Chapter 21 - Buffers and Neutralisation

Question 1

what is the equivalence point/ end point of a titration curve

Answer 1

• sharp vertical rise in the pH curve
• shows the point where the acid has been fully neutralised
• at this point [H+] = [OH-]

Question 2

how to chose a suitable indicator for a titration

Answer 2

• a suitable indicator must change colours entirely within the vertical part of the titration curve to be effective at determining the end point

Question 3

what is a buffer

Answer 3

• a chemical that resists the change in pH when small amounts of acid or base are added
• buffers do not stop change in pH they just resist it

Question 4

what is an acidic buffer

Answer 4

• resist the change in pH in order to keep the solution bellow pH 7
• they are made of a weak acid and its salt

Question 5

examples of two equilibrium reactions happening in an acidic buffer

Answer 5

Question 6

what happened when you add an acid to the acidic buffer

Answer 6

• [H+] ions increases
• H+ ions react with the CH3COO- ions in the solution
• there is a high concentration of these from the dissociation of the salt
• increase in [H+] and more CH3COOH being produced means equilibrium shifts to the left

Question 7

what happens when you add a base to the acidic buffer?

Answer 7

• OH- ions react with the H+ ions
• low concentration of H+ ions so they are replaced from the high concentration of the acid to counteract the change
• equilibrium shifts to the right to replace the reacted H+ ions

Question 8

calculating the pH of a buffer? assumptions?

Answer 8

• you need to know the Ka value, and the concentration of the weak acid and its salt
• assume [salt] = [A-]
• assume [HA start] = [HA equilibrium]