Chapter 21 - Buffers and Neutralisation Flashcards
Define a buffer solution
It minimises the pH changes when small amounts of an acid or a base are added.
What are the 2 components in a buffer solution
They contain a weak acid and its conjugate base.
What does the weak acid do
It removes the added alkali
What does the conjugate bas do
It remove the added acid
How can you prepare the weak acid buffer solutions?
- Preparation from a weak acid and its conjugate base
2. Preparation by partial neutralisation of weak acids
Preparation from a weak acid and its conjugate base
This needs a weak acid and its conjugate base.
Use an acid and one of it’s salts. When the acid is added to water, it will partially dissociate, and the acid itself is the weak acid.
Any weak acid salt is an ionic compound, so it will fully dissociate. So the salt is the conjugate base component. (As in CH3COO- is the component)
Preparation by partial neutralisation of weak acids
Adding an aqueous solution of an alkali to an excess f the weak acid.
The weak acid is partially neutralised by the alkali forming the conjugate base.
Some of the weak acid is left unreacted.
Action of the buffer solutions.
Adding more H+ to:
HA H+ +A-
- Add more H+
- H+ reacts with the conjugate base
- EQP moves to the left.
- Added acid is removed.
Action of the buffer solutions.
Adding more OH - to:
HA H+ +A-
- Adding more alkali
- A small concentration of the H+ reacts with the OH-.
- HA dissociates shifting right to restore the H+ ions.
How can you choose the components for a buffer solution
A buffer is most effective when there are equal concentrations of the weak acid and its conjugate base. So when:
- pH of buffer is same as pKa value of the HA.
- The operating pH is typically over about 2 pH units, centred at the pH of the pKa value.
Equation to calculate the pH of a buffer solution
{H+(aq)] = Ka x [HA]/[A-]
If the pKa is given and the [HA] =[A]
Then the pH of the buffer = pKa value.
What pH should the blood plasma be at?
Between 7.35 and 7.45.
How id the blood plasma pH controlled?
With the carbonic acid-hydrogencarbonate buffer system, which is the most important.
What happens if the pH falls below 7.35?
Develop acidosis.
Which causes fatigue, shortness of breath and in extreme cases, shock or death.