝑍𝑛𝑂)
(c) Iron (III) oxide (𝐹𝑒2 𝑂3 )
(d) Lead (II) oxide (𝑃𝑏(𝑂𝐻)2 )
Ionization of alkalis
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When alkalis dissolve in water, they produce hydroxide ions, OH- , as the only negatively charged ions.
e.g.
(a) 𝑁𝑎𝑂𝐻(𝑎𝑞) → 𝑁𝑎+ (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
(b) 𝐾𝑂𝐻(𝑎𝑞) → 𝐾 + (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
(c) 𝑁𝐻4 𝑂𝐻(𝑎𝑞) → 𝑁𝐻4 + (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
(d) 𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) → 𝐶𝑎2+ (𝑎𝑞) + 2𝑂𝐻 − (𝑎𝑞)
Strength of alkalis
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Strength of an alkali is the measure of its ability to produce hydroxide ions.
A weak alkali is an alkali that partially ionizes when dissolved in water and has a reversible ionization (⇌).
e.g. Ammonium Hydroxide
𝑁𝐻4 𝑂𝐻(𝑎𝑞) → 𝑁𝐻4 + (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
A strong alkali is an alkali that ionizes completely when dissolved in water.
e.g.
(a) Sodium hydroxide
𝑁𝑎𝑂𝐻(𝑎𝑞) → 𝑁𝑎+ (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
(b) Potassium hydroxide
𝐾𝑂𝐻(𝑎𝑞) → 𝐾 + (𝑎𝑞) + 𝑂𝐻 − (𝑎𝑞)
(c) Calcium hydroxide
𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) → 𝐶𝑎2+ (𝑎𝑞) + 2𝑂𝐻 − (𝑎𝑞)
Physical properties of alkalis
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Alkalis have PH numbers greater than 7.
Alkalis turn red litmus paper blue.
Alkalis have a bitter taste and feel soapy or slippery between fingers.
Chemical properties of bases and alkalis
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Alkalis and bases react with acids to form a salt and water only
e.g.
𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐻𝐶𝑙(𝑎𝑞) → 𝑁𝑎𝐶𝑙(𝑎𝑞) + 𝐻2 𝑂(𝑙)
𝐶𝑢𝑂(𝑠) + 𝐻2 𝑆𝑂4 (𝑎𝑞) → 𝐶𝑢𝑆𝑂4 (𝑎𝑞) + 𝐻2 𝑂(𝑙)
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Alkalis reacts with ammonium compounds to form a salt, water and ammonia gas.
e.g.
𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝑁𝐻4 𝐶𝑙(𝑎𝑞) → 𝑁𝑎𝐶𝑙(𝑎𝑞) + 𝐻2 𝑂(𝑙) + 𝑁𝐻3 (𝑔)
𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) + 𝑁𝐻4 𝑁𝑂3 (𝑎𝑞) → 𝐶𝑎𝑁𝑂3 (𝑎𝑞) + 𝐻2 𝑂(𝑙) + 𝑁𝐻3 (𝑔)
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Alkalis react with solutions of soluble salts to form precipitates of insoluble hydroxides.
e.g.
2𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐶𝑢𝑆𝑂4 (𝑎𝑞) → 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞) + 𝐶𝑢(𝑂𝐻)2 (𝑆)
2𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐹𝑒𝑆𝑂4 (𝑎𝑞) → 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞) + 𝐹𝑒(𝑂𝐻)2 (𝑆)
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Alkalis react with acidic oxides to form a salt and water only.
e.g.
2𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝑆𝑂3 (𝑔) → 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞) + 𝐻2 𝑂(𝑙)
𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) + 𝑆𝑂3 (𝑔) → 𝐶𝑎𝑆𝑂4 (𝑎𝑞) + 𝐻2 𝑂(𝑙)