Acids and Bases

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  • The difference between a strong acid and a weak acid is similar to the difference between a group of excellent netball players and that of terrible ones. A good netball player delivers the ball efficiently by passing it to another player or shooting into the net. She gets rid of the ball, just as a strong acid gives away or donates a proton. During a practice where everyone has a netball, nearly 100% of the excellent netball player will have passed the ball to another player or shot the it into the goal. Almost none will still be left holding the ball. This is similar to nearly 100% of the strong acid molecules dissolved in water giving away/ donating protons.
    HCl(aq) H+ (aq) + Cl- (aq)
    Almost none remain as protonated/un-ionized HCl(aq).
    On the other hand, a bad netball player delivers the ball
    Inefficiently. She is indecisive and tends to hold onto the ball, just like a weak acid does not readily donate its proton. At the end of practice, less than 100% of bad netball players will have pass the ball to others or shoot into the goal. For instance, only 10% of the netball player will pass the ball to others or shoot into the goal. The other 90% may be found holding the ball. Similarly, less than 100% of weak acid molecules dissolved in water may have delivered a proton. The rest of the molecules remain as protonated or un-ionized acid molecules. In conclusion, 100 aqueous strong acid molecules will successfully release 100 H+ ions into solution, whereas 100 aqueous weak acid molecules may only deliver 10 H+ ions.
    Give examples of strong and weak acids.
  • Acids and Bases

    1. 1. Acids and Bases
    2. 2. What are these? Lemon CokeVinegar
    3. 3. What is an Acid?  An acid is a substance that produces hydrogen ions, H+ in water.  An acid therefore can conduct electricity.  pH < 7  It has a sour taste.  It has a stinging feeling.  It is corrosive.
    4. 4. What are these? Window Cleaner Floor Cleaner Baking Soda
    5. 5. What is an Alkali?  An alkali is a substance that produces hydroxide ions, OH- in water.  An alkali can conduct electricity.  pH > 7  It has a bitter taste.  It is caustic.  It is corrosive.
    6. 6. Examples of Acids Names of acids Chemical formula Where can it be found? Hydrochloric acid HCl In gastric juice in the stomach Sulphuric acid H2SO4 In car battery Nitric acid HNO3 In the preparation of fertilizers and explosives Carbonic acid H2CO3 In fizzy drinks Citric acid C6H8O7 In oranges and lemons Acetic acid CH3COOH In vinegar
    7. 7. Acidic or not? Chemical name Name of compound Is it Acidic? H2SO4 (aq) Sulphuric acid Yes HNO3 (aq) Nitric acid Yes CH3COOH(aq) Ethanoic acid/acetic acid Yes H2O (s) water No
    8. 8. Strength of an Acid  The strength of an acid depends on the extent of ionisation.  The acid is stronger when more molecules are ionised.
    9. 9. Strong Acids : acid molecule : acid ions _ + _ _ _ _ + + + + _ + Dissolved in water
    10. 10. Strong Acids  A strong acid is one in which all the acid molecules become ions in water.  Examples: Hydrochloric acid, HCl (aq) Sulphuric acid, H2SO4 (aq) Nitric acid, HNO3 (aq)
    11. 11. Weak Acids : acid molecule : acid ions _ + _ + Dissolved in water
    12. 12. Weak Acids  A weak acid is one in which only a few acid molecules become ions in water.  Examples: Acetic acid, CH3COOH (aq) Citric acid, C6H8O7(aq) Carbonic acid, H2CO3(aq)
    13. 13. Reaction of Acids with Metals  General Equation: Metal + Acid  Salt + Hydrogen gas  Observation: The metal dissolves and effervescence occurs. The gas extinguishes a burning splint with a ‘pop’ sound.
    14. 14. Reaction of Acids with Metals  Mg(s) + 2HCl(aq) → Magnesium + Hydrochloric acid →  Ca(s) + H2SO4 (aq) → Calcium + Sulphuric acid →  Zn (s) + H2SO4 (aq) → ZnSO4(aq) + H2(g) → Zinc + sulphate Hydrogen gas MgCl2 (aq) + H2 (g) Magnesium chloride + hydrogen gas CaSO4 (aq) + H2 (g) Calcium sulphate + hydrogen gas Zinc + sulphuric acid
    15. 15. Reaction of Acids with Carbonates  General Equation: Metal + Acid  Salt + Water + Carbon Carbonate dioxide gas
    16. 16. Reaction of Acids with Carbonates  Observation: The metal dissolve and effervescence occurs. The gas forms a white precipitate with limewater.
    17. 17. Reaction of Acids with Carbonates  MgCO3 (s) + 2HCl(aq) → Magnesium + carbonate Hydrochloric acid →  → CaSO4(aq) + H2O(l) + CO2(g) Calcium + sulphate Water +→ Carbon dioxide gas MgCl2 (aq) + H2O (l)+ CO2(g) Magnesium chloride + water + carbon dioxide CaCO3 (s) + H2SO4 (aq) Calcium + sulphuric Carbonate acid
    18. 18. Reaction of Acids with Alkalis  General Equation: Alkali + Acid  Salt + Water
    19. 19. Reaction of Acids with Bases  Base: Metal oxides, Metal hydroxides  NaOH + HCl   KOH + H2SO4  Sodium hydroxide Hydrochloric acid Potassium hydroxide Sulphuric acid + + + +→ → NaCl (aq) + H2O (l) Sodium chloride water K2SO4 (aq) + H2O (l) Potassium sulphate water
    20. 20. Uses of Acids Acids Preserving food -acetic acid in vinegar Making industrial products -nitric acid in fertilisers -sulphuric acid in detergents Rust removal from iron and steel -Sulphuric acid -Hydrochloric acid Car battery -Sulphuric acid
    21. 21. Complete the Puzzle Across 1. Acids react with _______ to give salt and water only. 5. Only _______ solution of acids have acidic properties. 6. In a strong acid, the acid molecules are _______ ionised. 8. Ethanoic acid is an example of a _______ acid. 9. Acids have a _______ taste. Down 2. _______ acid is an example of a strong acid which is found in car battery. 3. _______ react with acids to give off a gas that forms a white precipitate with limewater. 4. When dissolved in water, acid molecules can ionise to give _______ ions. 7. Hydrogen gas is evolved when acids react with _______.
    22. 22. Answer Across 1. Acids react with bases to give salt and water only. 5. Only aqueous solution of acids have acidic properties. 6. In a strong acid, the acid molecules are completely ionised. 8. Ethanoic acid is an example of a weak acid. 9. Acids have a sour taste.
    23. 23. Answer Down 2. Sulphuric acid is an example of a strong acid which is found in car battery. 3. Carbonates react with acids to give off a gas that forms a white precipitate with limewater. 4. When dissolved in water, acid molecules can ionise to give hydrogen ions. 7. Hydrogen gas is evolved when acids react with metals.
    24. 24. Summary  An acid is a substance that produces hydrogen ions in water.  Only an aqueous solution of acid has acidic properties. HCl (aq)  H+ (aq) + Cl- (aq)  The greater the extent of ionisation, the stronger the acid.
    25. 25. Summary  An acid - pH < 7 - turn blue litmus paper red. - taste sour - stinging feeling - is corrosive (for strong acids) - can conduct electricity.
    26. 26. Summary  Reactions of acids with 1. metals Metal + Acid  Salt + Hydrogen gas 2. carbonates Metal + Acid  Salt + Water + Carbon Carbonate dioxide gas 3. bases Base + Acid  Salt + Water
    27. 27. Summary  An alkali - pH > 7 - turn red litmus paper blue - taste bitter - caustic - is corrosive (for strong alkalis) - can conduct electricity.
    28. 28. pH Indicators  Methyl orange:
    29. 29. pH Indicators  Universal Indicator
    30. 30. pH Indicators  Phenolphthalein

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