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Acids, Bases and Buffers

Acids, Bases and Buffers. History and Definitions. Your task is to research the history of acids and bases. Doing this you will need to find out about Arrhenius, Bronsted & Lowry and Lewis.

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Acids, Bases and Buffers

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  1. Acids, Bases and Buffers

  2. History and Definitions • Your task is to research the history of acids and bases. • Doing this you will need to find out about Arrhenius, Bronsted & Lowry and Lewis. • By the end of the lesson you will also need to definitions of acids and bases- with examples as equations.

  3. Bronsted-Lowry • Bronsted-Lowry acids and bases • A Bronsted-Lowry acid is any substance from which a proton can be removed • A Bronsted-Lowry base is any substance that can remove a proton from an acid • A single proton doesn’t really exist in a solution. Acids only release protons if a base can accept it.

  4. Conjugate Acid-Base Pairs • Instead of a floating proton in solution, water molecules accept protons to form hydronium ions, H3O+(aq) • This is sometimes called an oxonium ion.

  5. This is an acid This is a thing that can accept a proton, it’s the acids conjugate base Which one is the base and conjugate acid between these two?

  6. An acid-base pair is a set of two species that transform into each other by gain or loss of a proton

  7. Calculations • Practice questions on page 139.

  8. pH

  9. pH • Don’t ask what it means. Noone knows. • pH is all about the concentration of hydrogen ions in solution. • It is a logarithmic scale of concentration of hydrogen ions.

  10. pH calculations pH = -log[H+(aq)] [H+] = 10–pH

  11. Your Calculator

  12. Calculations • Attempt calculations on page 141 of text book.

  13. Strong and Weak Acids

  14. Strong Acids • Strong acids completely dissociate in aqueous solution. • Only a few exist, the rest are weak. • HCl -HI • HNO3 -HClO4 • H2SO4 • HBr

  15. Weak Acids • Weak acids only partially dissociate in aqueous solution, the equilibrium lies well to the left.

  16. Ka The Acid Dissociation Constant • A weak acid has the following equilbrium: HA H+ + A- • The expression for the acid dissociation constant is: Ka= • Units are always:

  17. KaContext • A strong acid has a high Ka value. • A weak acid has a small Kavalue. • Can also convert these into logs, which makes the numbers more manageable. pKa = -log10Ka Ka = 10-pKa • Taking logs inverts the values. High pKa is a weak acid and vice versa.

  18. pH of Strong Acids • For a strongacid: HA(aq) H+(aq) + A-(aq) • HA totally dissociates: [HA] = [H+] • Use pH = -log[H+] • A bottle of HCl has a concentration of 1.22 x10-3mol dm-3. What is the pH?

  19. pH of Weak acids • For a weak acid: HA(aq) H+(aq) + A-(aq) • HA only partially dissociates. • H+ and A- are formed equally. [H+]=[A-] • In our equation for Ka: [H+][A-] = [H+]2 • Due to the small partial dissociation we can assume that the equilibrium concentration of HA is the same as the start concentration. This gives us the equation:

  20. pH of Weak Acids Ka = [H+]2 [HA] Or [H+] = Ka x [HA]

  21. Weak Acid Practice • A sample of nitric acid, HNO2, has the concentration 0.055 mol dm-3. Ka = 4.70 x10-4 mol dm-3 at 25oC. Calculate the pH. • Ka= • [H]+ = • pH =

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