pH and buffers

What is meant by a weak acid and a bronsted-lowry acid

• partially dissociates H+ ions in aqueous solution

• AND a proton donor

How can an aqueous solution of an acid contain hydroxide ions

• water dissociates to form H+ and OH- ions

• H20 —> (reversible arrow) H+ + OH-

A student adds an excess of aqueous ethanoic acid to solid calcium carbonate. The resulting solution is able to act as a buffer solution. 

Explain why the buffer solution has formed. 

Solution contains CH3COO- and CH3COOH

A student adds an excess of aqueous ethanoic acid to solid calcium carbonate. The resulting solution is able to act as a buffer solution. 

Explain how this buffer solution controls pH when either acid or alkali is added 

STANDARD FORMAT: 
equation (in this case): CH3COOH —> H+ + CH3COO- 

CH3COO- reacts with added acid causing equilibrium to shift to the left 

CH3COOH reacts with added alkali causing equilibrium to shift to the right 

Explain what is meant by the term buffer solution.
Describe how a buffer solution based on methanoic acid can act as a buffer.

• a buffer solution is a solution that minimises changes to the pH of the solution on addition of small amounts of acid or alkali

Added alkali:

• HCOOH reacts with added OH-

Added acid: HCOO- reacts with H+

Carbonic acid, H2CO3, is a weak bronsted-lowry acid formed when carbon dioxide dissolves in water.
Explain how the carbonic acid-hydrogen carbonate mixture acts as a buffer in the control of blood pH.
In your answer you should explain how equilibrium allows the buffer solution to control the pH

• H2CO3 —> H+ + HCO3-

• some carbonic acid remains in solution

• when alkali is added, OH- react with H+/H2CO3 causing equilibrium to shift to the right

• when acid is added, H+ ions react with HCO3- so equilibrium shifts to the left

examples of acid-base conjugate pairs

HNO3 + HNO2 —> NO3- + H2NO2+ (removal of H to the one that doesnt have a plus charge and remember the plus charge)

Buffer solution made by mixing an excess of propanoic acid to an aqueous solution of sodium hydroxide.
- explain why the buffer solution formed when an excess of propanoic acid mixed with aqueous sodium hydroxide 

-explain how this buffer solution controls pH when acid or alkali added 

Normal eq: CH3CH2COOH + NaOH → CH3CH2COONa + H2O 

some propanoic acid remains 

EQUILIBRIUM EQUATION: CH3CH2COOH → H+ + CH3CH2COO- 

added alkali: 
CH3CH2COOH reacts with added alkali, equilibrium shifts to the right 

added acid: 
CH3CH2COO- reacts with added acid, equilibrium shifts to the left