Chapter 21 Flashcards
What is a buffer solution?
- A system that minimises pH changes on addition of small amounts of an acid or a base
What do buffer solutions consist of?
- Some undissociated acid (which is why WEAK acids must be used)
- Its conjugate base
How can buffer solutions be formed?
- By mixing a weak acid and a salt of the weak acid (e.g. ethanoic acid and sodium ethanoate)
- By reacting excess weak acid with an alkali (e.g. ethanoic acid and sodium hydroxide)
How do the 2 methods of forming buffer solutions provide weak acid and conjugate base?
- Method 1: the salt of the weak acid supplies the conjugate base (as it would break up in solution)
- Method 2:
- the acid and the alkali react to form the conjugate base (the salt formed would break up in solution)
- an EXCESS of acid is used to supply the weak acid (otherwise it would all be neutralised)
How do buffer solutions control pH when acid is added?
- The weak acid dissociates, creating an equilibrium
- If H+ is added, it reacts with the conjugate base to form the acid
- This shifts equilibrium to the left, which removes H+ ions
How do buffer solutions control pH when alkali is added?
- If alkali is added, OH- is added
- The OH- ions react with H+ ions to form water
- This decreases [H+], which shifts equilibrium to the right, and restores [H+]
Why do buffers only work when a small amount of acid or alkali is added?
- If too much alkali or acid is added, buffers cannot remove or replenish enough H+ ions
How do you calculate the pH of a buffer solution? What shortcut is there to this method?
- [H+]= Ka × [HA] / [A-]
- You can use the moles of HA and A- if it’s faster, since they would have the same volume
How are the most effective buffer solutions made? What special quality do these buffer solutions have?
- They contain equal concentrations of weak acid and its conjugate base (in the reaction method of making a buffer, you would add half as much as alkali as weak acid)
- Since [HA] = [A-], they would cancel out in the expression and [H+] = Ka, and pH = pKa
What approximation is used for weak acids, but not for buffers?
- [H+] = [A-]
- (A- was added in the buffer, so it’s no longer true)
What range of values should the pH of blood plasma be between?
- 7.35-7.45
How is the pH of blood maintained? Give the equation to demonstrate this.
- Using the carbonic acid-hydrogencarbonate buffer system
- H2CO3 <-> H+ + HCO3-
How do pH titration curves work? (Use the example of acid in the conical flask, and you’re adding base.)
- You plot volume of base added against pH
- Twice as much base as needed is used
- This causes the graph to be flat at first, have a steep rise, and then go flat again (s-shaped)
List 4 main features of pH titration curves (going from acidic to alkaline).
- At first, there is an excess of acid, so the pH slowly increases
- Then there is vertical line called the vertical section, where pH increases rapidly on addition of a very small amount of base
- The equivalence point is at the centre of the vertical section
- At the end, there is an excess of base, so the pH slowly increases
What is the equivalence point of a reaction?
- The point in a titration where the volume of one solution has reacted exactly with the volume of the other solution