SciPAD (Pages 299-303)

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[Page 299]

1) Describe the function of a buffer solution

A mixture/solution of a weak acid and its conjugate bases. Its function is to minimise changes in pH when small amounts of acid and base are added.

2) Describe, without reference to specific chemical species, how an:

a. Acidic buffer solution is prepared: from a weak acid and its conjugate base.

b. Alkaline buffer solution is prepared: from a base and its weak conjugate acid.

3) State the condition(s) necessary for pH to equal the pKa of the weak acid used in a buffer solution.

The weak acid and conjugate base need to in equimolar amounts.

4) A general equation for the reaction of any acid with water can be written: HA(aq) + H2O(l) --> H3O+(aq) + A+(aq)

a. Identify the symbols represeting the acid/conjugate base pair.

HA and A-

b. Write an equation for the reaction of the acid with OH-.

HA(aq) + OH-(aq) --> A-(aq) + H2O(l)

c. Write an equation for the reaction of the conjugate base with H3O+.

A-(aq) + H3O-(aq) --> HA(aq) + H2O(l)

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[Page 300]

5) Write the conjugate base of the following weak acids (needed to make a buffer solution):

a. HCOOH / HCOO- b. HF / F- c. NH4+ / NH3

6) Write the conjugate acid of the following weak bases (needed to make a buffer solution):

a. CH3COO- / CH3COOH b. NO2- /HNO2 c. CN- / HCN

7) For the following buffer solutions, write balanced chemical equations to show which species will react with hydronium ions and hydroxide ions:

a. propanoic acid / sodium propanoate (CH3CH2COONA):

Reaction with hydronium ions

CH3CH2COO-(aq) + H3O+(ag) --> CH3CH2COOH(aq) + H2O(l)

Reaction with hydroxide ions:

CH3CH2COOH(aq) + OH-(aq) --> CH3CH2COO-(aq) + H2O(l)

b. HA-(aq) / H2S(aq):

Reaction with hydronium ions:

HA-(aq) + H3O+(aq) --> H2S(aq) + H2O(l)

Reation with hydroxide ions:

H2S(aq) + OH-(aq) --> HS-(aq) + H2O(l)

c. Benzoic acid C5H6COOH(aq) / Sodium benzoate (C5H6COONA(aq)):

Reaction with hydronium ions:

C5H6COO-(aq) + H3O(aq) --> C5H6COOH(aq) + H2O(l)

Reation with hydroxide ions:

C5H6COOH(aq) + OH-(aq) --> C5H6COO-(aq) + H2O (l)

8) Bicarbonate, HCO3-, carobonic acid, H2CO3, work together as a buffering system that helps regulate the pH of b. Explain using equations how this system works.

When small amounts of acid (H3O+) are added, the HCO3-(aq) reacts with the H3O+ to produce H2CO3 and H2O. The [H3O+] has not changed o the pH remains unchanged.

HCO3-(aq) + H3O+(aq) --> H2CO3(aq) + H2O(l)

When small amounts of base (OH-) are added, the H2CO3(aq) reacts with the OH- to produce HCO3- and H2O. The [OH-] have not changed so the pH remains unchanged.

H2CO3(aq) + OH-(aq) --> HCO3-(aq) + H2O(l)