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Acids, Alkalis and Bases

Acids, Alkalis and Bases. Learning Objectives. By the end of the lesson, you should be able to: Give the definition of a strong and weak acid, and the properties of it. Identify the products of reactions between Acid and metals, carbonates, bases.

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Acids, Alkalis and Bases

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  1. Acids, Alkalis and Bases

  2. Learning Objectives • By the end of the lesson, you should be able to: • Give the definition of a strong and weak acid, and the properties of it. • Identify the products of reactions between Acid and metals, carbonates, bases. • Give the definition of a strong and weak Alkali, and the properties of it. • Identify the products of reactions between Bases and Ammonia salts • State the reactivity series and colour changes of each indicators.

  3. 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

  4. 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.

  5. Properties of Acids • An acid, when dissolved in water, forms a colourless solution. Solution of acids have the following properties: • Sour taste • Change the colour of indicators (E.g. Turns blue litmus paper red) • Contains hydrogen ions and conduct electricity • Reacts with metals, carbonates and bases. • *Note: Acids only behave as acids when they are dissolved in water.

  6. Reaction with metals • Acids react with metals to produce a salt and hydrogen • E.g. 2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g) • Observations: • Effervescence observed. Colourless, odourless gas evolved which extinguished a light wooden splint with a “pop” sound. • Grey solid reduced in size (when in excess). Grey solid dissolves to form a colourless solution. • Heat is liberated.

  7. Reaction with carbonates • Acids react with carbonates to produce a salt,carbon dioxide and water • E.g. Na2CO3 (s) + H2SO4 (aq) → Na2SO4 (aq)+ • CO2 (g)+ H2O (l) • Observations: • Effervescence observed. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate. • White solid dissolves to form a colourless solution.

  8. Reaction with bases • Acids react with bases (metal oxides and hydroxides) to produce a salt and water • E.g. MgO (s) + H2SO4 (aq) → MgSO4 (aq)+ H2O (l) • Observations: • White solid dissolves to form a colourless solution. • Heat is evolved. No visible change.

  9. Uses of acids

  10. Indicators

  11. Bases and Alkalis • Definition 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

  12. Strength of an alkali 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.

  13. Properties of bases • Alkalis have a soapy feel and bitter taste • Alkalis can change the colour of indicators (e.g. turn red litmus paper blue) • Bases can react with acids to form salt and water only- Neutralization • Example: • NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)

  14. Reaction with Ammonium Salts Bases 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.

  15. Uses of bases and alkalis • Magnesium Hydroxide: • In toothpaste to neutralize the acids on our teeth produced by bacteria • Ammonia Solution: • - Used for cleaning glass windows

  16. THE END

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