O.3 Flashcards
Buffer solutions
Despite an increasing concentration of CO2 in the atmosphere, the ocean doesn’t appear to get any more acidic, with more CO2 gas dissolving and dissociating. This is because the ocean is an example of what?
A buffer solution.
What is the definition of a buffer solution?
A solution that can resist a change in pH for a small addition of acid or alkali.
Why does the pH of buffer solution stay almost constant even though acid or alkali is added?
Because acid or alkali ions added are immediately removed.
What are two ways to make buffer solutions?
- A weak acid and its salt, e.g. ethanoic acid and sodium ethanoate.
- A weak base and its salt, e.g. ammonia solution and ammonium chloride.
What is the equilibrium that shows the dissociation of a weak acid?
[HA] ==== [H+] + [A-]
Where A- represent its salt.
Why does adding salt to a weak acid make a good buffer solution?
Because the addition of salt to the solution pushes the equilibrium well to the left, so essentially suppresses the ionisation of the acid, so it almost doesn’t dissociate at all.
What happens when a small amount of acid, H+ is added to the buffer solution?
The A- ions from the salt react with the extra H+ to make HA. This removes the extra H+ and the pH is reestablished.
What happens when a small amount of alkali, OH- is added to the buffer solution?
HA generates H+ ions that reacts with the added OH- ions to form water, pH is reestablished.
Why are both the weak acid and salt important for the buffer solution to normally function?
It needs the acid to act as a source of H+ when they are needed, and there must be plenty of salt to act as a sink for extra H+.
What are two assumptions we can hence make about the equilibrium system of a buffer solution?
- [A-] = [Salt], all the A- came from the salt, not the acid dissociation.
- [HA] = [Acid], no acid dissociated at all.
Using the assumption of a buffer solution, what is the expression of Ka we can deduce?
Ka = [H+] x [salt]/[acid]
What are two factors the concentration of H+ ions depend on in a buffer solution?
- The value of Ka.
- The ratio of [salt]:[acid].
What would happen if the concentrations of HA and A- in a buffer solution are very different?
It will affect its buffering abilities and reduce its effectiveness.
What is the relationship between pKa and pH when [HA] = [A-]?
When [HA] = [A-], Ka = [H+].
Therefore pKa = pH.
