What are conjugate acid-base pairs?

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We should start by appreciating that in reality many acid-base reactions are equilibrium reactions, even if it doesn’t appear obvious at first sight.

In the case of hydrochloric acid:

But with ethanoic acid:

If the ethanoate ion, CH3COO–, will readily accept a proton from H3O+ to complete the back reaction, it is behaving as a base. We can say that the ethanoate ion is the conjugate base of ethanoic acid and that together, they make a conjugate acid-base pair.

Water of course is behaving as a base in accepting a proton from ethanoic acid in the forward reaction but in the back reaction, H3O+ is behaving as an acid in donating a proton to the ethanoate ion. H3O+ is the conjugate acid to water’s base. Another conjugate acid-base pair.

Let’s look at another example.

It is really important to appreciate that we are describing the behaviour of a substance in the context of a particular reaction (as defined by the equation), we are not giving substances labels. The HSO3– ion can also be seen behaving as a base:

Practice questions

  1. Identify the conjugate pairs in each of these reactions

(a) NH3 (aq)   +    H2O (l)    ⇌    NH4+ (aq)    +     OH– (aq)

(b) HNO3 (aq)    +   OH– (aq)    ⇌    NO3– (aq)    +     H2O (l)

(c) H2O (l)     +     H2O (l)     ⇌      H3O+ (aq)     +    OH– (aq)

(d) HSO3– (aq)    +    H2O (l)     ⇌      SO32- (aq)    +  H3O+ (aq)

2.       Write an equation to show each of the following behaving as a base in solution – the product of the reaction will be the conjugate acid.

(a)  OH– (aq)         (b)    HSO3– (aq)     (c)   CO3 2- (aq)     (d)    HCO3– (aq)

3.         Write an equation to show the following behaving as an acid in solution – the product of the reaction will be the conjugate base.

(a)    HCO2H (aq)      (b)    H2O (l)       (c)   HCO3– (aq)     (d)  H2S (aq)

  1. When ethanoic acid (CH3COOH) is added to the stronger fluoroethanoic acid (FCH2COOH) the mixture establishes an equilibrium. Write an equation for the reaction and identify the two conjugate acid-base pairs. 

Answers

  1.   (a)  Base   NH3 (aq)  /  Conjugate acid NH4+ (aq)  ;  Acid  H2O (l)   /  Conjugate base   OH– (aq)

(b)  Base  OH– (aq)  /  Conjugate acid  H2O (l)  ;  Acid  HNO3 (aq)  /  Conjugate base   NO3– (aq)

(c)  Base  H2O (l)  /  Conjugate acid H3O+ (aq)  ;  Acid  H2O (l)  /  Conjugate base  OH– (aq)

(d)  Base  H2O (l)/  Conjugate acid H3O+ (aq) ;  Acid  HSO3– (aq)  /  Conjugate base  SO32- (aq)

2.     (a)   OH– (aq)  +    H+ (aq)   ⇌    H2O (l)

(b)  HCO3– (aq) +   H+ (aq)   ⇌   H2SO4 (aq)

(c)   CO3 2-(aq) +   H+ (aq)   ⇌    HCO3– (aq)

(d)  HCO3– (aq)  +   H+ (aq)   ⇌    H2CO3 (aq)

3. (a)    HCO2H (aq)  ⇌  H+ (aq)   +  HCO2– (aq)

(b)    H2O (l)  ⇌  H+ (aq)   +  OH– (aq)

(c)     HCO3– (aq)    ⇌  H+ (aq)   +   CO3 2–(aq)

(d)     H2S (aq)   ⇌  H+ (aq)   +  HS– (aq)

4. The stronger the acid, fluroroethanoic acid, will donate the proton to the weaker acid, ethanoic acid.

CH3COOH(aq) + FCH2COOH(aq) ⇌ CH3COOH2+(aq) + FCH2COO–(aq)