Buffers Flashcards
What is a buffer?
A chemical that resists the change in pH when small amounts of acid or base are added.
Explain the two types of buffers and how they are made.
Acidic buffer: Resists the change in pH in order to keep the solution below pH 7. They are made from a weak acid and its salt.
Basic buffer: Resists the change in pH in order to keep the solution above pH 7. They are made from a weak base and its salt.
Explain the equilibrium in acid buffer solutions using ethanoic acid CH3COOH and sodium ethanoate CH3COO-Na+
In any buffer solution there are 2 equilibrium equations that both co-exist in the same beaker.
1:
CH3COOH(aq) [HIGH] = CH3COO-(aq) [LOW] + H+ [LOW]
- The weak acid dissociated weakly so equilibrium is well over to the left.
2:
CH3COO-Na+(aq) [LOW] = CH3COO-(aq) [HIGH] + Na+(aq) [HIGH]
- Salts dissociate strongly (fully) so equilibrium lies well over to the right.
What happens if we add an acid to the acidic buffer?
The H+ ions will react with the CH3COO- ions in the solution. There is a high concentration of CH3COO- ions from the salt dissociation hence more CH3COOH is produced which means equilibrium shifts to the left.
What happens if we add a base to the acidic buffer?
The OH- ions react with the H+ ions in solution. There is a low concentration of H+ ions but they can be reproduced because of the high concentration of the weak acid CH3COOH to counteract the change (Le Chatelier’s Principle). This means more CH3COOH will dissociate to make more H+. Equilibrium shifts to the right to replace reacted H+ ions.
Explain the equilibrium in basic buffer solutions using ammonia NH3 and ammonium chloride NH4+Cl-
In any buffer solution there are 2 equilibrium equations that both co-exist in the same beaker.
1:
NH3(aq) [HIGH] + H2O(l) = NH4+(aq) [LOW] + OH-(aq) [LOW]
- Weak base produced very little OH- ions so equilibrium lies well over to the left.
2:
NH4+Cl-(aq) [HIGH] = NH4+(aq) [HIGH] + Cl-(aq) [HIGH]
- Salts dissociate strongly (fully) so equilibrium lies well over to the right.
What happens if we add a base to the basic buffer?
The OH- ions will react with the NH4+ ions in solution. There is a high concentration of NH4+ ions from the salt dissociation. More NH3 and H2O is produced which means equilibrium shifts to the left.
What happens if we add an acid to the basic buffer?
The H+ ions will react with the OH- ions in solution. There is a low concentration of OH- ions but they can be reproduced from a high concentration of NH3 and H2O to counteract the change (Le Chatelier’s Principle). Equilibrium shifts to the right to replace reacted OH- ions.
