S14E2 - Acid Strength and Acid-Base Conjugate Pairs

Another Bronsted-Lowry Acid-Base Reaction.

ex:  Let's look at the reaction between ammonia (a base) and hydrochloric acid:

As you can see in the second image below, the lone pair on NH₃ actively receives the H⁺ (proton) from the Bronsted-Lowry acid.

So while we say that the acid is the proton-donor, it's the Bronsted-Lowry base (NH₃) that actively "goes out and grabs" the proton (H⁺).

The result is an H⁺ transfter that generates the products - the conjugate base of our original acid, and the conjugate acid of our original base:

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Acid Dissociation Reaction Practice Problems.

ex:   HC₂H₃O₂

   HC₂H₃O₂ (aq)   ⇌   H⁺ (aq)   +   C₂H₃O₂ ^- (aq)

   - or, handwritten so it's easy to see:

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ex:   H₂SO₄

   H₂SO₄ (aq)   ⇌   H⁺ (aq)   +  HSO₄ ^- (aq)

   - or, handwritten so it's easy to see:

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ex:   [Al(H₂O)₆]³⁺

  Al(H₂O)₆³⁺ (aq)   ⇌   H⁺ (aq)   +  Al(H₂O)₅(OH)²⁺ (aq)

   - or, handwritten so it's easy to see:

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Acid Strength.

➞ consider the reaction of the monoatomic acid, HA, with water:

 HA (aq)   +   H₂O (l)   ⇌   H₃O⁺ (aq)   +   A^- (aq)

As stated already in the image above,

➞ If HA is a strong acid, then:
 - it has a weak conjugate base.
 - the value for K_a is very large: so equilibrium lies to the right.

➞ If HA is a weak acid, then:
 - it has a strong conjugate base.
 - the value for K_a is very small: so equilibrium lies to the left.

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NOTE - The strongest acid and strongest base always end up on the same side of a reaction, and equilibrium lies away from them.

So in the above general reaction, if HA is a stronger acid than H₃O⁺, then H₂O must be a stronger base than A^-

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Table of Ka Values for Some Common Monoprotic Acids.

monoprotic = acid has just one acidic proton (H⁺).

In Table 14-1 below, the strong acids HCl, HNO₃, HClO₄, H₂SO₄ are not listed because their "Ka values" are so high...

Here's an acid-strength practice problem where Table 14-1 (above) will come in handy.

ex:  Using Table 14-1, arrange the following bases from strongest to weakest base.

     F^-   ,   Cl^-   ,   NO₂^-   ,   CN^-

To do this problem, we have to remember:

➞ the "weakest base" is the conjugate of the "strongest acid."
➞ the "strongest base" is the conjugate of the "weakest acid."

So, we have:

     CN^-   >   NO₂^-   >   F^-   >   Cl^- 

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Stick around for the 3rd installment of SECTION 14 - Acids and Bases,

where we'll talk about the pH scale and pH calculations...