Understanding pH Changes

Question

I'm confused about how buffering works in weak acid/base systems. If I add a small amount of strong acid (like HCl) to a buffer made of acetic acid and sodium acetate, why doesn't the pH drop drastically? Can someone explain what's happening on a molecular level in a way that's easy to visualize? I'm studying for the AP Chem exam and this part keeps tripping me up.

By geometry dash meltdown

Answer 1

Let's begin with knowing and understanding the difference between a strong acid/base, and a weak acid/base.

A strong acid will ionize completely whereas a weak acid will not. This is the key to understanding how this acetic acid/acetate buffer works. In the example above, HCl is a strong acid and acetic acid is a weak acid. To understand why the pH does NOT change significantly when a strong acid is added to a weak acid buffer, you must understand this distinction between strong and weak. Now, let's look at what is happening on the molecular level when you add HCl (strong acid) to the acetic acid/acetate buffer.

The buffer essentially contains acetic acid (HAc) and the acetate anion (A^-). If you add HCl or any acid (H⁺), it will react with the A^- to produce HAc. And since HAc is a WEAK acid it will NOT ionize very much, thus essentially removing the added H⁺ from solution, and resisting a large change to the pH.

If you add a base (OH^-), it will react with the HAc to form H₂O and A^-, again, essentially removing the OH^- from solution, and resisting a large change to the pH.

Here it is in equation form:

CH₃COO^- + H⁺ ==> CH₃COOH (A WEAK ACID)

CH₃COOH + OH^- ==> CH₃COO^- + H₂O ( FORMATION OF CONJUGATE BASE AND WATER)

Hopefully this makes sense, and helps you understand how buffers work to resist large changes in pH. If not, please comment and indicate what is not clear.

Answer 2

Well the answer to this question is that when HCl( a very strong acid that produces many H+ proton ions ) is added to the buffer solution and due to the presence of its H + proton reacts with the weak acid salt (CH3OO-) .which ends up being used up in the reaction making the ratio of the weak acid salt in the buffer solution to decrease while more acetic acid is formed.Thus causing the sudden decrease in PH but note that it's doesn't fully drop down drastically as the buffer solution still does it work to resist major change in PH.