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Conjugate acids and bases

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Intros
Lessons
  1. What is a conjugate pair?
  2. Brønsted bases becoming Brønsted acids.
  3. Identifying conjugate acids and conjugate bases.
  4. Conjugate acid/base equations.
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Examples
Lessons
  1. Identify groups of conjugate pairs and understand the conjugate acid/base relationship.
    Identify three conjugate pairs in the list of compounds below.
    1. OH-, HCl, HSO4-, Cl2, NO3- H2O, H2PO4-, Cl-, NH4+, H2SO4.
    2. If OH- is a stronger base than Cl-, what does this tell us about their conjugate acids?
  2. Write simple acid-base equilibria using conjugate pairs.
    Write the reaction equation for the following two conjugate pairs. Identify the conjugate acid and base on both sides of the equation:
    1. HCl/Cl- with NH4+/NH3
    2. H3PO4/H2PO4- with H2O/OH-
Topic Notes
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In this lesson, we will learn:

  • To understand the relationship between conjugate acid-base pairs.
  • To identify conjugate acids and bases pairings.
  • How to write acid-base reaction equations using conjugate pairs.

Notes:

  • As we saw in Introduction to acid-base theory, the Brønsted-Lowry definition of acids and bases are complementary – Brønsted acids donate protons to Brønsted bases. Because of this, Brønsted acids and bases can be considered as part of a conjugate pair - a pair of chemical species that differ from each other by one hydrogen ion (H+).
    • An example is the base ammonia, NH3 and its conjugate acid, the ammonium ion, NH4+.
    • Any Brønsted acid or base can be thought of as part of a conjugate pair:
      • The conjugate \, base, without the extra proton as the base is in a state to accept a proton. Using the example above, that is ammonia, NH3.
      • The conjugate \, acid, with the extra proton as it can donate that proton to another species. Using the example above, that is ammonium, NH4+.
    • Conjugate pairs are interesting to chemists because acid-base interactions happen in aqueous solution and often establish equilibrium. A tip for spotting conjugate pairs: check the products of the equilibrium and see what gained H+ (the base) and what lost H+ (the acid) in the forward reaction. You will see which is the acid and which the base because the roles are reversed in the products. For this reason, different conjugate pairs often react in pairs! See the equation for two conjugate pairs A and B:

      [Conj. acid HA] + [ Conj. base B-] ⇌\rightleftharpoons [ Conj. base A-] + [Conj. acid HB]

    • Another example of conjugate pairs reacting is below:

      [Conj. acid HC+] + [ Conj. base D] ⇌\rightleftharpoons [ Conj. base C] + [Conj. acid HD+]

  • Organic acids and bases are part of conjugate pairs too.
    • Organic acids such as carboxylic acids contain the -COOH functional group. This dissociates into -COO- and H+ in the following reaction:

      RCOOH (aq) → CH3COO- (aq) + H+ (aq)
    • Organic bases contain amino groups, -NH2 where the nitrogen lone pair can accept a proton in the following reaction:

      RNH2(aq) + H+ (aq) → RNH3+(aq)

  • There is another relationship between conjugate acids and bases that chemists observe that the weaker the conjugate base, the stronger the conjugate acid. This is true in reverse as well – a very strong conjugate base will have a very weak conjugate acid.