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I Can-do Chemistry

With

Miss Williams

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Today’s Topic:

Acids and Bases

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Objectives:

  • Define acid, acid anhydride, base, alkali, acidic, basic, amphoteric and neutral oxides
  • Relate acidity to the pH scale
  • Discuss the strength of acids and alkalis
  • Discuss the reactions of non-oxidising acids
  • List examples of acids in living systems
  • Investigate reaction of bases with ammonium salts

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Basicity of Acids

  • Basicity is the number of H⁺ ions produced per molecule of acid when the acid dissolves in water.
  • Acids can be classified according to three types of basicity:
  • Monobasic acids
  • Dibasic acids
  • Tribasic acids

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  • Monobasic Acids

These produce one H⁺ ion per molecule:

​

E.g. HCl₍aq₎ H⁺₍aq₎ + Cl₍aq₎

Monobasic acids can only form normal salts.

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  • Dibasic Acids

These produce two H⁺ ions per molecule:

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E.g. H₂SO₄₍aq₎ 2H⁺₍aq₎ + SO₄²⁻₍aq₎

Dibasic acids can form both normal salts and acid salts.

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  • Tribasic Acids

These produce three H⁺ ions per molecule:

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E.g. H₃PO₄₍aq₎ 3H⁺₍aq₎ + PO₄³⁻₍aq₎

Tribasic acids can form both normal salts and acid salts.

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Acid Anhydrides

  • A compound that reacts with water to form an acid.
  • Many are acidic oxides of non-metals.
  • Examples are carbon dioxide (CO₂), sulfur dioxide (SO₂), sulfur trioxide (SO₃) and nitrogen dioxide (NO₂).

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Acid Anhydrides

CO₂ (g) + H₂O (l) H₂CO₃ (aq)

carbonic acid

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SO₂ (g) + H₂O (l) H₂SO₃ (aq)

sulfurous acid

​

⇀

↽

⇀

↽

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Acid Anhydrides

SO₃ (g) + H₂O (l) H₂SO₄ (aq)

sulfuric acid

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2NO₂ (g) + H₂O (l) HNO₂ (aq) + HNO₃ (aq)

nitrous acid nitric acid

​

⇀

↽

⇀

↽

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Acids in Living Systems

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Bases

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  • Chemically opposite to acids.
  • They include metal oxides e.g. calcium oxide (CaO), metal hydroxides, e.g. zinc hydroxide (Zn(OH)₂) and ammonia (NH₃).
  • They are proton acceptors.

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Alkalis

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  • This is a base which dissolves in water to form a solution that contains OH⁻ ions.
  • Most bases are insoluble and so, are not considered alkalis
  • All alkalis, however, are bases.

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Properties of Aqueous Solutions of Alkalis

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  • They have a bitter taste
  • They are corrosive
  • They feel soapy
  • They change red litmus to blue
  • They have a pH greater than 7
  • They conduct an electric current so are electrolytes

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Chemical Reactions of bases

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  • Bases react with acids to form a salt and water
  • Bases react with ammonium salts to produce a salt, ammonia and water.

Mg(OH)₂ (s) + 2NH₄Cl (s) MgCl₂ (s) + 2NH₃ (g) + 2H₂O (l)

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Distinguishing Between Acids and Alkalis

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Common Indicators

Indicator

Colour in an acidic solution

Colour in an alkaline solution

Litmus

Red

Blue

Methyl orange

Red

Yellow

Screened methyl orange

Red

Green

Phenolphthalein

Colourless

Pink

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Amphoteric Oxides and Hydroxides

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  • They can react with both acids and strong alkalis to form a salt and water.
  • The oxides and hydroxides of aluminum, zinc and lead are amphoteric.

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Amphoteric Oxides and Hydroxides

Amphoteric oxide

Amphoteric hydroxide

Salts formed when reacting with a strong alkali

Anion present in the salt

Aluminum oxide (Al₂O₃)

Aluminum hydroxide (Al(OH)₃)

aluminates

AlO₂⁻

Zinc oxide (ZnO)

Zinc hydroxide (Zn(OH)₂)

zincates

ZnO₂²⁻

Lead (II) oxide (PbO)

Lead (II) hydroxide (Pb(OH)₂)

plumbates

PbO₂²⁻

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Classification of Oxides

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Acidic Oxides

  • Oxides of some non-metals which react with alkalis to form a salt and water.
  • Examples include carbon dioxide (CO₂), sulfur trioxide (SO₃), nitrogen dioxide (NO₂) and silicon dioxide (SiO₂).

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Basic Oxides

  • Oxides of metals which react with acids to form a salt and water.
  • Examples include magnesium oxide (MgO), iron (III) oxide (Fe₂O₃) and copper oxide (CuO).

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Amphoteric Oxides

  • Oxides of some metals which react with both acids and strong alkalis to form a salt and water.
  • Examples include Aluminum oxide (Al₂O₃), zinc oxide (ZnO) and lead (II) oxide (PbO)

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Neutral Oxides

  • Oxides of some non-metals which do not react with acids or alkalis.
  • Examples include carbon monoxide (CO), nitrogen monoxide (NO) and dinitrogen monoxide (N₂O).

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Acids and Bases is Complete!!