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Strength Of Acid And Alkali

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Strength Of Acid And Alkali

  1. 1. Strength of Acids & Alkalis
  2. 2. Strong vs Weak Acids • The strength of an acid depends on the degree of ionisation/ dissociation of the acid in water. • Strong acid – Acid that Ionises completely in water to give high concentration of H+ ion. • Weak acid – Acid that partially ionises in water to give low concentration of H+ ion
  3. 3. Example of Strong & Weak Acid • Strong acid – Hydrochloric acid – All hydrogen chloride molecule ionises completely in water into H+ ions and Cl- ions – HCl  H+ + Cl- – No hydrogen chloride molecules in acid • Weak Acid – Ethanoic acid – Only few CH3COOH molecules ionises to form H+ ions and CH3COO- ions – CH3COOH H+ + CH3COO-
  4. 4. Ethanoic Acid Hydrochloric Acid CH3COOH H-Cl CH3COOH H-Cl CH3COOH H-Cl CH3COOH H-Cl CH3COOH CH3COOH H-Cl H-Cl H+ CH3COOH H+ Cl- H+ CH3COO- CH3COOH Cl- H+ CH3COOH Cl- H+ CH3COOH CH3COOH H+ Cl- Cl-
  5. 5. Strong vs Weak Alkali • The strength of an alkali depends on the degree of ionisation of the alkali in water. • Strong alkali – Alkali that Ionises completely in water to produce high concentration of OH- ion • Weak alkali – Alkali that Ionises partially in water to produce low concentration of OH- ion
  6. 6. Strong vs Weak Alkali • Strong alkali – Sodium hydroxide ionises completely when dissolved in water – NaOH  Na+ + OH- • Weak alkali – Only some ammonia molecules ionises in water to produce ammonium ions, NH4+ and hydroxide ions, OH- – NH3 + H2O ⇄ NH4+ + OH-
  7. 7. Sodium hydroxide Ammonia solution Na+ OH- Na+ OH- NH3 NH3 OH- Na+ OH- Na+ NH3 NH3 Na+ OH- Na+ OH- OH- Na+ OH- Na+ NH3l NH3 Na+ OH- Na+ NH4+ NH3 + OH- Na OH- OH- NH3 Na+ NH3 Na+ OH- NH3 OH- NH3
  8. 8. Compare hydrochloric acid & ethanoic acid Type of acid pH value Hydrochloric acid 1 Ethanoic acid 3 Between two acids, • Which one is more acidic? • Which one has higher concentration of H+ ions? • Which one has higher degree of ionisation? Compare the strength of the two acids and the degree of ionisation. What are the difference between strong acid & weak acid?
  9. 9. Strong acid vs Weak Acid Strong acid Weak Acid Lower pH value Higher pH value High degree of ionisation Low degree of ionisation Has higher concentration Has lower concentration of H+ ions of H+ ions Example: Example: hydrochloric acid Ethanoic acid Nitric acid Carbonic acid Sulphuric acid Sulphurous acid
  10. 10. Compare sodium hydroxide solution & ammonia solution Type of alkali pH value Sodium hydroxide 14 solution Ammonia solution 11 Between two alkalis, • Which one is more alkaline? • Which one has higher concentration of OH- ions? • Which one has higher degree of ionisation? Compare the strength of the two alkalis and the degree of ionisation. What are the difference between strong alkali & weak alkali?
  11. 11. Strong alkali vs Weak Alkali Strong alkali Weak Alkali higher pH value lower pH value High degree of ionisation Low degree of ionisation Has higher concentration Has lower concentration of OH- ions of OH- ions Example: Example: Sodium hydroxide Ammonia solution Potassium hydroxide methylamine Barium hydroxide hydrazine

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