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Conjugate Acids & Bases (HL IB Chemistry)

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Philippa Platt

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Conjugate Acids & Bases

  • A Brønsted-Lowry acid is a species that can donate a proton
  • A Brønsted-Lowry base is a species that can accept a proton
  • In an equilibrium reaction, the products are formed at the same rate as the reactants are used

CH3COOH (aq) + H2O (l)         ⇌    CH3COO- (aq)          +         H3O+ (aq)

         acid                 base                    conjugate base             conjugate acid

  • The reactant CH3COOH is linked to the product CH3COO- by the transfer of a proton from the acid to the base
    • Similarly, the H2O molecule is linked to H3O+ ion by the transfer of a proton
  • These pairs are therefore called conjugate acid-base pairs
    • A conjugate acid-base pair is two species that are different from each other by a H+ ion
      • Conjugate here means related
      • In other words, the acid and base are related to each other by one proton difference

Worked example

In the equilibrium reaction shown below, which species are a conjugate acid-base pair?

CH3CH2CH2COOH (aq)  +  H2O (l) ⇌ CH3CH2CH2COO- (aq)  +  H3O+ (aq)

 

   A.    CH3CH2CH2COOH and H2O

   B.    H2O and H3O+

   C.    H2O and CH3CH2CH2COO-

   D.   CH3CH2CH2COO- and H3O+

 

Answer:

  • The correct option in B
    • A conjugate acid-base pair differ only by an H+ ion

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.