Lecture Nineteen - Equilibrium and buffers Flashcards
What is a buffer?
Careful control of pH is essential in many cases.
Buffers are solutions that maintain an approximately constant pH, even when small amounts of acid or base are added.
They are usually made from - a weak acid and its conjugate base, or a weak base and its conjugate acid.
To understand how they work, one must understand the weak ion effect.
What is the weak ion effect?
This is how buffers work.
When small quantities of H3O+ or OH- are added to the buffer, they cause a small amount of one buffer component to convert into the other.
As long as the amounts of H3O+ and OH- are small compared to the concentrations of the acid and conjugate base in the bugger, the added ions will have little effect on the pH since they are consumed by the buffer components.
What is the Henderson-Hasselbalch Equation?
The Henderson-Hasselbalch Equation is a rearranged form of the Ka equation used to design buffers with particular pH ranges.
Ka = ([H3O+][A-])/[HA]
pH = pKa + log 10 ([A-]/[HA])
This equation allows us to solve for pH directly without having to calculate [H3O+] first.
Explain acid base titrations.
Acid base titrations are used to determine the concentration of an unknown acidic or basic solution.
An acid is slowly added to a base (or visa versa) using a burette and a conical flask.
The equivalence point represents the point at which the acid has completely reacted with the base.
Often detected using an indicator.
What is the pH range of a buffer?
The effective buffer range can be defined by the Henderson-Hasselbalch equation.
[acid][base] = 1/10 to 10/1
Substitute these values into the Henderson-Hasselbalch equation:
pH range = pKa + log10 ([10]/[1])
pH range = pKa +/- 1.
How can the pH of monoprotic solutions be found?
Since pKa(2) is so much greater than pKa(1), the pH of an aqueous solution can be determined from pKa(1) alone.
