12: Buffers, Titrations and pH Curves Flashcards
Acidic Buffers Basic Buffers pH Curves Indicators
What is a buffer?
A solution that resists changes in pH when small amounts of acid or alkali are added.
Describe the pH of an acidic buffer.
Have a pH of less than 7.
What is contained in an acidic buffer?
A mixture of a weak acid and a salt of that weak acid.
What is contained in a basic buffer?
A mixture of a weak base and a salt of that base.
Give an equation for the dissociation of ethanoic acid (CH3COOH) .
CH3COOH (reversible) CH3COO- + H+
Give an example of a salt used in a buffer made of ethanoic acid.
CH3COONa
What happens when H+ is added to an acidic buffer?
Extra H+ ions combine with the CH3COO- ions.
This reduces the H+ ion concentration closely back to its original level causing pH to remain constant.
What happens when -OH is added to an acidic buffer?
Extra -OH ions combine with the CH3COO- ions and H+ to make water, so the -OH concentration is reduced back to its original level and pH remains constant.
If ammonia is used as a weak base, name the salt that would be present in the buffer.
NH4Cl
What does the salt do when in the solution of ammonia?
Dissociates fully.
Give an ionic equation for the dissociation of NH4Cl.
NH4Cl -> NH4+ + Cl-
Give an equation for the ammonia molecules NH3 reacting with water molecules.
NH3 + H2O (reversible) NH4+ + OH-
What happens when a small amount of acid is added to a basic buffer? (What forms?)
What happens to the equilibrium position?
The H+ concentration increases, increasing the acidity of the solution.
Some H+ ions react with the -OH ions to make H2O.
Equilibrium moves to the right to replace -OH ions that have been used up and remove the extra H+ added, keeping pH constant.
What happens when a small amount of base is added to a solution?
The additional -OH ions react with which part of the buffer solution to produce water?
What happens to position of equilibrium?
-OH concentration increases making the solution more alkaline.
NH4+ ions react with the extra -OH to form NH3 and H2O.
Equilibrium shifts left to remove -OH ions from the solution, preventing change in pH.
What happens when you dilute a buffer with water?
The water slightly dissociates so the extra H+ and -OH ions push the equilibrium the same amount in both directions leaving it unchanged.
Explain why biological washing powders use buffers.
They contain enzymes, so buffers help to maintain the optimum pH for the enzymes so they don’t denature.
Why are buffers used in shampoos?
They increase the roughness of individual hairs surfaces to keep hair smooth and shiny.
Give an example of why buffers are used in the body.
An optimum pH is needed in the blood to ensure cells can work correctly.
Describe the shape of a pH curve with a strong acid, strong base.
Curve begins low and rises to 14
Describe the shape of a pH curve with a strong acid, weak base.
Curve begins low and ends near pH 8/9
Describe the shape of a pH curve with a weak acid, strong base.
Curve begins around pH 5 and ends around pH 14.
Describe the shape of a pH curve with a weak acid, weak base.
pH starts around 5 and ends around pH 9.
Describe the shape of a pH curve with a strong base, strong acid.
Curve starts at pH 14 and ends at pH 0.
Describe the shape of a pH curve with a strong base, weak acid.
Curve starts at pH 14 and ends at pH 5.
Describe the shape of a pH curve with a weak base, strong acid.
Curve starts around pH 8 and ends around pH 1.
Describe the shape of a pH curve with a weak base, weak acid.
Curve begins around pH 9 and ends at pH 5.
What is the midpoint on the vertical section of the pH curve called?
Equivalence point.
What is the equivalence point?
The point when the base or acid completely neutralises the solution.
How do you select the correct indicator needed for a titration?
The indicators pH range must fall within the vertical section on a pH curve.
