Conjugate Pairs
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Answers to Exercise 20
Answers to Exercise 21

Conjugate Pairs

A natural consequence of the Brønsted-Lowry definitions of acids and bases, perhaps a diabolical one, is that we end up with a very interesting relationship between acids and bases. An acid can donate a proton. The chemical that remains after the proton is donated is a base.

Let's consider the acid HF dissociating and giving up its proton. This equation does not show what is accepting the proton so that we can concentrate on what is left from the HF. What's left is fluoride ion. Because the fluoride ion no longer has a proton, it's no longer an acid. HF is an acid, but F- is not an acid. Being without a proton it could presumably accept a proton. If it can, we would consider the fluoride ion to be a base.

HF rtarrow.gif (850 bytes) H+ + F-

Let's expand the example. Let's look at HF donating its proton to a molecule of water, as is shown here. In this reaction, a proton is transferred from HF to H2O. That makes H2O a base in this reaction because it's accepting a proton. After it has accepted a proton, the water becomes a hydronium ion. It now has a proton, which it can donate, therefore it's an acid.

HF + H2O rtarrow.gif (850 bytes) H3O+ + F-

 

When a base accepts a proton, it becomes an acid because it now has a proton that it can donate. And when an acid donates a proton it becomes a base, because it now has room to accept a proton.

These are what we call conjugate pairs of acids and bases.

When an acid gives up its proton, what remains is called the conjugate base of that acid. When a base accepts a proton, the resulting chemical is called the conjugate acid of that original base. HF and F- are a conjugate acid-base pair. H2O and H3O+ are a conjugate pair, where H3O+ is the acid and H2O is the base.

Being able to recognize conjugate pairs of acids and bases becomes important dealing with comparisons between the strengths of acids and bases, buffered solutions, and the hydrolysis of various salts. Those topics will come up in later lessons.

Practice (Ex.20)

Now try your hand at identifying the conjugate acid-base pairs in the equations shown below and also in exercise 20 in your workbook. You can check your answers on the answer page when you continue with the lesson.

equation
H2SO4 + H2O rtarrow.gif (850 bytes) HSO4- + H3O+
HCl + OH- rtarrow.gif (850 bytes) Cl- + H2O
HCl + NH3 rtarrow.gif (850 bytes) Cl- + NH4+
NH3 + H2O rtarrow.gif (850 bytes) NH4+ + OH-

 

More Practice (Ex. 21)

Next work through the questions that are listed below and also given in exercise 21. After you've done those, check your answers on the answer page. If you have any problems at all in working through them, be sure to consult with the instructor. Then continue with the lesson.

What are the conjugate bases of these acids?

original acid conjugate base
HNO3  
H2O  
H3O+  
H2SO4  
HBr  
HCO3-  

What are the conjugate acids of these bases?

original base conjugate acid
OH-  
H2O  
HCO3-  
SO42-  
ClO4-  

 

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