STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 12)

by BerryBerryTeacher

in Berry Reference (Notes)

Buffering agents are chemicals that adjusts or maintain the pH values of a solution within the narrow limits (prevent change in pH) by absorbing or releasing hydrogen ions. Buffering agents then form the main components of a buffer solution, which also maintains the pH of solutions in order to resist any changes made to pH level. Part 12 of Berry Berry Easy notes on Ionic Equilibrium for STPM Form 6 Chemistry introduces the concept of biology buffer through examples and also explains the Henderson-Hasselbalch equation which is used to ascertain the pH level of a buffered solution. While this might seemed difficult, it can be easily understood if you understand the Le Chatelier’s principle.

[Tips: Do some calculation questions involving the Henderson-Hasselbalch equation. Most books do not have enough questions involving the Henderson-Hasselbalch equation, as such students do not get enough practise on it. Try to go online to find example of this questions. It'll help you a long way.]

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 12)

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage

Biology Buffer

Diet Cola is fairly acidic. If you consumed 12 can of diet cola, you might be expected that pH in your body would become lower than normal neutral condition and you might be dying. But surprisingly, in our body blood which is a buffer that can resist changes in pH.

Case 1:

HCl + NaCH3COO –> NaCl + CH3COOH

Buffer solution containing:

  • Weak acid = acetic acid
  • Conjugate base = sodium acetate

When we add hydrochloric acid (strong acid) to this solution, hydrochloric acid will be converted into a weak acid, CH3COOH.

Therefore, the overall pH of the solution relatively constant / pH value change only very slightly.

Case 2:

NaOH + CH3COOH –> NaCH3COO + H2O

Buffer solution containing:

  • Weak acid = acetic acid
  • Conjugate base = sodium acetate

When we add sodium hydroxide (strong base) to this solution, sodium hydroxide will be converted into a weak base, NaCH3COO.

Therefore, the overall pH of the solution relatively constant / pH value change only very slightly.

Henderson-Hasselbalch equation

  • pH = – log10 Ka + log10 ([base] / [acid])
  • pH = pKa + log10 ([A-] / [HA])
  • pH = pKa + log10 ([Moles conjugate base] / [Moles conjugate acid])

This equation is used to :

  • determine the pH of a buffered solution.
  • it shows that as [conjugate acid] increases, buffer pH decreases.
  • it also indicates that if the pH is greater than the pKa, the solution is rich in conjugate base.

Example:

If water is added to a buffer solution with pH = 3.96. What happen to the pH?

A The pH remains the same
B The pH increases slightly
C The pH decreases slightly
D If the pH is greater than 7, then it decreases. If the pH is less than 7, then it increases.

Solution:

  • Addition of water to a buffer solution equally dilutes the concentration of the weak acid and its conjugate base.
  • This means that the mole ratio of the weak base to the weak acid does not change upon the addition of water.
  • According to the Henderson-Hasselbalch equation, the pH of the solution does not change because pKa is constant and the fraction has not changed.
  • Therefore, the pH of the buffer does not change when it is diluted.

Answer: A

Stick with Berry Berry Easy for the upcoming post, Part 13 of Ionic Equilibrium for Chemistry – STPM Level which also involves buffer and how to create buffers.

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