BIOL 1406

PreLab 3.2

What are acids and bases?

Although several definitions for acids and bases exist, in this class we will define an acid as a proton donor, and a base as a proton acceptor. Remember, a proton is just another name for a hydrogen ion (H+). When an acid donates a proton to a base, the acid becomes a conjugate base (since it is now a proton acceptor) and the base becomes a conjugate acid (since it is now a proton donor):
 

General Example
Specific Example


Because acids are proton donors, they act to increase [H+] when dissolved in water. For example, when hydrochloric acid (HCl) is dissolved in water, it dissociates to form H+ and Cl- . The release of H+ ions increases [H+] in the solution. Also, remember that in any aqueous solution as [H+] increases [OH-] will decrease.

On the other hand, because bases are proton acceptors they act to decrease the [H+] when dissolved in water. Bases can decrease the [H+] in two ways:

  1. They can dissociate to form OH-. For example, sodium hydroxide (NaOH) is a strong base that dissociates to form Na+ and OH-. As [OH-] increases, [H+] decreases, because some of the H+ ions that were present in the water before the NaOH was added will combine with the extra OH- to form water.
  2. They can combine directly with H+ ions, thereby removing “free” H+ ions from the solution. As the number of H+ ions declines, more water molecules dissociate, increasing the number of OH- ions. For example (see figure above), ammonia (NH3) combines with H+ ions in water to form ammonium ions (NH4+), thereby decreasing the concentration of free H+ ions.

In sum, when an acid is added to an aqueous solution, [H+] increases and [OH-] decreases. When a base is added to an aqueous solution, [OH-] increases and [H+] decreases.

Acids and bases can vary in the extent to which they alter [H+] and [OH-]. Strong acids, such as hydrochloric acid (HCl) and sulfuric acid (H2SO4), almost completely dissociate in water. Therefore, they produce a relatively large increase in [H+] and a correspondingly large decrease in [OH-]. On the other hand, weak acids, such as acetic acid and citric acid, have a much lower level of dissociation. Therefore, they produce a relatively small increase in [H+] and a correspondingly small decrease in [OH-]. Similarly, strong bases, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), produce a large increase in [OH-] when dissolved in water, while weak bases such as ammonia (NH3) and Trizma® base produce a much smaller increase in [OH-]. Most biomolecules have properties of weak acids and/or bases.

YOUR TURN

A hydrogen ion is another name for a         

Hint Check your answer.
When an acid is added to an aqueous solution, increases and decreases.         Hint Check your answer.
When a base is added to an aqueous solution, increases and decreases.         Hint Check your answer.

 


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